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Terms in this set (99)

- The outer ear consists of the external pinna and the auditory canal, which collect sound waves and channel them to the tympanic membrane, which separates the outer ear from the middle ear
- In the middle ear, three small bones--the malleus (hammer), incus (anvil) and stapes (stirrup)--transmit vibrations to the oval window, which is beneath the stapes
- The middle ear also opens into the Eustachian tube, which connects to the pharynx and equalizes pressure between the middle ear and the atmosphere
- The inner ear consists of fluid-filled chambers, including the semicircular canals, which function in equilibrium, and the coiled cochlea, a bony chamber that is involved in hearing
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- Several organs in the inner ear of humans and most other mammals detect body movement, position and balnce
- Situated ina vestibule behind the oval window, the chambers called the utricle and saccule allow us to perceive position with respect to gravity or linear movement
- Each of these chambers contains a sheet of hair cells that project into a gelatinous material
- Embedded in this gel are many small calcium carbonate particles called otoliths
- When you tilt your head, the otoliths press on the hairs protruding into the gel, through the hair cell receptors, this deflection of the hairs is transformed into a change in the output of sensory neurons, signaling the brain that your head is at an angle (also responsible for ability to perceive acceleration)
- Three semicircular canals connected to the utricle detect turning of the head and other forms of angular acceleration
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- The outside of the human eye is surrounded by a mucous membrane called the conjunctiva, the sclera (a connective tissue) and the choroid (a thin, pigmented layer)
- At the front, the sclera forms the transparent cornea and the choroid forms the colored iris
- Just inside the choroid, the neurons and photoreceptors of the retina form the innermost layer of the eyeball, the optic nerve exits the eye at the optic diskt
- The lens, a transparent disk of protein, divides the eye into two cavities
- In front of the lens lies the aqueous humor, a clear watery substance
- Blockage of ducts that drain this fluid can produce glaucoma, a condition in which increased pressure in the eye damages the optic nerve, causing vision loss
- Behind the lens lies the jellylike vitreous humor
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-Light strikes the retina, passing through largely transparent layers of neurons before reaching the rods and cones, two types of photoreceptors that differ in shape and in function
- The neurons of the retina then relay visual information captured by the photoreceptors to the optic nerve and brain along pathways
- Each bipolar cell receives information from several rods or cones and each ganglion cell gathers input from several bipolar cells
- Horizontal and amacrine cells integrate information across the retina
- One region of the retina, the optic disk, lacks photoreceptors (this region forms a blind spot where light is not detected)
- Within the outer segment of a rod or cone is a stack of membranous disks in which visual pigments are embedded
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- Point-by-point information in the visual field is projected along neurons onto the visual cortex
- It is estimated that at least 30% of the cerebral cortex have a part in formulating what humans see
- Brain not only processes visual information, but also controls what information is captured
- One important type of control is focusing
- When you focus on a close object, your lens becomes almost spherical
- When you view a distant object, your lens is flattened
- Although our peripheral vision allows us to see objects over a nearly 180° range, the distribution of photoreceptors across the eye limits both what we see and how well we see it
- Overall the human retina contains about 125 million rods and about 6 million cones
- Most fish, amphibians, and reptiles (including birds) have very good color vision
- Humans and other primates also see color well, but are among the minority of mammals with this ability
- Many mammals are nocturnal, and have a high proportion of rods in the retina (an adaptation that gives these animals keen night vision)
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- In humans, the perception of color is based on three types of cones, each with a different visual pigment (photopsins)--red, green, or blue
- Photopsins are formed from the binding of retinal to three distinct opsin proteins --slight differences in the opsin proteins are sufficient for each photopsin to absorb light optimally at a different wavelength
- Although the visual pigments are designated as red, green, or blue, their absorption spectra in fact overlap and for this reason, the brain's perception of intermediate hues depends on the differential stimulation of two or more classes of cones (example, when both red and green cones are stimulated, we may see yellow or orange depending on which class is more strongly stimulated)
- Abnormal color vision results from alterations in the genes for one or more photopsin proteins
- Because the human genes for the red and green pigments are located on the X chromosome, a single defective copy of either gene can disrupt color vision in males
- Color blindness is more common in males than in females (5 to 8% of males, fewer than 1% of females) and nearly always disrupts perception of red or green (the gene for blue pigment is on human chromosome 7)
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- According the the sliding-filament model, the filaments do not change in length when the sarcomere shortens
- Instead the thin and thick filaments slide past each other, increasing their overlap
- The longitudinal sliding of the filaments relies on the interaction of actin and myosin
- Each myosin molecule has a long tail region and a golbular head region
- The tail adheres to the tails of other myosin molecules that form the thick filament,
- The head, which extends to the side, can bind ATP and hydrolyze it to ADP and inorganic phosphate
- Hydrolysis of ATP converts myosin to a high energy form which binds to actin, forming a cross bridge, and pulls the the thin filament toward the center of the sarcomere
- The cross-bridge is broken when a new molecule of ATP binds to the myosin head
- Muscle contraction requires repeated cycles of binding and release
- In each cycle, the myosing head freed from a cross-bridge cleaves the newly bound ATP and binds again to actin--because the thin filament moved toward the center of the sarcomere in the previous cycle, the myosin head now attaches to a new binding site farther along the thing filament
- Each of the approximately 350 heads of a thick filament forms and re-forms about five cross bridges per second
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- Calcium ions and proteins bound to actin play crucial roles in both muscle cell contraction and relaxation
- Tropomyosin, a regulatory protein, and the troponin complex, a set of additional regulatory proteins, are bound to the actin strands of thin filaments
- In a muscle fiber at rest, tropomyosin covers the myosin binding sites along the thin filament, preventing actin and myosin from interacting
- When Ca2+ accumulates in the cytosol, it bindss to the troponin complex, causing tropomyosin bound along the actin strands to shift position and expose the myosin-binding sites on the thin filament
- Thus, when the Ca2+ concentration rises in the cytosol, the thin and thick filaments slide past each other, and the muscle fiber vontracts
- When the Ca2+ concentration falls, the binding sites are covered, and contraction stops
- Motor neurons cause muscle contraction by triggering the release of Ca2+ into the cytosol of muscle cells with which they form synapses
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- The arrival of an action potential at the synaptic terminal of a motor neuron causes release of the neurotransmitter acetylcholine
- Binding of acetylcholine to receptors on the muscle fiber leads to a depolarization, triggering an action potential
- Within the muscle fiber, the action potential spreads deep into the interior, following infolding of the plasma membrane called transverse tubules
- The T tubules make close contact with the sarcaoplasmic reticulum (SR), a specialized endoplasmic reticulum
- As the action potentials spreads along the T tubules, it triggers changes in the SR opening Ca2+ channels
- Calcium ions stored in the interior of the SR flow through these open channels into the cytosol and bind to the troponin complex, initiation contraction of the muscle fiber
- When motor neuron input stops, the muscle cell relaxes, the filaments slide back to their starting position
- During this phase, proteins in the cell reset the muscle for the next cycle of contraction
- Relaxation begins as transport proteins in the SR pump Ca2+ in from the cytosol
- When the Ca2_ concentration in the cytosol drops to a low level, the regulatory proteins bround to the thin filament shift back to their starting position, once again blocking the myosin-binding sites
- At the same time, the Ca2+ pumped from the cytosol accumulates in the SR, providing the stores needed to respond to the next action potential
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- In vertebrates, each branched motor neuron may form synapses with many skeletal muscle fibers, although each fiber is controlled by only one motor neuron
- For the whole muscle, there may be hundreds of motor neurons ,each with its own pool of muscle fibers
- A motor unit consists of a single motor neuron and all the muscle fibers it controls
- When a motor neuron produces an action potential, all the muscle fibers in its motor unit contract as a group
- The strength of the resulting contraction depends on how many muscle fibers the motor neuron controls
- In most muscles, the number of muscle fibers in different motor units ranges from a few to hundreds
- The nervous system can thus regulate the strength of contraction in a muscle by determining how many motor units are activated at a given instant and by selecting large or small motor units to activate
- The force (tension) developed by a muscle progressivly increases as more of the motor neurons controlling the muscles are activated, a process called recruitment of motor neurons
- The nervous system regulates muscle contraction not only by controlling which motor units are activated, but also by varying the rate of muscle fiber stimulation
- A single action potential produces a twitch lasting about 100 msec or less
- If a second action potential arrives before the muscle fiber has completely relaxed the two twitches add together resulting greater tension (further summation occurs as the rate of stimulation increases)
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- A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment
- This is the main type of skeleton in most cnidarians, flatworms, nematodes, and annelids
- These animals control their form and movement by using muscles to change the shape of fluid-filled compartments
- In earthworms and many other annelids, circular and longitudinal muscles act together to change the shape of individual fluid filled segments which are divided by septa, these shape changes bring about peristalsis, a movement produced by rhythmic waves of muscle contractions passing from front to back
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