Download The Central Nervous System CNS

The Central Nervous System
CNS
Lecture 2
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Nervous System and Senses
• The nervous system consists of two types of cells:
1. Nerve cells are called neurons.
2. Various support cells are associated with the
neurons, most typically, Schwann cells.
• The parts of a neuron include the dendrite which
receives the impulse (from another nerve cell or from a
sensory organ), the cell body (numbers of which sideby-side form gray matter) where the nucleus is found,
and the axon which carries the impulse away from the
cell.
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• Wrapped around the axon are the Schwann cells, and
the spaces/junctions between Schwann cells are
called nodes of Ranvier.
• Collectively, the Schwann cells make up the myelin
sheath (numbers of which side-by-side form white
matter).
• Having an intact myelin sheath and nodes of Ranvier
are critical to proper travel of the nerve impulse.
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• Diseases which destroy the myelin sheath
(demyelinating disorders) can cause paralysis or
other problems.
• Schwann cells are analogous to the insulation on
electrical wires, and just as electrical wires short
out if there’s a problem with the insulation, so also,
neurons cannot function properly without intact
myelin sheaths.
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•
The outer layer of myelin is surrounded by a
neurilemma (neurilemmal sheath) made up of
the cytoplasm and nuclei of the Schwann cell.
•
Narrow gaps in the myelin sheath between
Schwann cells are called nodes of Ranvier.
•
The smallest axons lack a myelin sheath and are
unmyelinated fibers.
•
White matter in the CNS is due to myelin sheaths
in this area.
•
Unmyelinated nerve tissue in the CNS appears
gray.
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Types of Neurons
Neurons can be grouped in two ways:
1. on the basis of structural differences (bipolar,
unipolar, and multipolar neurons)
2.
by functional differences (sensory neurons,
interneurons, and motor neurons).
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Types of Neurons
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
Bipolar neurons are found in the eyes, nose, and
ears, and have a single axon and a single
dendrite extending from opposite sides of the
cell body.

Unipolar neurons are found in ganglia outside
the CNS and have an axon and a dendrite arising
from a single short fiber extending from the cell
body.

Multipolar neurons have many nerve fibers
arising from their cell bodies and are commonly
found in the brain and spinal cord.
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Another Classification of Neurons
• Sensory neurons (afferent neurons) conduct
impulses from peripheral receptors to the CNS and
are usually unipolar, although some are bipolar
neurons.
• Interneurons are multipolar neurons lying within
the CNS that form links between other neurons.
• Motor neurons (efferent neurons) are multipolar
neurons that conduct impulses from the CNS to
effectors.
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Motor Neuron
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Neuroglial Cells
1.
2.
3.
4.
Microglial cells
Oligodendrocytes
Astrocytes
Ependyma
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Functions of Neuroglial Cells
1. fill spaces
2.
support neurons
3.
provide structural frameworks
4.
produce myelin, and carry on phagocytosis
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•
Microglial cells are small cells that phagocytize
bacterial cells and cellular debris.
•
Oligodendrocytes form myelin in the brain and
spinal cord.
•
Astrocytes are near blood vessels and support
structures, aid in metabolism, and respond to
brain injury by filling in spaces.
•
Ependyma cover the inside of ventricles and
form choroid plexuses within the ventricles
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Nervous system Functions
The nervous system has three basic functions:
1. receive information from the sensory receptors
through the Sensory neurons.
2. transfer and interpret impulses through the
Interneurons.
3. send appropriate impulses/ instructions to the
muscles and glands through Motor neurons .
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