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Nervous System
A basic overview
Mrs. S. Taylor
What does it do?

Allows us to


Think, feel, remember, move, being aware of the
world around us
How?

Sensory receptors bring messages to the Central
Nervous System(CNS) from both inside and outside
sources.

CNS determines course of action and sends a
message out to where it need to go so that the
action can happen.
So... it is the master control?


Yep, it sure is and it is also our means of
communication, too.
How does it work? Well....

Sensory input – monitoring stimuli


Integration – interpretation of sensory input


This is all the data the receptors collect... you know... pain,
pressure, temperature... that sort of stuff
The data is brought together to create memories, sensations,
produce thoughts.. stuff like that and then we make decisions
about it, either consciously or subconsciously
Motor output – response to stimuli

This is how we act on what we have sensed... like running to
the restroom, jerking our hand back from a hot stove...
Okay... so what are the parts?

The major players are the Nerve cells
(technically called the neurons) that use
electrochemical impulses (nerve impulses) to
pass messages from one to another and to the
effectors (the things that respond to the
message)

They make up the brain, spinal cord, motor nerves,
sensory nerves... anything that sends and receives
messages in us has a nerve.
st
1
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major classification
Are they of the CNS (central nervous system)

Brain and spinal cord

Integration and command center
Or are they the PNS (peripheral nervous
system)

Paired spinal and cranial nerves (peripheral nerves)

Carries messages to and from the spinal cord and
brain
The PNS is further divided


Sensory (afferent) division

Sensory afferent fibers – carry impulses from skin,
skeletal muscles, and joints to the brain

Visceral afferent fibers – transmit impulses from
visceral organs to the brain
Motor (efferent) division

Transmits impulses from the CNS to effector organs
The motor division gets to do it
again....

Somatic nervous system


Conscious control of skeletal muscles (Voluntary)
Autonomic nervous system (ANS)

Regulates smooth muscle, cardiac muscle, and
glands (Involuntary)

Divisions – (once again...^-^)


Sympathetic- prepares the body for energy-expending,
stressful, or emergency actions
Parasympathetic- used in ordinary restful conditions, and
returning to that after the sympathetic division has had its
way.
Okay Okay Okay... we got that, but
what does the nerve look like?


Well... it has three main parts

Dendrites

Cell body

Axon
And some ancillary parts too

Nodes of Ranvier

Schwann cells & the Neurilemma

Myelin sheaths
Let's start with the main parts


Dendrites

They are the short, tapering, and diffusely branched
processes (extensions)

receive the messages from where ever (sensory
receptors, other nerves...) and carries them to the
Cell body
Axon

It is the slender processes of uniform diameter
arising from the hillock (If long called a nerve fiber)

Typically only one and it is usually unbranched

Takes messages from the Cell body to the next stop
(dendrites or the effector)
So, what about the Cell body?





Contains the nucleus and a nucleolus,
mitchondria, gogli apparatus... those sorts of
things
Is the focal point for the outgrowth of neuronal
processes (Those things we just finished talking
about)
Has no centrioles (hence its amitotic nature –
that means it can't divide to make new cells)
Has well-developed Nissl bodies (kind of like
rough ER)
Contains an axon hillock – cone-shaped area
from which axons arise
The Ancillaries?
First up... the Myelin sheaths



Whitish, fatty (protein-lipoid), segmented sheath
around most long axons
It functions to:

Protect the axon

Electrically insulate fibers from one another

Increase the speed of nerve impulse transmission
It is created by Schwann cells in PNS


Can repair the axon if damages
It is created by oligodendrocytes in the CNS

Axons usually cannot repair
Schwann cells?


A Schwann cell:

Envelopes an axon in
a trough

Encloses the axon
with its plasma
membrane

Has concentric layers
of membrane that
make up the myelin
sheath
Neurilemma –
remaining nucleus
and cytoplasm of a
Schwann cell
So, what are these node things?

The Nodes of Ranvier (AKS Neurofibral nodes)
Gaps in the myelin sheath between adjacent
Schwann cells

So, what if the axon is short?

It is called unmyelinated.


A Schwann cell surrounds nerve fibers but coiling
does not take place
In the brain they are called gray matter (the
myelinated are called white matter)