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198 terms

Ch. 11 The Nervous System

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What are Neurons?
Excitable cells that transmit electrical signals.
What are Supporting Cells?
Cells that surround and wrap neurons.
What is a Nerve Cell?
Structural units of the nervous system. Composed of a body, axon and dendrites. Long lived, amitotic, and have a high metabolic rate.
What is the Nerve Cell Body?
Perikaryon or Soma. Contains the nucleus and nucleolus, has well developed nissl-bodies (rough er), contains an axon hillock.
What is an Axon Hillock?
Cone shaped area from which axons arise.
What are the processes of the nervous system?
Axons and dendrites. Armlike extentions from the Soma.
What are processes called in the CNS?
Tracts.
What are processes called in the PNS?
Nerves.
What are Dendrites?
Short, tapering, and diffusing branced processes. They are the receptive, or input regions of the neuron.
What are Axons?
Slender processes of uniform, diameter arising from the hillock. Generate and transmit actions potentials. Secrete neurotransmitters from axonal terminals.
What are long axons called?
Nerve fibers.
What is the Axonal Terminal?
Branched terminus of an axon.
What is the Myelin Sheath?
Whittish, fatty (protein-lipoid), segmented sheath around most long axons. Protects axons. Electrically insulates fibers from one another. Increases speed of nerve impulse transmissions. Formed by Schwann cells.
What are Action Potentials?
Nerve Impulses. Electrical impulses that are carried along the lengths of the axons. Always the same regardless of the stimulus.
What are Passive, or Leakage Channels?
Type of plasma membrane ion channel that is always open.
What is a Chemically Gated Channel?
Type of plasma membrane ion channel that opens with a binding of a specific neurotransmitter.
What is a Voltage Gated Channel?
Type of plasma membrane ion channel that opens and closes in response to membrane potential.
What is a Mechanically Gated Channel?
Type of plasma membrane ion channel that opens and closes in response to physical deformation of receptors.
How does a Gated Channel Work?
EX: Na-/K+ Gated Channel. Closed when a neurotransmitter is not bound to the extracellular receptor. Na- cannot enter the cell and K+ cannot exit the cell. Open when a neurotransmitter is attached to the receptor. Na- enters the cell and K+ exits the cell.
How does a Voltage Gated Channel Work?
EX: Na- Channel. Closed when intracellular environment is negative. Na- cannot enter the cell. Open when the intracellular environment is positive. Na- can enter the cell.
What is Resting Membrane Potential?
The potential difference (-70mV) across the membrane of a resting neuron.
What is Depolarization?
The inside of the membrane becomes less negative.
What is Hyperpolerization?
The inside of the membrane becomes more negative then the resting potential.
What is Repolarization?
The membrane returns to its resting membrane potential.
What is the Absolute Refractory Period?
Time from the opening of the Na+ activation gates until the closing of inactivation gates. Prevents the neuron from generating an action potential. Ensures that each action potential is separate. Enforces one way transmission of nerve impulses.
What is the Relative Refractory Period?
The interval following the absolute refractory period when sodium gates are closed, potassium gates are open, repolarization is occuring. The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events.
What is EPSP?
Excitatory Postsynaptic Potentials.
What is IPSP?
Inhibitory Postsynaptic Potentials.
What are Neurotransmitters?
Chemicals used for neuronal communication with the body and the brain. Over 50 have been indentified.
What are the main kinds of Neurotransmitters?
Acetylcholine, Biogenic Amines, Amino Acids, Peptides, Novel Messengers.
What is Acetylcoline?
First neurotransmitter identified and best understood. Released at the neuromuscular junction. Synthesized and enclosed in the synaptic vesicles. Degraded by the enzyme acetylcholinesterase (ACheE). Released by all neurons that stimulate skeletal muscle and some neurons of the ANS.
What are Biogenic Amines?
Catecholamine - dopamine, norepinephrine (NE), and epinephrine. Indolamines - seratonin and histamine. Broadly distributed in the brain. Play roles in emotional behaviors and our biological clock.
What are Amino Acids?
Include: GABA - Gamma aminobutyric acid, Glycine, Aspartate, Glutamate. Is found only in the CNS.
What are Peptides?
Include: Substance P - mediator of pain signals. Beta Endorphin, dynorphin, and enkephalins. Act as a natural opiate; reduce pain perception. Binds to the same receptors as opiates and morphine. Gut-brain peptides - Somatostain, and cholectokinin.
What is the All Or None Phenomenon?
Action potentials either happen completely or not at all.
What is the role of the Sodium Potassium Pump?
Ionic redistribution back to resting conditions is restored by this. Restores at hyperpolerization.
Nervous System Functions
Sensory input - monitoring stimuli
Integration - interpretation of sensory input
Motor output - response to stimuli
Central nervous system (CNS)
Brain and spinal cord
Integration and command center
Peripheral nervous system (PNS)
Paired spinal and cranial nerves
Carries messages to and from the spinal cord and brain
PNS Functional Divisions
Sensory (afferent) division
Motor (efferent) division
Sensory (afferent) division
In the PNS, sensory afferent fibers carry impulses from skin, skeletal muscles, and joints to the brain (aka CNS)
Motor (efferent) division
In the PNS, transmits impulses from the CNS to effector organs
Motor Division: Two Main Parts
Somatic nervous system
Autonomic nervous system (ANS)
Somatic nervous system
Conscious control of skeletal muscles
Autonomic nervous system (ANS)
Regulates smooth muscle, cardiac muscle, and glands
Autonomic nervous system (ANS) divisions
sympathetic and parasympathetic
Neurons
excitable cells that transmit electrical signals
structural unit of the nervous system
Supporting cells
cells that surround and wrap neurons
Neuroglia
supporting cells of the nervous system
also called neuralgia or glial cells
Neuralgia functions
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth
Astrocytes
most abundant, versatile, and highly branched glial cells
cling to one or more neurons and their synaptic endings, and cover capillaries
Astrocytes functions
Support and brace neurons
Anchor neurons to their nutrient supplies
Guide migration of young neurons
Control the chemical environment
Microglia
small, ovoid cells with spiny processes
Microglia functions
Phagocytes that monitor the health of neurons
Ependymal cells
ciliated cells that range in shape from squamous to columnar
line the central cavities of the brain and spinal column
Ependymal cells functions
filters for CSF
Oligodendrocytes
branched cells that wrap CNS nerve fibers
help regulate how neuron fires
Schwann cells
aka neurolemmocytes
surround fibers of the PNS
help regulate how neuron fires
neuron characteristics
body, axon & dendrites
long, lived, high metabolic rate
do not undergo mitosis
Neuron plasma membrane functions
electrical signaling
cell-to-cell signaling during development
Nerve Cell Body (Perikaryon or Soma)
Contains the nucleus and a nucleolus
Is the major biosynthetic center
Is the focal point for the outgrowth of neuronal processes
Has no centrioles (hence its amitotic nature)
Has well-developed Nissl bodies (rough ER)
Contains an axon hillock - cone-shaped area from which axons arise
neuron processes
Armlike extensions from the soma
CNS neuron processes
tracts
PNS neuron processes
nerves
two types of neuron processes
axons and dendrites
Dendrites of Motor Neurons
'messenger' cells that receive stimulus
anchor the cell body
relay signal to cell body
Axons
slender processes that arise from hillock
one unbranched per neuron
axon collaterals
rare branching from the axon
axon functions
generate & transmit action potentials
secrete neurotransmittters from the axon terminals
myelin sheath
whitish, fatty, segmented sheath around most long axons
composed of 80% fat & 20% protein
myelin sheath functions
Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission
myelin sheath
Formed by Schwann cells in the PNS
The Schwann cell:
-Envelopes an axon in a trough
-Encloses the axon with its plasma membrane
-Has concentric layers of membrane that make up the myelin sheath
Nodes of Ranvier (Neurofibral Nodes)
Gaps in the myelin sheath between adjacent Schwann cells
They are the sites where axon collaterals can emerge
Unmyelinated Axons
grey matter neurons
Schwann cells are present but myelin sheath is not b/c they do not coil around axons
white matter
in the PNS, dense collections of myelinated fibers
'faster' than gray matter
gray matter
in the CNS, mostly soma and unmyelinated fibers
'slower' than white matter
action potentials
aka nerve impulses
Electrical impulses carried along the length of axons
Always the same regardless of stimulus
the underlying functional feature of the nervous system
Electrical Current
Reflects the flow of ions rather than electrons
potential on either side of membranes when:
The number of ions is different across the membrane
The membrane provides a resistance to ion flow
Types of plasma membrane ion channels
passive or leakage channels
chemically gated channels
voltage-gated channels
mechanically gated channels
passive, or leakage, channels
always open
chemically gated channels
open with binding of a specific neurotransmitter
voltage-gated channels
open and close in response to membrane potential
mechanically gated channels
open and close in response to physical deformation of receptors
Operation of a Gated Channel
Example: Na+-K+ pump
Closed when a neurotransmitter is not bound to the extracellular receptor
Na+ cannot enter the cell and K+ cannot exit the cell
Open when a neurotransmitter is attached to the receptor
Na+ enters the cell and K+ exits the cell
Operation of a Voltage-Gated Channel
Example: Na+ channel
Closed when the intracellular environment is negative
Na+ cannot enter the cell
Open when the intracellular environment is positive
Na+ can enter the cell
When gated channels are open:
Ions move quickly across the membrane
Movement is along their electrochemical gradients
An electrical current is created
Voltage changes across the membrane
Electrochemical Gradient
electrical and chemical gradients taken together
Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration
Ions flow along their electrical gradient when they move toward an area of opposite charge
Resting Membrane Potential (Vr)
potential difference (-70 mV) across the membrane of a resting neuron
Membrane Potentials: Signals
Used to integrate, send, and receive information
Membrane potential changes are produced by:
Changes in membrane permeability to ions
Alterations of ion concentrations across the membrane
Types of signals
graded potentials and action potentials
3 events that cause
Changes in Membrane Potential
depolarization
repolarization
hyperpolarization
Depolarization
the inside of the membrane becomes less negative
Repolarization
the membrane returns to its resting membrane potential
Hyperpolarization
the inside of the membrane becomes more negative than the resting potential
Graded Potentials
Magnitude varies directly with the strength of the stimulus
Short-lived, local changes in membrane potential
Decrease in intensity with distance
Sufficiently strong graded potentials can initiate action potentials
Current is quickly dissipated due to the leaky plasma membrane
Only travel over short distances
Action Potentials (APs)
'neuron firing' +35mV
Action potentials are only generated by muscle cells and neurons
They do not decrease in strength over distance
They are the principal means of neural communication
Action Potential: Resting State
Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates
Action Potential: Depolarization Phase
Na+ gates are opened; K+ gates are closed
Cell becomes more positive
Depolarization Phase Threshold
a critical level of depolarization (-55 to -50 mV)
At this point, the process becomes self-generating
Action Potential: Repolarization Phase
sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell and cell becomes more negative, as usual
Action Potential: Hyperpolarization
Potassium gates remain open, causing an excessive efflux of K+
The neuron is insensitive to stimulus and depolarization during this time
Action Potential:
Role of the Sodium-Potassium Pump
Repolarization
-Restores the resting electrical conditions of the neuron
-Does not restore the resting ionic conditions
Phases of the Action Potential
1 - resting state
2 - depolarization phase
3 - repolarization phase
4 - hyperpolarization
Propagation of an Action Potential
wave of impulse moving down the axon to the axon terminals
Action Potential Threshold
membrane is depolarized
membrane potential goes from resting state -70mV to -55mV
stimulus intensity
the CNS determines stimulus intensity by the frequency of impulse transmission
absolute refractory period
neuron cannot fire
time from the Na+ activation gates until the closing of inactivation gates
relative refractory period
Na+ gates are closed, K+ gates are open
repolarization is occurring
threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events
axon diameter
the larger the diameter, the faster the impulse
myelin sheath presence
dramatically increases impulse speed
saltatory conduction
Current passes through a myelinated axon only at the nodes of Ranvier avoiding fats
Action potentials are triggered only at the nodes and jump from one node to the next
Much faster than conduction along unmyelinated axons
Na+ channels are concentrated at these nodes
Multiple Sclerosis
scar tissue in myelin sheath causes too long of a jump resulting in muscle control
spreads to diaghram & causes respiratory failure
Central Nervous System (CNS)
This nervous system consists of the brain and spinal cord.
Peripheral Nervous System (PNS)
The section of the nervous system lying outside the brain and spinal cord
Sensory or Afferent
Part of the (PNS) that carry impulses towards the (CNS)
Motor or Efferent
Part of the (PNS) that carry impulses away from the (CNS)
Somatic Nervous System
This nervous system conduct impulses from the (CNS) to skeletal muscles.
Autonomic Nervous System (ANS)
This nervous system regulate the activity of smooth muscles.
Glial cells
These neurons are associated closely with much smaller cells., Cells that provide basic support systems for neurons and perform a variety of maintenance functions
Astrocytes
Star shaped cells found throughout the CNS, cleaning up debris in the extracellular space and removing neurotransmitters from the synaptic cleft, connects neurons to nearby capilaries, components of the blood-brain barrier
Microglia
Smallest neuroglial cells; phagocytic cells that engulf cellular debris, waste products and pathogens. increase in number as a result of infection or injury
Ependymal Cell
A glial cell that lines membranes within the brain and spinal cord and helps form cerebrospinal fluid
Oligodendrocytes
Provides myelination in CNS, electrically insulates certain axons, & speeds up rate of electrical signals
Satellite cells
Surrounds the neuron cell bodies in ganglia of PNS little is known of their function (PNS)
Schwann cells
Supporting cells of the peripheral nervous system responsible for the formation of myelin.
Neurons
Cells specialized for transmitting nerve impulses.
Perikaryon
Cytoplasm surrounding the nucleus of a neuron
Dendrites
The branching extensions of a neuron that receive messages and conduct impulses toward the cell body
Axon
The extension of a neuron, ending in branching terminal fibers, through which messages pass to other neurons or to muscles or glands
Axon collaterals
A branch of an axon
Telodendria
Series of fine, terminal extensions branching from the axon tip.
Axolemma
The plasma membrane of the axon
Myelin Sheath
A layer of fatty tissue segmentally encasing the fibers of many neurons; enables vastly greater transmission speed of neural impulses as the impulse hops from one node to the next.
Neurilemma
Portion of the Schwann cell which includes the exposed part of its plasma membrane.
Nodes of Ranvier
Small gaps in the myelin sheath of medullated axons
White matter
Regions of the brain and spinal cord containing a dense collection of myelinated fibers.
Gray Matter
Contains mostly nerve cell bodies and unmyelinated fibers.
Multipolar Neurons
Neurons having three or more processes.
Bipolar Neurons
Neruons having two processes
Interneurons
Central nervous system neurons that internally communicate and intervene between the sensory inputs and motor outputs
Potential difference
The difference in electrical charge between two points in a circuit expressed in volts.
Depolarization
Sodium rushes into neuron through membrane, potassium ruses out; results in a change in charge
Hyperpolarization
The movement of the membrane potential of a cell away from rest potential in a more negative direction.
Resting State
Na+ and K+ channels are closed
Depolarizing phase
Increase in Na+ permeability and reversal of membrane potential.
Repolarizing phase
Decrease in Na+ permeability and an increase in K+ permeability
Sodium potassium pump
Helps establish a difference in charge across the membrane (membrane potential) moves ions and molecules against the concentration gradient (takes energy)
Refractory period
The time after a neuron fires or a muscle fiber contracts during which a stimulus will not evoke a response
Axodendritic synapses
Synapses between the axon endings of one neuron and the dendrites of other neurons.
Axosomatic synapses
Between axon endings of one neuron and cell bodies of other neurons
Presynaptic neuron
A neuron conducting impulses toward the synapse
Postsynaptic neuron
A neuron on the receiving end of a synapse
Electrical Synapses
Made of connexons- tubular proteins that allow passage of an impulse in 2 directions at once- are located in gap junctions where 2 cells come together- more rapid than a chemical synapse
Chemical Synapses
A synapses that is specialized to release and the reception of chemical neurotransmitters.
Synaptic Vesicles
Tiny sacs in a terminal button that release chemicals into the synapse.
Synaptic Cleft
A space between two connecting neurons where neurotransmitters are released.
Biogenic Amines
Simple hormones; water soluble; derived from amino acids; e.g, epinephrine, norepinephrine, dopamine
Diverging circuits
One incoming fiber triggers responses in ever-increasing numbers of neurons farther along in a circuit.
neuroblast
A cell from which a nerve cell develops.
Growth cone
Growing tip of an axon that has a prickly, fanlike structure that gives an axon the ability to interact with its environment.
Reflex
Rapid, automatic responses to stimuli, in which a particular stimulus always causes the same response.
Neuropathy
Any disease of nervous tissue, but particularly degenerative disease of the nerves.
Central Nervous System
integrates and coordinates incoming and outgoing neural signals and carries our higher learning functions (thinking and learning)
Nucleus
collection of nerve cell bodies in the CNS
Gray Matter
unmyelinated axons, cell bodies, and dendrites; found in horns and commissures of spinal cord; surrounds ventricles, in cortex, in nuclei of brain
White Matter
The portions of the central nervous system that are abundant in axons rather than cell bodies of neurons. The colour derives from the presence of the axon's myelin sheaths
Peripheral Nervous System
-consists of nerve fibers and cell bodies outside the CNS that conduct impulses to or away from the CNS
-Also, organized into nerves that connect the CNS with peripheral structures
Nerve fibers consist of:
a. bundle of nerve fibers outside the CNS (or a "bundle of bundled fibers" or fasicles)
b. the connective tissue coverings that surround and ind the nerve fibers and fascicles together
c. blood vessels (vasa nervorum) that nourish the nerve fibers and their coverings
Types of nerve connective tissue coverings
Endoneurium
Perineurium
Epineurium
Endoneurium
Delicate connective tissue immediately surrounding the neurilemma cells and axons
Perineurium
Layer of dense connective tissue that encloses a fascicle of nerve fibers, providing an effective barrier against penetration of the nerve fibers by foreign substances
Epineurium
a thick connective tissue sheath that surrounds and encloses a bundle of fascicles, forming the outermost covering of the nerve
-includes fatty tissue, blood vessels, and lymphatics
Types of nerves
-Cranial: exit the cranial cavity through the foramina in the cranium
-Spinal: exit the vertebral column through the intervertebral foramina
Astrocyte
Ependymal cell(s)
Microglia
Mylinated axon
Neuron
Oligodendrocyte
Trigger zone (aka threshold)
-55 mV; If the stimulus reaches the trigger zone then the action potential happens.
Depolarized graded potentials
Becomes more positive.... so is excitatory
Hyperpolarizing graded potentials
Becomes more negative... so is inhibitory
Larger vs Shorter Neurons
Larger neurons conduct APs faster
2 Key Parameters that influence speed of AP
Diameter of the neuron (larger --> faster) and the Resistance of the axon to ion leakage (more myelination --> faster)
Significance of multiple sclerosis
Myelin breaks apart, slowing down AP's causing disease --> fatigue, muscle weakness, difficulty walking

a) APs appear to jump from one node of ranvier to the next
b) In demyelinating diseases, conduction slows due to current leakage out of the previously insulated regions between the nodes.
Draw an action potential***
- label voltages (-55mV, -70mV, +30mv)
- know what channels open and closes at each step
What is the synaptic cleft and what happens at the synapse?
Area between the presynaptic neuron and the postsynaptic membrane. The presynaptic and postsynaptic cells makes the synapse.
The majority of synapses are chemical synapses.
They are proteins or neurohormones that are made in the cell body then travels via axonal transport to a synaptic vesicle
What is the role of calcium at the end of a neuron?
Calcium triggers exocytosis of the synaptic vesicle contents (in that the synaptic vesicle fuses with the membrane) and therefore signaling the release of neurocrine neurotransmitters at the synapse.
What are the 2 major neurocrines secreted by the nervous system, what types of receptors do they have, and where are those receptors?
Acetylcholine (cholinergic; nicotonic- skeletal muscles, autonomic neurons, CNS | muscarinic- smooth and cardiac muscle, endocrine and exocrine glands, CNS) and norephinephrine (adrenergic; smooth and cardiac muscle, endocrine exocrine glands, CNS)
Why are neurotransmitter activities normally rapidly terminated?
Neurotransmitter activity is rapidly terminated 1) by removal or inactivation of neurotransmitter or 2) moved back into presynaptic cell

In order for activity to last longer, stimulation most occur continuously.
What is plasticity?
If a cell body still exists, plasticity allows neurons to make new connections and relearn how to function.
What are the anatomic and functional categories of neurons?
a)unipolar neurons have a single process called the axon. During development, the dendrite fused with the axon.
b) Bipolar neurons have 2 relatively equal fibers extending off the central cell body.
c) Axaxonic CNS interneurons have no apparent axon.
d) Multipolar CNS interneurons are highly branched but lack long extensions.
e) A typical multipolar efferent neuron has 5 to 7 dendrites, each branching 4 to 6 times. A single long axon may branch several times and end at enlarged axon terminals.
How are organelles quickly moved down an axonal transport?
What are the different glial cells and their functions?
- Provide support
- Outnumber neurons 10-50 to 1

A)PNS has 2 types:
1.Schwann cells- wrap around neuron (speed up electrical impulse)
2. Satellite cells- supportive capsules around nerve cells in ganglia (cluster of nerve bodies found outside of CNS)

B) CNS has 4 types
1. oligodendrocytes- like Schwann cells in PNS
2. microglia- immune cells in CNS
3. astrocytes- take up and release chemicals
4. ependymal cells- one source of stem cells

Schwann and oligodendrocytes support and insulate axon by forming myelin (nodes of ranvier)
What is the difference between the absolute refractory period and the relative refractory period?
During the absolute refractory period, no stimulus can trigger another action potential.

During the relative refractory period, only a large than normal stimulus can initiate a new action potential.