A. TYPES OF NERVE CELLS

1. Neurons are the functional units of the nervous system. (cell body, dendrites, axon hillock, axon, telodendria, synaptic knobs, neurotransmittors) (Fig. 48.2 & 48.4)

2. Supporting cells act to support cells of the CNS:

a. glial cells are the supporting cells of the CNS:

(1) oligodendrocytes form myelin sheaths that insulate some axons in CNS.

(2) astrocytes line the capillaries and form blood-brain barrier.

b. Schwann cells surround the axons of many peripheral nerve cells and form an insulating layer called a myelin sheath (for speed, also). (Fig. 48.5)

B. MEMBRANE POTENTIALS OF NEURONS

1. Neuronal membranes typically have resting potentials of about -70 mv. (Fig. 48.6a)

2. This electromotive potential is generated by:

a. an abundance of negatively charged proteins along inside edge of membrane.

b. an ATP-driven Na⁺-K⁺ pump (protein) distributes these ions unevenly (Fig 48.7)

3. Compare this resting potential to the potential energy of dominos on end.

4. Stimuli such as mechanical pressure, chemical, electrical shock, radient, or temp change can alter the membrane potential:

a. some stimuli locally hyperpolarize membrane (graded potential) (open K⁺ channels) (Fig 48.8)

b. some stimuli locally depolarize the membrane (graded potential) (open Na⁺ channels)

c. if depolarization goes beyond a certain threshold, then potential becomes kinetic, and is called an "action potential". (dominos falling)

C. PROPAGATION OF ACTION POTENTIALS BY NEURONS

1. The action potential is due to ion fluxes through specific channels in membrane called ion "gates".

2. These gates are actually membrane proteins that become permeable to ions in response to changes in the membrane potential (i.e., voltage sensitive gates).

3. The basic gating events during an action potential: (Fig 48.9)

a. when the stimulus is of a magnitude to cause depolarization to a certain threshold, the voltage change causes the gates of Na⁺ channels to open.

b. the flood of Na⁺ into cell even makes the inside positive (» +35-50 mv).

c. within less than 2 msec, the Na⁺ gates are closed and inactivated (refractory).

d. simultaneously, K⁺ gates open and let this ion out to restore resting potential.

e. this rapid sequence is an all-or-none phenomenon when graded depolarization goes beyond the threshold potential.

4. Collectively, the membrane around the dendrites and nerve cell body is integrating the various signals that reach a given neuron at a given time. If there is adequate depolarization, and the threshold potential at the hillock is exceeded, then the action potential is propagated down the axon.

5. Propagation occurs when the Na⁺ influx of an action potential behaves like a tilted domino that has enough kinetic energy to upset the balance of the adjacent domino. (Fig 48.10)

6. Rate of transmission is constant for a given neuron (depending on its diameter) [speed of 1 cm/sec (thin axons) to 120 meters/sec (thick axons)] (Schwann cells for saltatory conduction) (Fig 48.11)

7. Intensity of original stimulus is transmitted not by speed of impulse, but by frequency of impulse (i.e., by frequency of the action potentials).

II. THE SYNAPSE, AND TRANSMISSION BETWEEN CELLS

1. A synapse is the intercellular junction that connects neurons to other neurons, muscle cells, and other cells like glandular secretory cells.

2. Electrical synapses are gap junctions that couple presynaptic and postsynaptic cells.

3. Chemical synapses consist of synaptic clefts between cells, and they require chemical secretion and coupling of neuronal cells. (Fig 48.12)

a. t he chemicals (neurotransmitters) are stored in synaptic vesicles.

b. these neurotransmitters cross the cleft and react with ion channel receptors.

c. at least 10 different neurotransmitters have been identified. (see Table 48.1)

d. neurotransmitters are usually rapidly inactivated in synaptic cleft.

e. some are excitatory (EPSP), and some are inhibitory (IPSP).

4. The nerve cell body and dendrites integrate multiple synaptic inputs in a process called summation. (Fig 48.13a)

5. Temporal summation is when the nerve cell body responds to two or more closely timed stimuli (Fig 48.14)

6. Spatial summation is the addition of all EPSPs and IPSPs. (Fig 48.14)

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