Nervous tissue is composed of
neurons, which send and receive
electrical signals throughout the body and the
supporting cells that protect and nourish the
neurons. Bathing the
nervous tissue is the
cerebrospinal fluid, an
extracellular fluid with a strictly controlled
solute composition that cushions the delicate
tissues of the
nervous system.
Supporting Cells
The
neuroglia (glia or glial cells) protect, surround and nourish the
neurons. There are several types of
glia, each with specialized
functions. Unlike most
neurons, which lose their ability to divide after maturity,
glia can reproduce freely and therefore be replaced regularly.
Glia also cannot transmit electrical impulses, which is the main
function of
neurons.
Supporting cells of the CNS:
Astrocytes
The
projections of the star-shaped
astrocytes wrap around the
nerve fibers of
neurons and
blood capillaries, anchoring them together.
Astrocytes form the living “
blood-brain barrier” that separates the
blood system from the delicately balanced
cerebrospinal fluid.
Blood composition is not as rigidly controlled as the
extracellular fluid around
neurons, and may contain substances harmful to the
functioning of
neurons.
Astrocytes are very common and make up over half the
cells in neural
tissue.
Microglia
The phagocytic microglia have many thin
projections and wander around the
CNS clearing debris including dead
nervous tissue cells and
pathogens.
Ependymal cells
The
ciliated ependymal
cells line cavities of the
CNS and circulate the
cerebrospinal fluid. This ensures that nutrients and gases are distributed evenly throughout the
nervous tissue of the
CNS.
Oligodendrocytes
The flattened
projections of oligodendrocytes wrap around the
nerve fibers of
CNS neurons, forming a fatty insulating layer called the
myelin sheath. The insulation of
CNS neurons by the oligodendrocytes increases the efficiency with which the
neurons transmit electrical impulses. Oligodendrocytes can wrap their processes on many
CNS nerve fibers.
Supporting cells of the PNS:
Schwann Cells
Schwann cells are flattened and wrap around the
nerve fibers of PNS
neurons, insulating the
neurons and increasing the efficiency with which they transmit electrical impulses.
Satellite Cells
Satellite cells support and cushion the
neurons of the PNS, performing a similar role to the
astrocytes of the
CNS.
Neurons
Neurons (nerve cells) are specialized in transmitting electrical impulses.
Electrical signals sent from one part of the body to another are used to control the physiological and behavioral responses of the body.
Neurons have two main parts: A
cell body and many
processes (projections).
Cell body
The
cell body contains all the
membrane-bound organelles that carry out the metabolic
functions of the
neuron.
Neurons possess all the organelles found in a typical animal
cell with the exception of centrioles, since most mature
neurons cannot divide.
The
cell body of
neurons possesses several distinguishing features:
Nucleus – transparent, contains the genetic information and a distinct
nucleolusNissl substance – the rough ER of a
neuron; very abundant
Neurofibrils –
Intermediate filaments that maintain
cell shape
Processes
The
processes (also called
projections or
fibers) of
neurons are often very numerous and range in length from a fraction of the length of the
cell body to up to over a meter in length.
Dendrites are the
processes that carry messages from other
cells or sources of stimuli
to the cell body.
Axons are the
processes that generate electrical impulses starting at the
axon hillock and transmit those impulses
away from the cell body towards the
axon terminals.
The
axons of some
neurons are branched, forming
collateral branches, but all axons end in several (up to thousands) of axon terminals. Axon terminals contain
vesicles of
neurotransmitters, which are released into the
synaptic cleft when stimulated by an electrical impulse from the
axon.
The
synaptic cleft is the small space between an
axon terminal and the next
neuron. The
presynaptic neuron releases
neurotransmitters at its axon terminal, which are received by the
postsynaptic neuron. The combination of a
presynaptic neuron,
synaptic cleft and postsynaptic neuron is a
synapse.
Myelin covers long
axons in a whitish, shiny
myelin sheath. The
myelin sheath is formed by layers of fatty
plasma membrane with very little cytoplasm in between layers, creating a tightly wrapped insulating layer. The fatty insulating layer increases the efficiency and speed of
transmission of electrical impulses.
In the
CNS, the
myelin sheath is formed by the wrapping of flattened
processes of oligodendrocytes, which can wrap around several dozen
axons each. Because
CNS neurons do not have ready access to a
functional cell body, their regeneration ability is largely non-existent. If
CNS neurons are damaged, they typically do not regenerate.
In the PNS, the
myelin sheath is formed by the wrapping of many Schwann cells tightly around a single
axon. In the outermost layer of the
myelin sheath created by Schwann cells is the Schwann cell cytoplasm, found in a region called the
neurilemma. The
neurilemma plays a role in regenerating damaged PNS
neurons. In between Schwann cells, the
nodes of Ranvier allow materials to pass quickly into and out of a PNS
axon.
Neurons – Functional classifications
Neurons are classified
functionally according to the direction in which they carry information.
Sensory (afferent) neurons transmit electrical impulses from
sensory receptors to a
neuron of the
CNS.
The
dendrites of sensory
neurons are associated with
sensory receptors, and send electrical impulses when the
sensory receptors are activated by changes in the
variable they are designed to
detect. For example, if you
touch a cold drink the
temperature receptors in the skin of your hand will activate nearby sensory
neurons to send electrical impulses to the
CNS.
The
cell bodies of sensory
neurons are found in
ganglions in the PNS, and the
axons of sensory
neurons synapse with
neurons in the
CNS.
Sensory receptors come in various types. There are several types of
cutaneous receptors:
There are also specialized
receptors for each of the body’s
special senses (
vision,
hearing,
taste,
smell and balance).
Interneurons (association neurons) transmit electrical impulses from sensory
neurons to efferent neurons. Their
cell bodies and
projections are in the
CNS.
Interneurons are also responsible for
integration (decision-making) processes that occur in the
CNS.
Motor (efferent) neurons transmit electrical impulses from
interneurons to
effectors. The
dendrites and
cell bodies of efferent neurons are found in the
CNS, whereas their
axons extend through the PNS to
effectors such as muscles and
glands.
Neurons – Structural Classifications
The structural classification of a
neuron depends on its
number of processes.
| Neuron type |
Number of processes |
Location and occurrence |
| Multipolar |
Several (3+) |
Motor and association neurons; most common |
| Bipolar |
2 |
Sensory receptor cells in eyes and nose; rare |
| Unipolar |
1* |
Sensory neurons of PNS |
* In unipolar
neurons, the short single
process quickly divides into two longer
processes:
distal (
peripheral) and
proximal (central)
processes.
Dendrites are only found on the end of the
peripheral process, and the remainder of the central and peripheral processes
function as
axons. This allows the axons to conduct impulses both towards and away from the
cell body; a feature unique to unipolar
neurons.
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1 - Anatomy and Physiology Basics