Page 57 - 84 human physiolofy part-1

The Nervous System

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

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 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 Nerve Synapse
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.

Summation

When excitatory synapses exceed the amount of inhibitory synapses there are, then the excitatory
synapses will prevail over the other. The same goes with inhibitory synapses, if there are more

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