Human Physiology/The Nervous System

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length of their necks. Much of what is known about axonal function comes from studying the squids giant axon, an

ideal experimental preparation because of its relatively immense size (0.5

–

1 millimeters thick, several centimeters

long).

Function

Sensory afferent neurons convey information from tissues and organs into the central nervous system. Efferent

neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor

neurons. Interneurons connect neurons within specific regions of the central nervous system. Afferent and efferent

can also refer generally to neurons which, respectively, bring information to or send information from brain region.

Classification by action on other neurons

Excitatory neurons excite their target postsynaptic neurons or target cells causing it to function. Motor neurons and

somatic neurons are all excitatory neurons. Excitatory neurons in the brain are often glutamatergic. Spinal motor

neurons, which synapse on muscle cells, use acetylcholine as their neurotransmitter. Inhibitory neurons inhibit their

target neurons. Inhibitory neurons are also known as short axon neurons, interneurons or microneurons. The output

of some brain structures (neostriatum, globus pallidus, cerebellum) are inhibitory. The primary inhibitory

neurotransmitters are GABA and glycine. Modulatory neurons evoke more complex effects termed

neuromodulation. These neurons use such neurotransmitters as dopamine, acetylcholine, serotonin and others. Each

synapses can receive both excitatory and inhibitory signals and the outcome is determined by the adding up of

summation.

Excitatory and inhibitory process

Nerve Synapse

The release of a excitatory neurotransmitter

(ACHe) at the synapses will cause an inflow

of positively charged sodium ions (Na+)

making a localized depolarization of the

membrane. The current then flows to the

resting (polarized) segment of the axon.

Inhibitory synapse causes an inflow of Cl-

(chlorine) or outflow of K+ (potassium)

making the synaptic membrane

hyperpolarized. This increase prevents

depolarization, causing a decrease in the

possibility of an axon discharge. If they are

both equal to their charges, then the

operation will cancel itself out. There are

two types of summation: spatial and temporal. Spatial summation requires several excitatory synapses (firing several

times) to add up,thus causing an axon discharge. It also occurs within inhibitory synapses, where just the opposite

will occur. In temporal summation, it causes an increase of the frequency at the same synapses until it is large

enough to cause a discharge. Spatial and temporal summation can occur at the same time as well.

The neurons of the brain release inhibitory neurotransmitters far more than excitatory neurotransmitters, which helps

explain why we are not aware of all memories and all sensory stimuli simultaneously. The majority of information

stored in the brain is inhibited most of the time.