I. Overview of the Nervous System (p. 454; fig. 13.1; TR 445)

A. Fundamental Types and Properties of Neurons (p. 454)

1. There are three general classes of neurons, which correspond to the three major aspects of nervous system function. (fig. 13.2; TR 446)

a. Sensory (afferent) neurons are specialized to detect changes in their environment called stimuli.

b. Interneurons (association neurons) lie within the central nervous system where they receive signals from other neurons and carry out the integrative function of the nervous system.

c. Motor (efferent) neurons send signals to muscle and gland cells that carry out the body’s responses to stimuli.

2. Nerve cells exhibit excitability, conductivity, and secretion of neurotransmitters and other chemical messengers.

B. Subdivisions of the Nervous System (p. 455)

1. The two major subdivisions of the nervous system are the central nervous system, or CNS, and the peripheral nervous system, or PNS.

a. The CNS consists of the brain and spinal cord, and is composed of two types of nervous tissue—gray matter and white matter.

b. The PNS consists of the nerves leading to and from the CNS.

2. The motor component of the nervous system is divided into the somatic nervous system, which innervates skeletal muscle, and the autonomic nervous system, which innervates cardiac and smooth muscle to control body function.

II. Cells of the Nervous System (p. 456)

A. Structure of a Neuron (p. 456; fig. 13.3; TR 447, 448)

1. The control center of the neuron is its soma or perikaryon (cell body). It contains the nucleus, nucleolus, Nissl bodies (rough ER), many other organelles, and supportive neurofibrils.

2. Mature neurons lack centrioles and do not undergo mitosis past adolescence.

3. Major cytoplasmic inclusions are glycogen granules, lipid droplets, melanin, and lipofuscin.

4. Dendrites, cellular extensions from the cell body, have receptors for neurotransmitters and receive signals from other neurons.

5. On one side of the soma is the axon hillock, which gives rise to the axon. Axons vary greatly in length and end in a synaptic end bulb through which neurotransmitters are passed to the next neuron.

6. Neurons are classified structurally according to the number of processes extending from the soma: multipolar, bipolar, unipolar, and anaxonic. (fig. 13.4; TR 449)

a. Neurons with one axon and several dendrites are multipolar (the most common type).

b. Neurons with one axon and one dendrite are bipolar.

c. Unipolar neurons have only a single process leading away from the soma.

d. Anaxonic neurons have multiple dendrites, but no axon.

B. Axonal Transport (p. 458)

1. Axonal transport is the two-way transport of materials along an axon that may be fast or slow. Movement away from the soma is anterograde transport and employs a motor protein called kinesin.

2. Movement toward the soma is retrograde transport and employs a motor protein called dynein.

C. Neuroglia (p. 459; fig. 13.5; table 13.1; TR 450)

1. Neuroglia outnumber neurons 50 to 1. These are the helper cells of nervous tissue; they bind neurons together and provide a supportive framework, among other functions. There are six types of neuroglia.

a. Oligodendrocytes form discontinuous myelin sheaths in the CNS and wrap several cells at once.

b. Abundant astrocytes are star-shaped cells found in the CNS.

i. Protoplasmic astrocytes help form the blood-brain barrier.

ii. Fibrous astrocytes form a physically supportive framework for the CNS.

c. Ependymal cells produce and circulate cerebrospinal fluid.

d. Microglia are small mobile macrophages that develop from monocytes and wander freely through the CNS.

e. Schwann cells (in the PNS) form a neurilemma around all cells they cover, and a myelin sheath around neuron fibers they cover in successive wrappings. They are necessary for the regeneration of cut neurons.

f. Satellite cells surround cell bodies in the PNS, but little is known of their function.

D. Myelin (p. 461)

1. All axons in the PNS have a sheath of Schwann cells (and thus a neurilemma, made up of the outer layer of Schwann cells) around them.

2. When a Schwann cell is wrapped successively around an axon, it becomes a myelin sheath. (fig. 13.6; TR 451)

3. Gaps between adjacent Schwann cells are called nodes of Ranvier; the covered segments are internodes.

4. Discontinuous myelination in the CNS is contributed by oligodendrocytes.

E. Unmyelinated Nerve Fibers (p. 462; fig. 13.7; TR 452)

1. Not all nerve fibers are myelinated, but even unmyelinated ones are associated with Schwann cells.

2. The speed at which a nerve signal travels depends on the diameter of the nerve fiber and the presence or absence of myelin.

a. Large fibers conduct impulses more rapidly than small ones.

b. Myelinated fibers are faster than nonmyelinated ones.

F. Regeneration of Nerve Fibers (p. 463; fig. 13.8; TR 453)

1. Peripheral nerve fibers can sometimes regenerate if the soma is not damaged and some of the neurilemma remains intact.

2. The neurilemma forms a regeneration tube through which the growing axon reestablishes its original connection.

3. If the nerve originally led to a skeletal muscle, the muscle atrophies in the absence of innervation but regrows when the connection is reestablished.

III. Electrophysiology of Neurons (p. 463)

A. Electrical Potentials and Currents (p. 464)

1. An electrical potential is a difference in the concentration of charged particles between one point and another.

2. An electrical current is a flow of charged particles from one point to another.

3. When living cells have electrical potential, we say they are polarized.

B. The Resting Membrane Potential (p. 465; fig. 13.9; TR 454)

1. At resting potential, neurons are polarized, with a resting membrane potential of –70 mV.

2. The sodium-potassium pump contributes some of the RMP, but accounts for 70% of the energy requirements of the nervous system.

3. An electrical charge is called a potential because it has the potential to make charged particles move.

C. Local Potentials (p. 466; fig. 13.10; TR 455)

1. A local potential is a small deviation in the RMP caused by a stimulation.

2. Local potentials have the following attributes: They are graded, decremental, ineffective beyond a short distance, reversible, and are either excitatory or inhibitory.

D. Action Potentials (p. 467; figs. 13.11, 13.12; TR 456, 457)

1. An action potential can be generated only in places where the plasma membrane has an adequate density of voltage-gated ion channels.

2. An action potential can also occur if local potentials arrive from multiple points of origin on the cell and their combined effect is great enough to reach action potential.

3. The action potential consists of rapid dramatic changes in membrane voltage.

4. The axonal hillock is the generator potential, and it must rise to threshold (the minimum voltage needed to open voltage-regulated sodium and potassium gates). When it reaches threshold, the neuron "fires."

5. At threshold, potassium gates open slowly and sodium gates open quickly, depolarizing the membrane. The polarity of the membrane becomes reversed in comparison to the RMP.

6. Membrane depolarization causes sodium gates to close, and the voltage stops rising.

7. Potassium gates are now fully open; potassium ions rush out of the cell, causing membrane voltage to drop rapidly, reaching hyperpolarization.

8. The original distribution of sodium and potassium is restored, and RMP is reestablished.

9. The action potential obeys the all-or-none law and lasts from the time threshold is first reached to the time the voltage returns to threshold.

E. Local potentials and action potentials are compared in table 13.2.

F. The Refractory Period (p. 469; fig. 13.13; TR 458)

1. The absolute refractory period, when the membrane is insensitive to additional stimulation, lasts from threshold until repolarization is one-third complete.

2. A relative refractory period follows and lasts until the membrane voltage returns to threshold. During this period, a new stimulus can trigger action potential only if it is stronger than a threshold stimulus.

3. The refractory period applies only to the patch of membrane where the action potential has just occurred.

G. Signal Conduction in Nerve Fibers (p. 470; fig. 13.14; TR 459)

1. An unmyelinated fiber has voltage-gated sodium ion channels along its entire length.

2. In unmyelinated fibers, the impulse travels at a nondecremental rate of up to 2 m/sec.

3. Saltatory conduction occurs in myelinated fibers. Action potential is reached at each node of Ranvier, and travels to the next node. (fig. 13.15; TR 460)

4. This type of conduction can travel at speeds up to 120 m/sec.

IV. Synapses (p. 472)

A. The Discovery of Neurotransmitters (p. 472)

1. Originally it was thought that neurons communicated by electrical connections between them until it was discovered by Otto Loewi in 1921 that a space, the synapse, occurred between two adjacent neurons. Later, acetylcholine was identified as the first known neurotransmitter.

2. We now know that neurons, muscle cells, and neuroglia do communicate through gap junctions with electrical signals. Much of the communication within the nervous system is accomplished using neurotransmitters.

B. Structure of a Chemical Synapse (p. 473; figs. 13.16, 13.17; TR 461)

1. The presynaptic neuron houses vesicles filled with neurotransmitter in its synaptic knob.

2. The postsynaptic neuron contains no specializations other than proteins that function as receptors and ion gates.

C. Neurotransmitters and Related Messengers (p. 473; fig. 13.18; TR 462)

1. More than 100 different chemicals have been identified as neurotransmitters, falling into three major categories: acetylcholine, amino acid, and biogenic amines.

2. Neuropeptides sometimes modify the actions of neurotransmitters. (table 13.3).

D. Synaptic Transmission (p. 474)

1. A synapse where transmission is mediated by acetylcholine is a cholinergic synapse.

2. The presynaptic neuron transmits an impulse to its synaptic knob, from which ACh stored in synaptic vesicles is released to the cleft.

3. At the postsynaptic neuron, ACh binds to ligand-gated channels, causing them to open and sodium and potassium to cross the membrane. This produces a local postsynaptic potential (PSP). If strong enough, the PSP opens voltage-gated ion channels, causing the neuron to fire. This is also called an ionotropic effect. (fig. 13.19; TR 463)

4. Biogenic amines and neuropeptides have a metabotropic effect mediated by a second messenger (like cAMP), which in turn activates a protein kinase that triggers other cellular reactions. A number of hormones also function in this manner. (fig. 13.20; TR 464)

E. Cessation of the Signal (p. 477)

1. ACh binds to its receptors for only a very short time, then dissociates from the receptor.

2. Ways to rid the synapse of additional neurotransmitter include diffusion, reuptake by the synaptic knob, and chemical degradation by enzymatic activity (acetylcholinesterase or monoamine oxidase [MAO]).

F. Neuromodulators (p. 477)

1. Neuromodulators are hormones, neuropeptides, and other messengers that modify synaptic transmission.

2. They can stimulate a neuron to raise or lower the number of receptors in the postsynaptic membrane or alter the rate of neurotransmitter synthesis, release, reuptake, or breakdown.

V. Neural Integration (p. 478)

A. Postsynaptic Potentials (p. 478)

1. Neural integration refers to the information-processing, decision-making, and memory mechanisms of neurons. This ability is based on the postsynaptic potentials produced by a neurotransmitter.

2. An excitatory postsynaptic potential (EPSP) is the likelihood of the postsynaptic cell reaching action potential; if a neurotransmitter instead makes the postsynaptic membrane hyperpolarize, it is called an inhibitory postsynaptic potential (IPSP). (fig. 13.21; TR 465)

3. Glutamic and aspartic acid are excitatory and tend to produce EPSPs; glycine and GABA are inhibitory and generally produce IPSPs.

4. Other neurotransmitters are inhibitory in some cases, and excitatory in others, depending on the receptors in target cells.

B. Summation, Facilitation, and Inhibition (p. 479)

1. Summation is the process of adding up incoming information and responding to the net effect of it. This occurs in the trigger zone of the neuron.

2. Summation can be temporal or spatial. (fig. 13.22; TR 466)

a. Temporal summation occurs when a single synapse generates EPSPs at such short intervals that each is generated before the previous one decays. (fig. 13.23; TR 467)

b. Spatial summation occurs when EPSPs from several different synapses add up to threshold at the axon hillock.

3. Facilitation is a process in which one neuron enhances the effect of another one.

4. Presynaptic inhibition, the opposite of facilitation, is a mechanism in which one presynaptic neuron suppresses another one. (fig. 13.24; TR 468)

C. Neural Coding (p. 481)

1. The way in which the nervous system converts information to a meaningful pattern of action potentials is called neural coding. (fig. 13.25; TR 469)

2. The nervous system employs a phenomenon known as recruitment, by which it is able to judge stimulus strength by which neurons, and how many of them, are firing.

D. Neuronal Pools and Circuits (p. 481)

1. Neurons actually function in much larger groups (thousands to millions of interneurons) called neuronal pools.

2. As an input fiber enters a neuronal pool, it branches and synapses with numerous neurons. It can produce EPSPs at all points of contact with the cell; these can summate to make it fire.

3. These postsynaptic neurons are in the discharge zone of the input fiber; neurons in its facilitated zone receive only a few contacts from the input fiber. (fig. 13.26; TR 470)

4. A neuronal circuit is the connection pathway among a series of neurons.

5. There are four principal kinds of neuronal circuits: diverging, converging, reverberating, and parallel-discharge. (fig. 13.27; TR 471)

E. Memory and Synaptic Plasticity

1. The basis of memory is the memory trace (engram), a pathway where new synapses have been formed or existing synapses have been modified.

2. Immediate memory is the ability to hold something in mind for just a few seconds.

3. Short-term memory lasts from a few seconds to a few hours.

4. Long-term memory can last up to a lifetime.

a. Declarative memory is the memory of events and facts than can be put into words.

b. Procedural memory is the retention of motor skills.