Human Physiology/The Nervous System

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General functions of the CNS

Brain, brain stem, and spinal chord.

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

the 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