Chapter 11
Fundamentals of the Nervous System
Lecture # 26, 27, 28
Objectives 1. List the basic functions of the nervous system.
2. List the types of supporting cells and cite their functions.
3. Define neuron, describe its important structural components, and relate each to a functional role.
4. Differentiate between a nerve and a tract, and between a nucleus and a ganglion.
5. Explain the importance of the myelin sheath and describe how it is formed in the central and
peripheral nervous systems.
6. Classify neurons structurally and functionally.
7. Explain how action potentials are generated and propagated along neurons.
8. Define absolute and relative refractory periods.
9. Define salutatory conduction and contrast it to conduction along unmyelinated fibers.
10. Define synapse. Distinguish between electrical and chemical synapses structurally and in their
mechanisms of information transmission.
11. Distinguish between excitatory and inhibitory postsynaptic potentials.
12. Describe how synaptic events are integrated and modified.
13. Define neurotransmitter and name several classes of neurotransmitters.
Functions of Nervous System:
1. Monitor changes inside and outside of the body (stimuli);gathered information is sensory input.
2. Processes and interprets what should be done – integration.
3. Causes response by activating effector organs (motor output).
Supporting Cells (Neuroglia)
I. CNS
A. Astrocytes: star-shaped, most abundant and versatile, forms blood-brain barrier by making exchanges
between neurons and capillaries. Mops up K+ and neurotransmitters.
B. Microglia: monitor health of neurons; phagocytic for microorganisms or neuronal debris.
C. Ependymal: line cavities of brain and spinal cord to circulate CSF (cerebrospinal fluid)
D. Oligodendrocytes: wrap around neuron fibers to provide myelin sheath (1 cell to many neurons) in CNS.
II. PNS
A. Satellite cells: surround neuron cell bodies within ganglia.
B. Schwann cells: wrap around axons of neurons (1 neuron has many Schwann cells) in PNS.
Characteristics of neurons: basic functional unit (BFU) of nervous system
1.
2.
3.
4.
conduct nerve impulses
extreme longevity
amitotic (except olfactory (smell) and hippocampal (memory)
high metabolic rate – require constant and abundant supplies of glucose and oxygen. Neurons will die w/in 5
minutes w/o oxygen.
Structures of Neurons
I. Cell body: called the prokaryon or soma; located w/in CNS; clusters of cell bodies are called nuclei (in CNS), or
ganglia in PNS.
II. Processes: bundles of processes in CNS are called tracts; in PNS they are nerves.
A. Dendrites: receive the signal, provide enormous surface area for reception of stimuli
B. Axons: each neuron has only one axon w/ many terminal branches w/ axon terminals. Axons generate nerve
impulses away from cell body. Impulse is generated at axon hillock. Neurotransmitters are released from
vesicles at end.
III. Myelin Sheath and Neurilemma: myelin is a whitish, fatty substance that protects and electrically insulates
fibers and increases speed of transmission up to 150x faster. Present only on axons. Schwann cells wrap themselves
around axons many times to form sheath. Neurilemma is plasma membrane of Schwann cell and its squished up
cytoplasm and nucleus.
A. Nodes of Ranvier: spaces between Schwann cells that allow salutatory conduction. Impulse travels from
node to node dowm a myelinated axon.
Types of Neurons
I. Structural
A. Multipolar: motor neurons and interneurons; possess many dendrites with only long axon. Cell body found
w/in CNS
B. Bipolar: in eye and nasal cavity for olfaction. “Two axons” with cell body in middle. Sensory neuron.
C. Unipolar: most sensory neurons. Two axons with cell body out to side. Cell body found in dorsal root
ganglion of spinal cord.
II. Functional
A. Sensory: carry nerve impulses from sensory receptors to CNS. Sometimes referred to as afferent neurons.
Their dendrites many be the sensory receptors (as in touch, pain) or other sense cells may be involved (as in rods and
cones in eye).
B. Motor: carry impulses from CNS to effector (muscle or gland). Sometimes referred to as efferent neurons.
C. Interneurons: found only in CNS, relay information up and down from brain and spinal cord and from in to
out of spinal cord.
Neurophysiology
I. Membrane Ion Channels: known as active gates, these chemically gated channels (which have receptors for
neurotransmitters) open when chemical binds. Voltage gated channels open in response to changes in membrane
potential.
Passive gates are leakage channels that are always open. Each channel is specific for a particular type of ion.
II. Resting membrane potential: about –70 mV. Same characteristics as in muscle cell membrane potential.
III. Depolarization: a reduction in membrane potential; the inside becomes less negative (close to zero).
Depolarization increases the probability of producing nerve impulses.
IV. Hyperpolarization: occurs when the membrane potential increases (more negative inside than at rest).
Hyperpolarization reduces the probability of producing nerve impulses.
V. Graded potential: short-lived, local changes in membrane potential that can be depolarizations or
hyperpolarizations. Their magnitude varies with stimulus strength.
VI. Action potential: events are identical to those in a muscle cell. Also called a NERVE IMPULSE and only axons can
generate one. Channels along the axon open in response to changes in potential that occur along the dendrite and cell
body membranes. The transition from local graded potential to action potential occurs at the axon hillock in motor
neurons.
Threshold and All-or-none phenomenon: not all local depolarizations produce action potentials. They muscle
reach threshold (when membrane has been depolarized about 15-20 mV). Subthreshold stimuli (brief, weak stimuli) are
not translated into a nerve impulse. Strong stimuli depolarize to threshold; quick, weak ones must be applied longer. But
once threshold is reached, an action potential occurs along the ENTIRE axon length.
Refractory Period: when polarized, the neuron cannot respond to another stimulus
I. Absolute: The period from the opening of the sodium channels to the closing of the gates. Each action potential is a
separate all or none event, and the action potential is in one direction from beginning of axon to end.
II. Relative: follows absolute refractory period. Sodium gates are closed, potassium gates are open and repolarization
is occurring. A threshold stiumuls at this point won’t cause an action potential, but an exceptionally strong one can
reopen the sodium gates.
Saltatory conduction and conduction velocities: in the saltatory conduction of myelinated fibers the
electrical signal jumps from node to node along the axon and conduction is faster than in unmyelinated fibers. In
addition, the larger the axon diameter, the faster the transmission (less resistance).
The Synapse: the presynaptic neuron carries the signal to the synapse, the postsynaptic neuron carries signal away
from synapse
I. Information transfer across synapse: can be electrical through connexin proteins, or chemical (as is discussed
here)
A. Calcium gates open: when impulse reaches the axon terminal, voltage gated calcium channels open and
calcium acts as the messenger to cause synaptic vesicles to bind with presynaptic membrane.
B. Neurotransmitter released: from synaptic vesicles (by exocytosis). Calcium is quickly removed after this.
C. Neurotransmitter binds: after diffusing across the synaptic cleft, it binds reversibly to receptors clustered
on the postsynaptic membrane.
D. Local potential generated: as ion channels open up. This may excite or inhibit the postsynaptic neuron.
The more intense the stimulus, the more NT is released.
Termination of Neurotransmitter effects: If the NT is not removed form the postsynaptic receptors, it will block
the reception of additional messages. The terminating mechanisms may be:
1. Degradation of NT by enzymes on the postsynaptic membrane or in synapses (fate of ACh).
2. Reuptake by astrocytes or presynaptic terminal for storage or destruction (fate of NE).
3. Diffusion away from synapse.
EPSP: Local graded depolarization events at excitatory postsynaptic membranes. Each EPSP lasts a few milliseconds in
order to help trigger an action potential at the axon hillock. If currents reaching the hillock are strong enough to
depolarize to threshold, voltage gated sodium channels open and the action potential is generated.
IPSP: The binding of inhibitory neurotransmitters induces hyperpolarization of the postsynaptic membrane by making
permeability to K+ or Cl- to increase. The charge of the inner part of membrane becomes more negative. The
postsynaptic neuron is more and more less likely to fire. Large depolarizing currents are required to induce an action
potential.
Summation by postsynaptic neuron: Temporal summation occurs when one or more presynaptic neurons transmit
impulse in rapid-fire order (rate increase). Bursts of NT are released in quick succession. One EPSP comes right after
another, and they summate at the postsynaptic neuron (may occur in the same manner with IPSP’s).
Spatial summation occurs when the postsynaptic membrane is stimulated at the same time by a large number of axon
terminals from the same or (usually) different neurons (voltage increase). Many EPSP’s (or IPSP’s) occur at once.
Neurotransmitters: the means by which each neuron communicates with others.
Acetylcholine (ACh): generally excitatory (especially in skeletal muscles).
 Release is inhibited by botulism toxin, binding inhibited by curare and some snake venom
 Decreased ACh in Alzheimers, receptors destroyed in myasthenia gravis.
Norepinephrine (NE): generally excitatory, release enhanced by amphetamines, uptake blocked by cocaine
Dopamine: uptake blocked by cocaine, deficient in Parkinson’s disease, may be involved in schizophrenia
Serotonin: inhibitory, blocked by LSD, role in sleep, appetite, nausea. Prozac blocks its uptake.
GABA: inhibitory, augmented by alcohol.
Glutamate: excitatory, learning and memory, the “stroke neurotransmitter”
Endorphins: inhibitory, inhibits substance P that causes pain, mimicked by morphine, heroin and methadone
Substance P: excitatory, mediates pain transmission