1. The two major subdivisions of the nervous system are the central nervous system, or CNS, which includes the brain and spinal cord, and the peripheral nervous system (PNS), which includes the nerves leading to and from the CNS.

2. Forty-three nerve pairs make up the PNS, including 12 pairs of cranial nerves, and 31 pairs of spinal nerves.

1. The three broad categories of nervous system function include: sensory function to detect internal or external changes; integrative function, to "decide" a course of action; and a responsive function, employing motor neurons to make the necessary adjustment.

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

1. The nervous system transmits messages at great speed (1-10 msec), using both electrical impulses and neurotransmitters. Its effects are relatively local, and the response stops when the stimulus ceases. Prolonged stimulation results in adaptation.

2. The endocrine system, by contrast, sends chemical messages (hormones) into the bloodstream that are much slower to act. Responses can be systemic, are slow to adapt, and last long after the stimulation ceases.

3. Both the nervous and endocrine systems function together to coordinate the body's activities.

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.

2. 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.

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

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

5. Abundant astrocytes are star-shaped cells found in the CNS. Protoplasmic astrocytes help form the blood-brain barrier. Fibrous astrocytes form a physically supportive framework for the CNS.

6. Ependymal cells produce and circulate cerebrospinal fluid.

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

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. Large fibers conduct impulses more rapidly than small ones; myelinated fibers are faster than nonmyelinated ones.

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.

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.

1. Small (type C) nerve fibers have Schwann cells but lack myelination.

1. Neurons are classified according to the number of extensions arising from the soma.

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

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

4. Somatic sensory neurons are unipolar.

1. Axonal transport is the two-way transport of materials in an axon. It 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 called retrograde movement and employs a motor protein called dynein.

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.

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.

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, irreversible, and either excitatory or inhibitory.

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

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

3. 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".

4. 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.

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

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

7. The original distribution of sodium and potassium are restored and RMP is reestablished.

8. 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.

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, which lasts until the membrane voltage returns to threshold, follows. 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.

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 0.5 to 2.0 m/sec.

1. Saltatory conduction occurs in myelinated fibers. Action potential is reached at each node of Ranvier, and travels to the next node.

2. This type of conduction can travel at speeds up to 130 m/sec.

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.

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.

1. More than 100 different chemicals have been identified as neurotransmitters. Major neurotransmitters and neuropeptides (that sometimes modify the actions of neurotransmitters) are described in Table 13.6, p. 448.

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.

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.

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).

1. To qualify as a neurotransmitter, a substance must be synthesized by a presynaptic neuron, released in response to stimulation, bind to specific receptors on postsynaptic cells, and alter the physiology of the same.

2. Other means of communication between neurons include neuropeptides (gut-brain peptides), neuromodulators (including hormones and neuropeptides, depending on their source), and inorganic gases.

1. Neural integration refers to the information-processing, decision-making, and memory mechanisms of neurons. This ability is based on the postsynaptic potentials (PSPs) 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).

3. Glutamic and aspartic acid are excitatory and tend to produce EPSPs; glycine and GABA are inhibitory and generally produce ISPSs. Other neurotransmitters are inhibitory in some cases, and excitatory in others, depending on the receptors in target cells.

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.

1. The way in which the nervous system converts information to a meaningful pattern of action potentials is called neural coding.

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

1. Synaptic potentiation (presynaptic facilitation) occurs at synapses when thought or actions occur repeatedly, producing a well-worn pathway. Prolonged use of the pathway causes more calcium to accumulate in the synaptic knob, making impulse transmission easier in the future. This is the process involved in learning.

2. In presynaptic inhibition, one neuron suppresses the release of neurotransmitter by another.

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. 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.

1. A neuronal circuit is the connection pathway among a series of neurons. Four principal kinds of circuits have been identified: diverging circuits, converging circuits, reverberating (oscillating) circuits, and parallel after-discharge circuits.