Download THE NERVOUS SYSTEM

THE NERVOUS
SYSTEM
INTRODUCTION TO THE
NERVOUS SYSTEM



Communication
system
100 Billion nerve
cells in the brain
alone
2 main divisions
to a vertebrate
nervous system:
TWO MAIN DIVISIONS

Central Nervous
System (CNS)


nerves, brain, spinal
cord
coordinating centre

Peripheral Nervous System (PNS)
 nerves that carry info between the
CNS and the organs
 Somatic Nerves
 skeletal muscle, bones and
skin
 sensory and motor somatic
nerves
 Autonomic Nerves
 special motor nerves that
control the internal organs
 sympathetic nervous system
and parasympathetic
nervous system
NERVE CELLS
Two cells types: neurons and glial cells
 Glial cells (neuroglial cells): structural support and
metabolism of nerve cells (do not conduct)

Neurons: nerve cells that conduct nerve impulses
(functional unit). A nerve is a bundle of many
neurons.
Types of Neurons

Sensory Neurons: (afferent neurons)
carry impulses from sensory receptors
to the CNS


Sensory receptors ex.
photoreceptors in eyes (light),
chemoreceptors in nose and skin
(chemicals), thermoreceptors in skin,
hypothalamus (heat/cold).
Ganglia (singular – ganglion):
clusters of sensory nerve cell bodies
located outside of the CNS.

Motor Neurons: (efferent
neurons) carry impulses from the
CNS to effectors (muscles,
organs, glands...i.e. Things that
produce a response)

Interneurons: link neurons
within the body (found mostly in
the CNS).
NERVE CELL ANATOMY



Cell body: nucleus and
majority of cytoplasm.
Dendrites: projections
of cytoplasm that carry
impulses TOWARD the
cell body
Axon: extension of
cytoplasm that carries
nerve impulses AWAY
from the cell body.



Myelin Sheath:
insulated covering (fatty
protein) over the axon
of some nerves,
“myelinated”
 prevents loss of
charge
Schwann cells: type of
glial cell that produces
myelin sheath
nodes of Ranvier:
regularly occurring gaps
between sections of the
myelin sheath

nerve impulses jump from one
node to another – increases
speed of the impulse



nonmyelinated nerves carry
impulses at a slower rate
axon diameter also effects speed –
narrower = faster.
Neurilemma: PNS axons have
a thin membrane surrounding
the axon. Promotes
regeneration of damaged
axons.


Cells without myelin sheath or neurilemma do not
regenerate, damage is permanent. (“grey matter”
in brain and spinal cord; white matter has myelin
sheath).
PNS has greater ability to regenerate than the CNS
(spinal cord injuries). Presence of a growth
inhibitor in the CNS. Scientists are looking for
ways around this (stem cells)
REFLEX ARC



Simplest nerve
pathway.
Involuntary,
unconscious.
Neural circuit through
the spinal cord that
provides a framework
for a reflex action.
Ex. Accidentally touching a hot
stove (see handout)
1.
2.
3.
4.
5.
Heat detected by
temperature receptors in
skin
nerve impulse carried by a
sensory neuron to the spinal
cord
interneuron in spinal cord
passes the impulse to a
motor neuron
motor neuron carries
impulse to muscles
arm/hand
causes the muscles to
contract and pull hand away



Happens in less than a second – information has
not even travelled to the brain. When it does, the
sensation of pain will become noticeable and you
may scream!
If you had to wait to feel the pain first – the burn
would be a lot worse.
HOMEWORK: Handout Questions #1-9