General functions of the CNS

CNS:
The "Central Nervous System", comprised of brain, brainstem, and
spinal cord.
The central nervous system (CNS) represents the largest part of the
nervous system, including the brain and the spinal cord. Together, with
he peripheral nervous system (PNS), it has a fundamental role in the
control of behavior.
The CNS is conceived as a system devoted to information processing,
where an appropriate motor output is computed as a response to a
sensory input. Many threads of research suggest that motor activity
exists well before the maturation of the sensory systems, and senses
only influence behavior without dictating it. This has brought the
conception of the CNS as an autonomous system.

Structure and function of neurons

Structure



Neurons are highly specialized for the processing and transmission of
cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there
is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of
a neuron can vary in size from 4 to 100 micrometers in diameter.
The soma (cell body) is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most
protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter. The dendrites of a neuron are
cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a
dendritic tree. This is where the majority of input to the neuron occurs. However, information outflow (i.e. from
dendrites to other neurons) can also occur (except in chemical synapse in which backflow of impulse is inhibited by
the fact that axon do not possess chemoreceptors and dendrites cannot secrete neurotransmitter chemical). This
explains one way conduction of nerve impulse. The axon is a finer, cable-like projection which can extend tens,
hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away
from the soma (and also carry some types of information back to it). Many neurons have only one axon, but this
axon may - and usually will - undergo extensive branching, enabling communication with many target cells. The part
of the axon where it emerges from the soma is called the 'axon hillock'. Besides being an anatomical structure, the
axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This
makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in neurological terms it
has the greatest hyperpolarized action potential threshold. While the axon and axon hillock are generally involved in
information outflow, this region can also receive input from other neurons as well. The axon terminal is a specialized
structure at the end of the axon that is used to release neurotransmitter chemicals and communicate with target
neurons. Although the canonical view of the neuron attributes dedicated functions to its various anatomical
components, dendrites and axons often act in ways contrary to their so-called main function.
Axons and dendrites in the central nervous system are typically only about a micrometer thick, while some in the
peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is
not much larger than the cell nucleus it contains. The longest axon of a human motor neuron can be over a meter
long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal
columns, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire
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.

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 inhibitory synapses than excitatory,
the synapses will be inhibited. To determine all of this is called summation.
Classification by discharge patterns:
Neurons can be classified according to their electrophysiological characteristics (note that a single action potential is
not enough to move a large muscle, and instead will cause a twitch).
Tonic or regular spiking:  Some neurons are typically constantly (or tonically) active. Example: interneurons in
neurostriatum.
Phasic or bursting: Neurons that fire in bursts are called phasic.
Fast spiking: Some neurons are notable for their fast firing rates. For example, some types of cortical inhibitory
interneurons, cells in globus pallidus.
Thin-spike: Action potentials of some neurons are more narrow compared to the others. For example, interneurons
in prefrontal cortex are thin-spike neurons.
Classification by neurotransmitter released:
Some examples are cholinergic, GABAergic, glutamatergic and dopaminergic neurons.

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