Neurotransmitters

CELLS OF THE NERVOUS SYSTEM What are neurons and what do they do? A. Neurons

1. There are two main cell types in the nervous system.

a. Neurons are specialized to respond rapidly to signals and send signals of their own. b. Glial cells hold neurons together, guide their growth, secrete and absorb chemicals to

maintain a stable chemical environment, and send a limited number of signals between

neurons.

2. All body cells have some features in common.

a. An outer membrane selectively allows only some substances to pass in and out.

b. The cell body contains the nucleus.

c. Mitochondria turn oxygen and glucose into energy.

3. Neurons have special features that permit effective signal communication:

a. An axon is a cell fiber that carries signals away from the cell body. Most neurons have

just one axon. b. A dendrite is a cell fiber that receives signals from other neurons and carries information

toward the neuron's cell body. Most neurons have many dendrites. B. Action Potentials

1. Action potentials are electrochemical pulses that shoot down the neuron's axon. They are "all-or-none:" A neuron either fires an action potential at full strength, or does not fire at all.

2. After an action potential, there is a brief refractory period, during which a neuron cannot fire another action potential.

3. The speed of an action potential on a given axon is constant, but different neurons show different speeds relative to each other.

a. Action potentials travel faster on axons with large diameter or axons wrapped with myelin, a white, fatty substance. Such axons are often found in the pathways that carry the most "urgent" information

1. Multiple sclerosis is a disease in which the myelin is destroyed by a person's own immune system. Symptoms result because the correct timing of neuron signaling is thrown off. C. Synapses and Communication Between Neurons

1. Two neurons communicate at a synapse, a connection with only a narrow gap separating the two