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Chapter 41

Animal Form and Function


• Anatomy: study of an organism's physical structure
• Physiology: study of how the physical structures in an organism function
• There is a great diversity in anatomical and physiological traits observed in animals

Form, Function, and Adaptation

• Biologists who study animal anatomy and physiology are studying adaptations
o Adaptation: heritable trait allowing survival and reproduction in a given environment
genetic change that occurs over generations in response to natural selection in a population.
o Acclimatization or acclimation: phenotypic change that occurs in an individual in response to short-term change in environmental conditions
ability to acclimatize is itself an adaptation

Adaptations and Structure/Function

• If structure found in animal is adaptive (helps to survive and produce offspring), is common that its composition correlates closely with its function
o if mutant allele (leading to adaptation) alters structure and functions more efficiently, it leads to greater fitness, resulting in increased frequency of allele in population over time!
• Correlations between structure/function at molecular level:
o ex.) protein shape correlates with protein function
• Correlations between structure/function occur at cellular level:
o ex.) cells that secrete enzymes contain lots of rough ER and golgi
o ex.) absorptive cells have large surface area


• Animals are multicellular—bodies contain distinct types of cells that are specialized for different functions.
• Tissue: group of similar cells that function as a unit
• Embryonic tissue gives rise to four adult tissue types, all of which have a structure that highly correlates with its function:
1. Connective.
2. Nervous.
3. Muscle.
4. Epithelial.

Connective Tissue

• Connective tissue: holds body together; provides framework for growth and development
o consists of cells loosely arranged in a liquid, jellylike, or solid extracellular matrix
each type secretes its own distinct type of matrix
o Four categories of connective tissue:
1. Loose connective tissue
2. Dense connective tissue
3. Supporting connective tissue
4. Fluid connective tissue

Nervous Tissue

• Nervous tissue: consists of nerve
cells (neurons) and several types
of supporting cells

Muscle Tissue

• Muscle tissue: functions in movement
o Three types of muscle tissue:
1. Skeletal muscle.
2. Cardiac muscle.
3. Smooth muscle.

Epithelial Tissues

• Epithelial tissues: cover outside of body; line surfaces of organs; form glands
o provide protection; interface between interior and exterior
o regulate transfer of heat, water, nutrients, and other substances between interior and exterior
o has polarity (sidedness)
apical side faces toward environment
basolateral side faces animal's interior
each side has a distinct structure and function
o Organ: specialized functions; consists of several tissues
o Gland: secretes specific molecules or solutions

Organs and Organ Systems

• Cells with similar functions are organized into tissues
• Tissues are organized into specialized structures called organs
• Organs are part of larger units called organ systems
• Organ systems consist of groups of tissues and organs that work together to perform one or more functions.

Organs and Organ Systems

• Structure/function of each component in the body is integrated with other components
o each level of organization is integrated with other levels of organization
o organism as a whole is greater than the sum of its parts

Body Size Affects Animal Physiology: Surface Area/Volume Relationships

• The rate at which gases, nutrients, and waste products diffuse across membranes depends in part on the amount of surface area available for diffusion.
• Rate at which nutrients are used and waste products are produced depends on the volume of the cell.
• As cell gets larger, volume increases faster than surface area

Comparing Mice and Elephants

• Metabolic rate: overall rate of energy consumption by individual
• Basal metabolic rate (BMR): rate at which O2 consumed at rest, with empty stomach, normal temperature and moisture conditions (measured in mL O2 consumed/gm body mass/hour)
o On per-gram basis, small animals have higher BMRs than large animals
elephant has more mass than mouse, but 1 gram of elephant tissue consumes much less energy than 1 gram of mouse tissue
as organism's size increases, mass-specific metabolic rate must decrease, or surface area available for exchange of materials would fail to keep up with metabolic demands

Adaptations Increase Surface Area

• Function of cells and tissues depends on diffusion; structure has shape that increases surface area relative to volume
o Flattening, folding, and branching are effective ways to increase surface area/volume ratio
1. Fish gills have lamellae (flattened sheetlike structures)
2. Mammalian small intestine has villi (folds)
3. Capillaries (small blood vessels) are highly branched


• Homeostasis: maintenance of a relatively constant internal state regardless of environment changes
• Constancy of physiological state achieved by two processes:
o Conformation to the external environment
Ex.) body temperature of Antarctic rock cod closely matches that of surrounding seawater
o Regulation - physiological mechanism adjusts internal state to keep it within tolerable limits
Ex.) dog maintains body temperature of about 38°C whether its cold or hot outside

Why Is Homeostasis Important?

• Temperature, pH, and other physical and chemical conditions have dramatic effect on structure and function of enzymes
o Most enzymes function best under fairly narrow range of conditions
• Molecules, cells, tissues, organs, and organ systems function at an optimal level when homeostasis occurs.

Role of Regulation and Feedback

• Animals have regulatory systems that constantly monitor internal conditions (temperature, blood pressure, blood pH, and
blood glucose...)
o If one variables changes, the system acts quickly to modify it
o Each variable has a set point (normal or target value)
• Homeostatic system is based on three general components:
o Sensor
o Integrator
o Effector

Role of Regulation and Feedback

• Sensor: structure that senses external or internal environment
• Integrator: component of nervous system that evaluates incoming sensory information and "decides" if response is necessary to achieve homeostasis
• Effector: structure that helps restore desired internal condition
• Homeostatic systems are based on negative feedback in which effectors oppose the change in internal conditions

Animals Regulate Body Temperature

• Heat exchange is critical in animal physiology because individuals that get too hot or too cold may die
o Overheating can cause proteins to denature and cease functioning; can also lead to dehydration
o Low body temperatures can slow down enzyme function and energy production
• Many animals control body temperature via thermoregulation
o Endotherm: produces adequate heat to warm its own tissue
o Ectotherm: relies on heat gained from the environment

Temperature Homeostasis in Endotherms

• Thermoregulation: important aspect of homeostasis in some animals
o Temperature receptors in skin (sensors) sense external environment
o Information from sensors are interpreted by hypothalamus (integrator)
o Variety of responses occur, ranging from metabolic to behavioral (effectors)
Ex.) within cells, dramatic temperature spikes denature proteins and may activate heat-shock proteins that speed the refolding of proteins (key step in recovery process)

Concepts to Remember

• Structure has profound influence on function at variety of levels: molecules, cells, tissues, organs, and organ systems
• Role of surface area/volume relationship
• Homeostasis: maintain relatively constant internal environment
o have systems that sense changes in internal conditions and trigger responses that return conditions to normal
• Some animals have sophisticated systems for generating and conserving heat and regulating body temperature.

Chapter 42

Water & Electrolyte Balance in Animals


• Chemical reactions occur in aqueous solutions
o if water/solute balance is disturbed, chemical reactions (and life itself) may stop
• Electrolyte: compound that dissociates into ions when dissolved in water
o maintaining electrolyte balance is crucial
cells require precise concentrations (Na+, Cl-, K+, and Ca2+) to function
• Water balance, electrolyte balance, and excretion of waste products are tightly integrated processes.

Osmoregulation and Osmotic Stress

• Electrolytes and water move by diffusion and osmosis
o Diffusion: movement from regions of higher concentration to lower concentration (along concentration gradients)
Solutes move down concentration gradients across selectively permeable membranes by diffusion
o Osmosis: diffusion of water through selectively permeable membrane from areas of higher to lower water concentration
o Osmolarity: concentration of dissolved substances in a solution (measured in moles per liter)

Osmotic Stress

• Diffusion & osmosis affect animals differently based on habitat
o Different environments pose different challenges in maintaining water and electrolyte balance
• Osmotic stress: occurs when concentration of dissolved substances in cell or tissue is abnormal
• Osmoregulation: control by living organisms of water and salt concentration in their bodies
• Osmoconformers: organisms (ex. sponges and jellyfish) who do not osmoregulate because tissues are isotonic to seawater
o maintain fairly constant ionic and osmotic environments that nearly match electrolyte concentrations found in seawater

Osmotic Stress in Seawater

• Osmoregulation required in marine fish because tissues are hypotonic to salt water (contain fewer solutes)
o lose water by osmosis; gain electrolytes by diffusion
under osmotic stress
Solution = active transport to lose excess salts

Osmotic Stress in Freshwater

• Tissues of freshwater fish are hypertonic to
surrounding water (solution inside cells contains more solutes)
o cells gain water through osmosis; lose electrolytes by diffusion
Solution = active transport to gain salts

Osmotic Stress on Land

• Land animals constantly lose water to environment, just as many marine animals do - by evaporation rather than osmosis.
• Land animals also lose water when produce urine, sweat, or pant.

Active and Passive Transport

• Solutes move across membranes by passive or active transport.
• Passive transport: diffusion along electrochemical gradient; does not require expenditure of energy
o Ex.) facilitated diffusion of solutes via proteins called channels or carriers
• Active transport: ATP powers movement of solute against electrochemical gradient
o Uses membrane proteins called pumps


• Once pump establishes concentration gradient, co-transport can occur
• Energy released when solute is transported along concentration gradient can be used by co-transporter to transport another molecule against concentration gradient
o Symporters: move solutes in the same direction
o Antiporters: move solutes in opposite directions

Water and Electrolyte Balance in Aquatic Environments (Sharks)

• Sharks - model organism in researching marine osmoregulation
o similar salt-secreting systems found in wide array of species, including humans
shark rectal glands secrete a concentrated salt solution
actively transported against concentration gradient
epithelial cells along inner surface (lumen) of shark rectal gland contain sodium-potassium pumps

A Molecular Model for Salt Excretion

• Salt (NaCl) excretion is a multistep process:
1. Na+/K+ ATPase creates electrochemical gradient favoring diffusion of Na+ into cell
a. allows transport of other ions without additional ATP
2. Na+, Cl-, and K+ enter the cell
a. powered by Na+ electrochemical gradient
3. Chloride channels allow Cl- to diffuse down its concentration gradient into the lumen of the gland
4. Na+ diffuses into the lumen of the gland
a. along its electrochemical gradient

Common Mechanism of Salt Excretion

• In many animals, epithelial cells that transport Na+ and Cl- ions contain same combo of membrane proteins found in sharks
o Ex.) marine fish that excrete salt from gills
o Ex.) mammals that transport salt in their kidneys
• Research on shark rectal gland also had unforeseen benefit for biomedical research - cystic fibrosis research
o Cystic fibrosis transmembrane regulator (CFTR) was identified in humans and found to be 80% identical to shark chloride channel
o Subsequent studies supported the hypothesis that cystic fibrosis results from defect in a chloride channel

Types of Nitrogenous Wastes

• Ammonia (NH3) is a by-product of catabolic reactions
o strong base that readily gains a proton to form an ammoniumm ion (NH4+) - which is eventually toxic to cells
Different species get rid of ammonia safely and efficiently in different ways.
Fish detoxify ammonia by diluting it to a low concentration and excrete it as watery urine
Humans convert ammonia to less toxic urea and excrete it in urine
Birds, reptiles, and terrestrial arthropods convert ammonia to uric acid, which is excreted as dry paste

Why Do Nitrogenous Waste Vary among Species?

• Type of nitrogenous waste produced by animal correlates with its evolutionary history
• Waste production also correlates with habitat that species occupies, and thus the amount of osmotic stress it endures
• Is fitness trade-off between energetic cost of excreting urea or uric acid and benefit of conserving water

Maintaining Homeostasis: The Excretory System (An Example: Insects)

• To maintain homeostasis, insects must carefully regulate the composition of hemolymph (bloodlike fluid) because:
1. Nitrogenous wastes have to be removed before build up to toxic concentrations.
2. Excess electrolytes must be excreted before they lead to osmotic stress.
3. Water balance must be regulated constantly.

Maintaining Homeostasis: The Excretory System

• To maintain water and electrolyte balance, insects have:
o Malpighian tubules (excretory organ)
o Hindgut (posterior portion of digestive tract)

Filtrate Forms in Malpighian Tubules

• Malpighian tubules:
o Have large surface area
o In direct contact with hemolymph
o Empty into hindgut
o Responsible for forming filtrate from hemolymph
o "pre-urine" (filtrate) then passes into hindgut, where it's processed and modified prior to excretion

Selective Reabsorption of Electrolytes and Water in the Hindgut

• If shortage of electrolytes and water, they are reabsorbed and returned to hemolymph
o Ions are transported back into
hemolymph from filtrate - water follows by osmosis
forms concentrated urine
results in water conservation and nitrogenous waste elimination

Selective Reabsorption of Electrolytes and Water: Active Transport in Hindgut

• Chloride Pump: Cl- pumped into epithelial cells from hindgut lumen
o K+ follows through potassium channels along electrochemical gradient - water follows by osmosis
• Na+/K+-ATPase: Na+ pumped into hemolymph; K+ pumped into epithelial cells
o sets up gradient
o favors movement of Cl-, K+, and H2O into hemolymph

Overview of Water Regulation and Electrolyte Balance

• Several general principles have emerged from insect studies:
o Water is not pumped directly—moves only by osmosis due to gradients set up by active transport of ions
o Formation of filtrate is not particularly selective
o Reabsorption is highly selective for certain molecules and ions
o Reabsorption is tightly regulated in response to osmotic stress - ion channels are activated and deactivated

Water and Electrolyte Balance in Terrestrial Vertebrates

• Terrestrial vertebrates must carefully regulate the osmolarity of
their tissues.
o Most drink water to replace water they lose
o Ingest electrolytes in food
o Osmoregulation occurs primarily in the kidney
§ Responsible for water and electrolyte balance and
excretion of nitrogenous wastes

Structure of the Kidney

• Kidneys occur in pairs located in back side of body
• Renal artery brings blood containing wastes into kidney
• Renal vein carries cleaned blood away
• Urine formed in kidney is transported via ureter to storage organ (bladder); then transported to body surface via urethra and excreted

Structure of the Kidney

• Most of kidney's mass is made of small structures (nephrons)
o Nephron: basic functional unit of kidney
Maintains water and electrolyte balance
Shares important functional characteristics with insect excretory system:
Water not transported actively—only moves by osmosis
Cells in kidney set up strong osmotic gradients
Regulate osmotic gradients and specific channel proteins to exert precise control over loss or retention of water and electrolytes

Kidney Function: An Overview

• Nephrons have 4 major regions
1. Renal corpuscle: filters blood; forms "pre-urine"
2. Proximal tubule: reabsorbs nutrients, vitamins, valuable ions, and water
3. Loop of Henle: establishes strong osmotic gradient in tissues outside loop;
4. Distal tubule: ions and water reabsorbed in
manner regulated by hormones

Kidney Function: An Overview

• Collecting Duct: closely associated with nephrons; under control of hormones - maintains homeostasis with respect to water
o urea leaves base of collecting duct and contributes to osmotic gradient set up by loop of Henle
• Blood Vessels: juxtaposed with nephron - play key role
o bring "dirty" blood into nephron and take away molecules and ions that are reabsorbed from initial filtrate
o serves nephron by wrapping around each of its four regions

Filtration: Renal Corpuscle

• Urine formation begins here
• Glomerulus: cluster of capillaries; bring blood to nephron from renal artery
o capillaries have large pores
o surrounded by unusual cells whose membranes fold into a series of slits and ridges
• Bowman's capsule: region of nephron surrounding glomerulus

Filtration: Renal Corpuscle

• Pressure much higher in glomerulus than surrounding capsule
o pressure differential forces water and solutes out of blood through pores in glomerulus
results in filtrate formation ("pre-urine")\
up to 25% of water and solutes present in blood is removed
filtering large volumes from blood allows waste to be removed effectively - pairing process with reabsorption allows waste excretion to occur with minimal water and nutrient loss

Reabsorption: Proximal Tubule

• Proximal tubule: filtrate enters here after Bowman's capsule
o Filtrate contains water and small solutes such as urea, glucose, amino acids, vitamins, and electrolytes
some waste products and also valuable nutrients
• Epithelial cells of proximal tubule have prominent series of small projections (microvilli) facing lumen
o Microvilli greatly increase epithelial surface area
• Reabsorption occurs in proximal tubule
o Active transport of selected molecules out of filtrate
Causes water to follow via osmosis along osmotic gradient
valuable solutes and water are returned to body

Reabsorption: Proximal Tubule

• Selective reabsorption requires four molecular mechanisms:
1. Na+/K+-ATPase in basolateral membranes removes intracellular Na+, creating gradient for Na+ entry from lumen
2. Na+-dependent cotransporters in apical membrane use gradient to remove valuable ions and nutrients from filtrate
3. Solutes move into cell and diffuse into blood vessels
4. Water follows ions from proximal tubule into cell and then into blood vessels
Reabsorption: Proximal Tubule

Ion and Water Movement Driven by "Master Gradient"

• Water leaves proximal tubule through water channels (transmembrane proteins) called aquaporins and across cell membrane
1. Filtration step in renal corpuscle is based on size
2. Reabsorption step in proximal tubule selectively retrieves small substances that are valuable
a. pumps and co-transporters in proximal tubule recover water, nutrients, and electrolytes but leave wastes

Creating Osmotic Gradient: Loop of Henle

• Loop of Henle functions as counter current exchanger and multiplier; sets up osmotic gradient
o Countercurrent = fluid flows in opposite directions
o Exchanger = exchange fluids from nephron to blood
o Multiplier = sets up concentration gradient
o Osmotic Gradient = osmolarity of fluid inside loop of Henle is low in cortex and high in medulla -osmolarity in tissues surrounding loop mirrors this gradient

Establishment of the Osmotic Gradient

• Loop of Henle has three distinct regions:
o Descending limb: highly permeable to water; impermeable to solutes
o Thin and thick ascending limb: highly permeable to Na+ and Cl-; moderately permeable to urea; impermeable to water
• Loop of Henle maintains osmotic gradient because water leaves the descending limb and salt leaves the ascending limb!

Loop of Henle: More Details

1. Thick ascending limb: Na+ and Cl- are actively transported out
o increases osmolarity outside descending limb...
2. Descending limb: loses water
o movement of water is passive, down gradient
o creates concentrated fluid inside loop of henle at bottom of descending limb...
3. Thin ascending limb: loses Na+ and Cl- passively along gradient (does not lose water because impermeable)

Loop of Henle: In Summary

• The countercurrent flow of material is self reinforcing
o presence of osmotic gradient stimulates water and ion flows that in turn maintain the osmotic gradient.

Role of the Vasa Recta

• Water and salt that move out of Loop of Henle quickly diffuse into vasa recta (associated network of blood vessels)
o as result, water and electrolytes are returned to the body

Regulating Water and Electrolyte Balance

• Once filtrate passes through Loop of Henle, enters distal tubule
o fluid is now slightly hypotonic to blood
contains mainly urea and other waste products
o fluid that enters distal tubule is relatively constant in composition over time...
• Following distal tubule, fluid moves to collecting duct
o urine that leaves collecting duct is highly variable in osmolarity and in Na+ and Cl- concentration...

Collecting Duct Leaks Urea

• Collecting Duct: osmotic gradient in tissue is partially established here
o urea diffuses out of innermost section of the collecting duct
creates steep osmotic gradient in space
surrounding nephron
high in inner medulla and low in outer medulla

Urine Formation Is Under Hormonal Control

• Activity of distal tubule and collecting duct is highly regulated and altered in response to osmotic stress
o amount of Na+, Cl-, and water that is reabsorbed here varies with the animal's condition
o Changes in the distal tubule and collecting duct are controlled by hormones
If Na+ levels in blood are low, adrenal glands release the hormone aldosterone - leads to activation of sodium pumps and reabsorption of Na in the distal tubule
If an individual is dehydrated, the brain releases antidiuretic hormone (ADH)

How Does ADH Work?

• Two important effects of ADH on epithelial cells in collecting duct:
1. Insertion of aquaporins into the apical membrane
o cells more permeable to water; large amounts of water are reabsorbed
2. ADH increases permeability
to urea
o Increases osmolarity of surrounding fluid and thus water loss from the filtrate

How Does ADH Work?

• In response to ADH:
o Water leaves collecting duct passively—following concentration gradient maintained by loop of Henle
o water is conserved; urine strongly hypertonic relative to blood
• When ADH is absent:
o few aquaporins found in collecting duct epithelium
o collecting duct relatively impermeable to water
o results in hypotonic urine
• Fun Fact: Alcohol inhibits ADH

Concepts to Remember

• Different habitats pose different challenges to animals with regard to maintaining water and electrolyte balance.
• In marine animals (ex. sharks) specialized cells remove excess salt - same types of cells are found in the kidneys of mammals.
• In terrestrial insects, hindgut and Malpighian tubules responsible for excreting water-soluble waste products and achieving homeostasis
• In terrestrial vertebrates, kidney is responsible for excreting water-soluble waste products and achieving homeostasis
o Renal Corpuscle = filtration
o Proximal Tubule = reabsorption
o Loop of Henle creates and maintains osmotic gradient = reabsorption
o Collecting Duct - regulated by hormones

Chapter 43

Animal Nutrition


• Animals are heterotrophs
o obtain energy and nutrients from other organisms rather than making their own food)
• Ingestion: first of four processes needed to obtain energy from food
o must be followed by digestion, absorption, and

Nutritional Requirements

• Animals get chemical energy and carbon-containing building blocks they need from carbohydrates and fats
• Animals also require other nutrients (substances an organism needs to remain alive)
o Food: any material that contains nutrients
• Humans require several other essential nutrients (nutrients that cannot be synthesized and must be obtained in the diet)

Essential Nutrients

• Essential amino acids: cannot be synthesized by humans; must be obtained from food
• Vitamins: vital for health; required only in small amounts; several function as coenzymes
• Electrolytes: inorganic ions that influence osmotic balance; required for normal membrane function
• Inorganic substances: often are important components of cofactors or structural materials (ex. calcium, phosphorus, magnesium and iron)

Nutrient Digestion and Absorption: An Introduction

• Processes necessary for animal to obtain energy from food:
o Ingestion
o Digestion: breakdown of food into small enough pieces to allow for absorption
o Absorption: uptake of nutrients
o Elimination

Digestive Tract

• Digestive tract / gastrointestinal (GI) tract: begins at mouth and ends at anus; digestion takes place here
• Digestive tracts come in two general designs:
1. Incomplete digestive tract: single opening where food is ingested and wastes are eliminated
2. Complete digestive tract: has two openings— starts at mouth and ends at anus; interior of tube communicates directly with external environment via these openings

Complete Digestive Tract

• Advantages of complete digestive tract:
1. Can feed on large pieces of food
2. Chemical and physical processes
are separated within canal; occur
independently and in prescribed sequence
3. Material can be ingested and
digested continuously

Beginning: Mouth and Esophagus

• Enzymes in saliva break down some food components
o Salivary amylase: most important in breakdown of carbs
o Lipase: begins breakdown of lipids
• Salivary glands: release water and mucins (glycoproteins)
o Mucus: slimy substance formed when mucins contact water
makes food soft and slippery enough to be swallowed
• Esophagus: muscular tube connecting mouth and stomach
o Peristalsis: wave of muscle contractions that propels food to stomach
Automatic reaction (reflex) to act of swallowing


• Stomach: muscular pouch bracketed on both ends by valves (sphincters)
o muscular contractions churn, mix and break down food
o lumen is highly acidic
predominant acid is HCl

Stomach: Protein Digestion

• Gastric juice: contains HCl and pepsin; pH as low as 1.5
o Chief cells: specialized stomach cells synthesize and secrete pepsin precursor (pepsinogen)
Pepsinogen converted to active pepsin by contact with acidic environment of stomach
Pepsin: enzyme that digests proteins
Secretion of inactive form prevents destruction of proteins in chief cells
o Parietal cells: secrete HCl that denatures proteins
• Mucous cells: secrete mucus - lines epithelium and protects stomach from damage by HCl

Parietal Cells: Secretion of HCl

• Carbonic anhydrase: high concentration in parietal cells
o catalyzes forming of carbonic acid (H2CO3) from CO2 & H2O
o carbonic acid immediately dissociates into bicarbonate ion (HCO3-) and a proton
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
Protons formed actively pumped into lumen of stomach
Co-transport (antiport) of HCO3- and Cl
Passive transport of Cl- from parietal cell into stomach

The Small Intestine

• Small intestine: six-meter-long tube; receives partially digested food from stomach
o food mixes with secretions from pancreas and liver
o enormous surface area for absorption of nutrients due to projections (villi), which in turn have projections (microvilli)
increases efficiency of nutrient absorption
each villus contains blood vessels and a lymphatic vessel
nutrients pass quickly from epithelial cells to transport systems
o at the end of small intestine, digestion is complete
most nutrients—along with much water—has been absorbed

Protein Processing in Small Intestine

• Protease: generic name for any enzyme that digests polypeptides to monomers
o many types - each specific for different kind of polypeptide
o synthesized in inactive form by pancreas
transferred through pancreatic duct to small intestine -activated in small intestine
• Pancreatic enzymes activated sequentially by other enzymes
o chain of events starts with enterokinase (located in small intestine)
• Nucleases: digest RNA and DNA; secreted by pancreas
o RNA and DNA found within cells of ingested food

Carbohydrate Digestion and Absorption

• Pancreatic amylase: continues digestion of carbohydrates that began in mouth; produced by pancreas
• Two general principles about nutrient (carb) absorption:
1. Highly selective
o plasma membranes of microvilli responsible for bringing specific nutrients into cell
2. Active Process
o requires expenditure of ATP to bring nutrients into epithelium against concentration gradient

Carbohydrate (Glucose) Absorption:
Active Transport and Co-transport

1. Na+/K+ Pump in epithelial cells
o generates electrochemical gradient that favors
Na+ entry
2. Glucose enters cell with sodium via cotransporter
3. Glucose diffuses into nearby blood vessels via passive transport

Digesting Lipids: Bile Salts

1. Hydrophobic fats enter small intestine in large globules that must be broken up (emulsified) before digestion can begin
o Bile salts: small lipids that emulsify fat into small globules
synthesized in liver
secreted in complex solution (bile) that's stored in gallbladder
2. Pancreatic lipase: breaks bonds in complex fats
o results in release of fatty acids and other small lipids
lipids then bind receptor protein, enter epithelial, are further processed and diffuse into lymphatic vessels and blood

Hormones Signal Secretion from Glands

• Secretin: hormone produced in small intestine in response to arrival of food from stomach
o induces flow of bicarbonate ions from pancreas to small intestine
bicarbonate neutralizes acid arriving from stomach
• Cholecystokinin: hormone produced in small intestine
o stimulates secretion from pancreas and liver
• Hormones involved in stomach function as well
o Gastrin: hormone produced by stomach
stimulates parietal cells to secrete HCl

Water Absorption

A: Osmosis

Cecum (aka. Appendix)

• Cecum: evagination of digestive tract located at start of large intestine
o dramatically reduced in size in humans
functions in defense against invading bacteria and viruses
because size and function differ from typical cecum, it is called the appendix

The Large Intestine

• Primary function is to compact remaining waste and absorb enough water to form feces
o processes occur in colon (main section of large intestine)
aquaporins play major role in water absorption
o feces are held in rectum (final part of large intestine) until ready to be excreted

Nutritional Homeostasis: Glucose

• When digestion is complete, nutrients enter bloodstream and are delivered to cells that need them.
• Too much of a nutrient, or too little, can be problematic or even fatal!
• People with diabetes experience abnormally high levels of glucose in their blood
o Over lifetime, chronic glucose imbalance can lead to reduced circulation in legs, blindness, and heart failure
o caused by problems with a hormone (insulin)

Glucose Homeostasis

• When blood glucose levels are high:
o Insulin is secreted by pancreas and binds receptor on target cell surface:
cells increase rate of glucose uptake
cells synthesize storage molecules (glycogen and fat)
blood glucose levels decrease
• When blood glucose levels fall too low:
o Glucagon is secreted by pancreas and binds receptors on target cell surface:
cells in liver catabolize glycogen
cells in liver synthesize glucose from non carbohydrate compounds
blood glucose levels rise

Diabetes Can Take Several Forms

• Type I diabetes: affected individuals do not synthesize insulin
o Insulin-producing cells of pancreas are mistakenly attacked by immune system
o treated with insulin injections and careful attention to diet
• Type II diabetes mellitus: cells of affected individuals lose ability
to respond to insulin (still not well understood)
o managed through prescribed diets, monitoring blood glucose levels, and drugs that increase cellular responsiveness to insulin

Concepts to Remember

• Digestion occurs in digestive tract (compartmentalized into organs with specialized functions): ingestion and digestion of food, absorption of nutrients and water, or excretion of wastes
o Enzyme activity - active and inactive forms
o Active and passive transport and co-transport
o Hormone Regulation
• Lack of homeostasis with respect to nutrients such as glucose can cause disease - diabetes.
o Insulin and glucagon regulate glucose levels through negative feedback

Chapter 44

Gas Exchange and Circulation

Introduction: Respiratory and Circulatory Systems

• O2 is required for cellular respiration and CO2 is produced
o must be continuously exchanged with the environment to support ATP production
o must be transported throughout the body (along with wastes, nutrients, and other molecules)
• Gas exchange involves four steps: ventilation, gas exchange, circulation, and cellular respiration.

Four Steps of Gas Exchange

1. Ventilation: air or water moves through a specialized gas exchange organ, such as lungs or gills.
2. Gas exchange: CO2 and O2 diffuse between air or water and blood at ventilatory surface.
3. Circulation: dissolved O2 and CO2 are transported thru body
4. Gas exchange: O2 and CO2 exchange occurs between blood and tissues where cellular respiration occurs

Air as Respiratory Media

• Gas exchange between environment and cells is based on diffusion
o O2 is high in the environment and low in tissues
O2 tends to move from the environment into tissues
o CO2 is high in tissues and low in the environment.
CO2 tends to move from tissues to the environment

Partial Pressure

• Partial pressure: pressure of particular gas in mixture of gases
o Calculate partial pressure of particular gas: multiply fractional composition of gas by total pressure of entire mixture
Ex.) partial pressure of O2 (Po2) at top of Mt. Everest = % O2 in atmosphere (0.21) * atmospheric pressure (250) = 53 mm Hg
o O2 and CO2 diffuse between environment and cells along partial-pressure gradients (from high to low)

Respiration: Vertebrate Lungs

• Air enters trachea and is carried to lungs via bronchi which then branch into bronchioles and empty into alveoli
Alveoli: tiny sacs specialized for gas exchange
greatly increase surface area for gas exchange
provide interface between air and blood
approximately 150 million alveoli per lung in humans

Ventilation of Human Lung

• Inhalation: accomplished by downward motion of muscular sheet (diaphragm) and outward motion of rib muscles
o results in increased lung space and decreased pressure which draws air into lungs along a pressure gradient
• Exhalation: passive process driven by elastic recoil of the lungs as the diaphragm and rib muscles relax Boyle's Law: at constant temp,
inverse relationship between volume and pressure

O2 and CO2 Transport in Blood

• Blood: connective tissue consisting of platelets, red blood cells and white blood cells in a watery extracellular matrix (plasma)
o Red blood cells: transport oxygen from lungs to body tissues, and carbon dioxide from tissues to lungs
contain oxygen-carrying molecule (hemoglobin)
consists of four polypeptide chains (tetramer), each of which binds a non-protein group (heme)
each heme contains an iron ion that can bind oxygen
each hemoglobin molecule can thus bind up to four oxygen molecules
• In blood, 98.5% O2 bound to hemoglobin; 1.5% is in plasma

Hemoglobin: Cooperative Binding

• Po2 greater in blood leaving lungs than in other tissues
o diffusion gradient - unloads O2 from hemoglobin to tissues
o oxygen-hemoglobin equilibrium (or oxygen dissociation) curve: plots % saturation of hemoglobin versus Po2 in tissues
sigmoidal (S-shaped) curve
cooperative binding: binding of each O2 molecule to subunit of hemoglobin causes conformational change that makes remaining subunits more likely to bind O2 (loss of bound O2 makes additional losses more likely)
makes hemoglobin extremely sensitive to changes in Po2 of tissues (large change in % saturation in response to small change in tissue Po2)

CO2 Transport

• CO2 produced by cellular respiration enters blood and is quickly converted to bicarbonate ions (HCO3-) and protons (H+)
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-
o Carbonic anhydrase: catalyzes formation of H2CO3
o H2CO3 quickly converted to HCO3- and protons
most CO2 is transported in blood in the form of HCO3-

Carbonic Anhydrase Activity: Leads to Bohr Shift

• Protons produced decrease the pH
o alters hemoglobin's conformation so more likely to release O2 at all values of Po2 = Bohr shift
therefore, increase O2 uptake into tissue if high CO2

Carbonic Anhydrase Activity: Leads to Increased CO2 Uptake

• Pco2 in blood drops when CO2 is converted to bicarbonate
o maintains strong partial-pressure gradient favoring entry of CO2 into red blood cells

The Lungs: Exhalation of CO2

• In lungs, hemoglobin releases protons, which combine with bicarbonate to form CO2
o in alveoli, partial-pressure gradient favors diffusion of CO2 from blood to atmosphere
o hemoglobin picks up O2 during inhalation, and the cycle begins again

The Circulatory System - Humans

• Provides large surface area for diffusion of gases
o carries blood into close contact with every cell in body
• Closed circulatory system: blood flows in continuous circuit through body under pressure generated by the heart
o generates enough pressure to maintain high flow rate
o blood flow is directed in a precise way
o contains array of blood vessels - each has distinct structure and function

Blood Vessels

• Blood vessels are classified as arteries, capillaries, or veins.
o Arteries: take blood away from heart under high pressure - have tough, thick-walls
Aorta: large artery into which the heart ejects blood
elastic walls allow it to expand when blood enters under high pressure from heart
o Arterioles: small arteries
o Capillaries: smallest vessels - walls are just one cell thick
• allow gases and other molecules to exchange with tissues
• Veins: return blood to heart under low pressure - have thinner walls and larger interior diameters than arteries
• Venules: small veins

Basic Heart Structure and Function

• Atrium: receives blood returning from circulation
• Ventricle: generates force to propel blood through body
• Atria are separated from ventricles by atrioventricular valves

The Heart Pumps Blood

• Blood flows through the heart in a specific sequence:
1. Blood returns from body (de-oxygenated) to right atrium
2. Blood enters right ventricle through right AV valve
3. Blood pumped through pulmonary valve into pulmonary artery and to lungs
4. Blood returns to left atrium from lungs (oxygenated) via pulmonary veins
5. Blood enters left ventricle through left AV valve
6. Blood pumped through aortic valve into aorta and to body

• One-way valves ensure blood follows in only one direction
Q: What makes the "lub-dub" sound you hear in a stethoscope?
• Contraction phase of atria and the ventricles (systole) is coordinated with relaxation phase (diastole)
• Cardiac cycle: consists of one complete systole and one complete diastole.


Measuring Blood Pressure

1. Squeeze cuff until exceeds systolic pressure...then...
2. Release pressure slowly until systolic pressure just exceeds pressure of cuff and blood will flow; indicates systolic pressure
3. Slowly release cuff until no sounds; indicates diastolic pressure
• Blood pressure: force that blood exerts on walls of blood vessels"
• Systolic pressure: maximum pressure in arteries generated by ejection of blood from left ventricle during systole"
• Diastolic pressure: minimum pressure in arteries between contractions (during relaxation of the ventricles as they fill with blood)

Electrical Activation of the Heart

• Sinoatrial (SA) node (pacemaker): specialized group of cells that initiate contraction in the heart within the right atrium
o receives input from nervous system that regulates heart rate
ex.) epinephrine - fight or flight response
o electrical impulse generated by SA node is rapidly conducted throughout the right and left atria - then conducted to atrioventricular (AV) node, which passes it to the ventricles
signal spreads quickly from cell to cell because cardiac muscle cells form physical and electrical connections with each other
connected by specialized structures that allow
electrical signals to pass directly from one cell to the next

Electrical Activation of the Heart

• Key events in the heart's electrical activation include:
1. SA node originates signal
2. Signal from SA node is propagated over atria, which contract simultaneously and fill ventricles
3. Signal is conducted to AV node, which relays signal to ventricles after they fill completely with blood
4. Electrical impulse is rapidly transmitted through both ventricles, causing them to contract as the atria relax
5. Ventricles relax and the cells recover

Electrocardiogram (EKG)

• Electrocardiogram (EKG): graph that corresponds to the electrical activity associated with cardiac muscle contraction

Patterns in Blood Pressure and Blood Flow

• Blood pressure drops dramatically as blood moves through capillaries, due to increased total cross-sectional area
o decreases blood flow rate - allows sufficient time for gases, nutrients, and wastes to diffuse between tissues and blood

Concepts to Remember

• Animals must take in O2 and expel CO2 to sustain cellular respiration and stay alive.
• Lungs maximize rate of O2 and CO2 diffusion by
• large, thin surface area of alveoli
• steep partial-pressure gradient favors entry of O2 and elimination of CO2
• Hemoglobin: O2 carrying protein; extremely efficient at taking up O2 in lungs and delivering it to tissues (cooperative binding).
• Circulatory system uses pressure generated by heart to transport blood and other substances throughout the body
• Cardiac Cycle
• Electrical Activity
• Blood Pressure

Chapter 45

Electrical Signals in Animals


• Animal movements are triggered by electrical signals conducted by nerve cells (neurons) .
• Complex processes (moving, seeing, and thinking) are based on seemingly simple events: flow of ions across membranes
• Neurons transmit electrical signals; muscles can respond to electrical signals by contracting.

Overview: Nervous System

• Sensory neuron: receives information transmitted by sensory receptors (located throughout body)
• Central nervous system (CNS): comprised of brain and spinal cord; integrates information from many sensory neurons
• Peripheral nervous system (PNS): all components of nervous system outside the CNS
1. sensory information from receptors in PNS is sent to CNS
2. information is processed by the CNS
3. response is transmitted back to appropriate part of body

Anatomy of a Neuron

• Most neurons have the same three parts:
1. Dendrite: receives electrical signals from axons of adjacent cells
2. Cell body (soma): includes the nucleus; integrates incoming signals and generates an outgoing signal
3. Axon: sends signal to the dendrites of other neurons
• Each neuron makes many connections with other neurons

Membrane Potentials

• Voltage: difference in electrical potential; created by difference of electrical charge between two points
• Electrical potential: exists across membrane if positive and negative charges on ions that exist on two sides of plasma membrane do not balance each other
o Membrane potential: separation of charges when electrical potential exists across a plasma membrane
a form of electrical potential measured in millivolts (mV)
typically are 70-80 mV in neurons
are always expressed as inside-relative-to-outside (which makes then negative)

Electrical Potential, Currents, and Gradients

• When a membrane potential exists, the ions on both sides of the membrane have potential energy
o ions move across membranes in response to concentration gradients as well as charge gradients
electrochemical gradient: combination of electric gradient and concentration gradient

Resting Membrane Potential

• Resting (membrane) potential: voltage of neuronal membrane when cell is at rest
o represents energy stored in ion concentration gradients

Role of Na+/K+-ATPase

• Na+/K+-ATPase imports 2 K+ ions and exports 3 Na+ ions
o results in higher K+ concentration inside the cell and higher Na+ outside the cell
o results in inside of neuron being negatively charged relative to extracellular environment; negative resting potential

Action Potential

• Action potential: rapid, temporary change in membrane potential
o Three phases:
1. depolarization - membrane potential
becomes less negative (or positive)
2. repolarization - membrane potential returns to normal
3. hyperpolarization - membrane potential becomes more negative

Starting an Action Potential

1. Cell must become sufficiently depolarized to reach threshold potential (this leads to further depolarization)
o Resting membrane potential = -70 to -80 mV
o Threshold membrane potential = approx -55 mV
o Action potentials are all or nothing - if cell reaches threshold, action potential WILL occur because:
Na+ voltage gated channels in the axon membrane open in response to reaching threshold potential - ions rush into axon along electrochemical gradients, causing further

Step 2: Repolarization

2. Rapid repolarization
o when membrane potential reaches about +40 mV:
Na+ voltage gated channels close
K+ voltage gated channels open
K+ ions flow out of the axon, changing membrane potential from positive back to negative

Step 3: Hyperpolarization

3. Hyperpolarization
o repolarization event results in the membrane briefly becoming more negative than the resting potential
4. Repolarization - Na/K pump restores the resting membrane potential (it never stopped working - but was overpowered by
voltage gated channels!)

An "All-or-None" Signal That Propagates

• Action potentials are all-or-nothing events:
o no such thing as partial action potential
o all action potentials for a given neuron are identical in magnitude and duration
• Action potentials are propagated down the length of the axon

How Do Voltage-Gated Channels Work?

• Voltage-gated channels: ion channels that open and close in response to changes in membrane voltage
o shape of protein (voltage-gated channel) changes in response to charges present at membrane surface
shape change "opens" the channel to admit ions

Propagation of Action Potential

1. When Na+ enters cell at onset of action potential, it spreads away from the sodium channels
2. Na+ leads to depolarization of adjacent portions of membrane
3. Nearby voltage-gated Na+ channels open in response to depolarization.
4. Positive feedback occurs; full-fledged action potential results Na+ channels more likely to open as membrane depolarizes - leads to opening of additional Na+ channels, further depolarizing the membrane (reinforces the signal!)
Example of Positive feedback: occurrence of event makes the same event more likely to occur

One-Way Propagation of Signal

• Action potential continuously regenerated as moves down axon
o signal does not diminish as it moves, because response is all or none
• Action potentials do not propagate back up the axon - it is a one-way signal, because:
o Na+ channels are refractory - once opened and closed, they are less likely to open again for a short period of time
o Na+ channels downstream of the site are not in refractory state - results in one-way propagation of action potential

The Synapse

• What happens when an action potential arrives at the interface between cells?
o Neurotransmitters: molecules that transmit information from one neuron to another (or from a neuron to a target cell)
o Synapse: interface between two neurons
Presynaptic neuron: cell sending the signal
Synaptic vesicles: store neurotransmitters in axon of presynaptic neuron
Postsynaptic neuron: cell receiving the signal

Model of Synaptic Transmission

1. Action potential (AP) arrives at end of axon
2. AP triggers entry of Ca2+ into presynaptic cell (voltage gated)
3. Synaptic vesicles fuse with presynaptic membrane and release neurotransmitter into synaptic cleft (gap between cells)
4. Neurotransmitters bind receptors on postsynaptic membrane, initiating action potential if threshold is reached
5. Response ends as neurotransmitter is broken down and taken back up by presynaptic cell

Neurotransmitters: Ligand-Gated Channels

• Neurotransmitters:
1. present at synapses and released in response to AP
2. bind to receptor on postsynaptic cell
3. taken up in presynaptic cell or degraded
• Many neurotransmitters function as ligands that bind receptors called ligand-gated ion channels
o in response to binding, channel opens and ions enter along electrochemical gradient
neurotransmitter's chemical signal is transduced to electrical signal (leads to change in membrane potential of postsynaptic cell)
• Some neurotransmitters bind to receptors that activate signal transduction pathways.

Postsynaptic Potentials

• Synapses lead to depolarization or hyperpolarization of postsynaptic cell membrane
o Excitatory postsynaptic potentials (EPSPs): causes membrane to depolarize, increasing likelihood of AP
o Inhibitory postsynaptic potentials (IPSPs): causes membrane to hyperpolarize, decreasing likelihood of AP

Postsynaptic Potentials Are Graded

• EPSPs & IPSPs are not all-or-none events; are graded in size
o size depends on amount of neurotransmitter released at synapse
o signals are short lived because neurotransmitters do not bind irreversibly to channels in postsynaptic cell

Summation and Threshold

• Neurons make hundreds or thousands of synapses
o EPSPs and IPSPs lead to short-lived surges of charge in postsynaptic cell
o Summation: additive nature of EPSPs and IPSPs
if IPSP and EPSP occur close together in space/time, changes in membrane potential can cancel each other out
if several EPSPs occur close together, they sum and make neuron likely to fire an AP
charge spreads through dendrites and cell body to axon, where Na+ voltage-gated channels are located - if membrane depolarizes past threshold, an AP begins and is propagated down axon to next synapse!

Vertebrate Nervous System

• Peripheral nervous system (PNS): made up of neurons outside the CNS; consists of two systems with distinct functions:
o Afferent division: transmits sensory information to the CNS
monitors conditions inside and outside the body
carries out sensory functions
o Efferent division: carries commands from CNS to the body
carries out motor functions
sends signals that allow body to respond appropriately
further divided into two systems:
somatic system: controls movement; carries out voluntary responses under conscious control
autonomic system: controls internal processes; carries out involuntary responses not under conscious control

Mapping Functional Areas in the Cerebrum

• functions are localized to specific brain areas
o study mental abilities of people who have suffered brain damage, or lesions
o electrically stimulate portions of the cerebrum of conscious patients

Concepts to Remember

• Neurons: cells transmit electrical signals used in communication
o plasma membranes carry a voltage (membrane potential) due to differences in ion concentrations across membrane
• Action potential: are all-or-none changes in membrane potential; serve as electrical signals
o Depolarization - inflow of Na+ ions
o Repolarization (and Hyperpolarization) - outflow of K+ ions
• At synapse, electrical signal from presynaptic cell triggers release of neurotransmitter, which arrives at postsynaptic and triggers change in membrane potential - summation.
• PNS neurons receive sensory information and transmit it to the CNS, which processes the information and sends signals to muscles, glands, or other tissues via PNS neurons.

Chapter 46

Animal Sensory And Movement

Sensory Organs Convey Information to the Brain

• Sensing changes in environment and moving in response is fundamental to how animals work!
o Each type of sensory information is detected by a sensory neuron OR a specialized receptor cell that makes a synapse with a sensory neuron.
o Transduction requires a sensory receptor cell to convert the stimulus (light, sound, tension etc.) into an electrical signal
o Sensory receptors are located throughout the body and are categorized by the type of stimulus.

Types of Sensory Receptors

• Nociceptors: sense harmful stimuli
• Thermoreceptors: detect changes in temperature
• Mechanoreceptors: respond to distortion caused by pressure
• Chemoreceptors: perceive the presence of specific molecules
• Photoreceptors: respond to particular wavelengths of light
• Electroreceptors: detect electrical fields

Sensory Organs Convey Information to the Brain

• Ability to sense change in environment depends on:
1. Transduction (conversion of external stimulus to internal signal in form of an action potential)
2. Amplification of signal
3. Transmission of signal to central nervous system (CNS).

Sensory Transduction

• Recall: resting membrane potential is approx -70/80 mV
o Depolarization leads to an action potential (AP)
o Hyperpolarization makes it more difficult to have an AP
• All sensory receptors transduce sensory input (light, sounds, touch, odors etc.) to a change in membrane potential
o different stimuli are transduced to common signal type that
can be interpreted by the brain!
if sensory stimulus induces large change in membrane potential, there is change in firing rate of APs sent to brain
amount of depolarization or hyperpolarization is proportional to intensity of the stimulus

Transmitting Information to the Brain

• Receptor cells tend to be highly specific
• Each type of sensory neuron sends its signal to a specific
portion of the brain


• Hearing: ability to sense sound (pressure waves)
o Frequency: number of pressure waves occurring in one
second (we perceive differences in frequency as different
o Virtually all animal pressure-sensing systems are based on a mechanoreceptor cell that responds to pressure
direct physical pressure on plasma membrane or distortion by bending changes conformation of ion channels in membrane, causing them to open or close
Humans - ion channels that respond to pressure are found in hair cells

Signal Transduction in Hair Cells

• Ion channels open in response to pressure
changes and the cell depolarizes.
o hair cells are bathed in high extracellular K+
opening K+ channels leads to K+ influx and cell depolarization
results in new pattern of action potentials from sensory neuron to brain

Vertebrate (Human) Eye

• Light enters eye through cornea, passes through pupil, and strikes curved, clear lens
o Cornea & lens focus incoming light onto retina in back of eye
Retina comprises three distinct, synapsing cell layers:
1. Light-sensitive photoreceptors
o form layer at back of retina
2. Bipolar cells
o intermediate layer of connecting neurons
3. Ganglion cells
o form front or innermost layer of the retina
o axons project to brain via optic nerve

Photoreceptors: Rods and Cones

• Rods and Cones: photoreceptors (specialized cells) in the eye
o Rods: sensitive to dim light but not colors
o Cones: sensitive to colors and less sensitive to dim light
rods and cones have segments packed with large quantities of opsin (transmembrane protein)
each opsin molecule is associated with a molecule of
the pigment retinal (two-molecule complex=rhodopsin
Retinal changes shape when it absorbs a photon of
light, leading to a change in opsin's conformation
leads to series of events that culminates in different
stream of action potentials being sent to the brain

How Do Rods and Cones Detect Light?

• Light does not open ion channels or trigger the release of a
neurotransmitter to a sensory neuron.
• Molecular basis of vision is shape change in retinal that shuts down existing ion channel è decreases amount of
neurotransmitter being released to the sensory neuron è
decreases electrical activity sent to the brain
• Decrease in neurotransmitter indicates to the postsynaptic cell, bipolar cells, that the rod absorbed light.
o Result: new pattern of action potentials is sent to brain via
the ganglion cells

Taste - Chemoreceptors

• Chemoreceptor Cells: detect presence of particular molecule (protein receptors recognize and bind the molecule) è in response the cells undergo a change in membrane potential
o information about the presence of a particular chemical is
transduced to an electrical signal in the body
• Taste buds: structures that contain clusters of taste-sensing
o scattered around mouth & throat, but mostly found on tongue
o each contains about 100 chemoreceptor cells that synapse
to sensory neurons

Olfaction: Detecting Molecules in the Air

• When odor molecules reach the nose, they diffuse into mucus layer in roof of nose and bind to membrane-bound receptor proteins, activating olfactory receptor neurons
o Axons from these neurons project to brain where olfactory
signals are processed and interpreted


• Information received from the environment is useless unless
animal can respond in appropriate way...usually by moving
• Muscles pull against resistance (skeleton) to produce
o Muscles only exert force by contracting - pairs of muscles
work together to move a bone back and forth
Flexor muscles swing two long bones toward each other
Extensor muscles straighten two long bones out
movements of paired muscles are coordinated by motor
neurons that originate in brain or spinal cord

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