Transcript Neural Tissue - Decker

Neural Tissue
Chapter 12
I. Overview of the Nervous System
Provides swift, but brief responses
to stimuli
A.
As opposed to what??
Consists of:
B.
Neural tissue:
1.
a.
Neurons- functional units
Neuroglia (aka Glial Cells)
b.
i.
ii.
iii.
iv.
v.
Separate & protect neurons
Provide a supportive framework for neural tissue
Act as phagocytes
Help regulate interstitial fluid
Far outnumber neurons (account for ~ ½ the
volume of the nervous system).
2.
Blood vessels
3.
Connective tissues
Organs of the nervous system
C.
1.
2.
3.
4.
Brain
Spinal cord
Sensory receptors for eyes, ears,…
Nerves
Major anatomical divisions of the
nervous system:
D.
Central Nervous System (CNS)
1.
Consists of:
a.
Brain
Spinal cord
i.
ii.
Responsible for:
b.
i.
ii.
iii.
iv.
v.
Integrating, processing, & coordinating sensory data
and motor commands
Intelligence
Memory
Learning
Emotion
Peripheral Nervous System (PNS)
2.
Consists of neural tissue outside the CNS
(nerve fibers (axons))
a.
i.
ii.
iii.
Cranial nerves- connected to the brain
Spinal nerves- connected to the spinal cord
Peripheral nerves- aka ‘nerves’
Responsible for:
b.
i.
ii.
Delivering sensory information to the CNS
Carrying motor commands to peripheral tissues &
systems
Functional divisions of the PNS
c.
i.
Afferent division- brings sensory information to the
CNS from receptors in peripheral tissues & organs
ii.
Efferent division- carries motor commands from the
CNS to effectors (muscles & glands)

Somatic nervous system (SNS)- controls skeletal
muscle
*
Conscious- voluntary
*
Subconscious- involuntary (reflex)

Autonomic nervous system (ANS or visceral motor
system)- provides automatic, involuntary
regulation of smooth muscle, cardiac muscle &
glands
*
Sympathetic division- fight or flight activities
*
Parasympathetic division- activities that
conserve energy
II. Neurons
Representative Neuron Structure
A.
(Multipolar, most common type in CNS)
Soma- cell body
1.
a.
b.
c.
d.
Large nucleus & noticeable nucleolus
Many mitochondria
Clusters of ribosomes & RER (Nissl bodies)
give the cell a grayish appearance, hence the
name “gray matter”
Lack centrioles (can’t divide)
Dendrites- sensitive processes
2.
a.
Extend from soma
b.
Highly branched w/ dendritic spines
c.
Neurons receive info from other neurons via
synaptic connections at the dendritic spines
Axon- long, cytoplasmic process
3.
a.
Connects to the soma at a axon hillock
(thickened region)
b.
May have side branches (collaterals) to
communicate w/ multiple cells
c.
End in a series of fine extensions (telodendria)
*
Telodendria end at synaptic terminals (synaptic bulbs)
* Synaptic terminals are a part of a synapse
Synapse- site of intercellular
communication
4.
There are 2 cells involved:
a.
i.
Presynaptic cell

Has the synaptic terminal

Sends a message
ii.
Postsynaptic cell

Receives the message
b.
Involves the release of chemicals
(neurotransmitters) by the synaptic terminal
c.
May occur on a dendrite, the soma, or along the
axon
Neuron Classification
B.
Structural Classification
1.
Anaxonic Neurons
a.
i.
Small
ii.
Dendrites & axons look alike
iii.
Located in the brain and special sense organs
iv.
Function is poorly understood!
Bipolar Neurons
b.
i.
Highly branched dendrites at one end, one
axon at the other end with the soma in the
middle
ii.
Short (30 mm end-to-end)
iii.
Rare- found in special sense organs
iv.
Relay info about sight, smell, hearing
Unipolar Neurons (Pseudounipolar
Neuron)
c.
i.
Dendrites and axon are continuous, with
the soma off to one side.
ii.
Most sensory neurons of the PNS are
unipolar
iii.
Axons may extend a meter or more!
Multipolar Neurons
d.
i.
Several dendrites and a single axon w/ one or more
branches
ii.
Most common type in the CNS
iii.
One function- controls skeletal muscle
iv.
Can be as long as unipolar neurons
Functional Classification
2.
Sensory Neurons
a.
i.
Form the afferent division of the PNS (deliver info from
sensory receptors to the CNS)

Somatic sensory neurons- monitor exterior
conditions

Visceral sensory neurons- monitor internal
conditions
ii.
Unipolar
iii.
Axons are known as afferent fibers
iv.
~10 million in the body
Motor Neurons (efferent neurons)
b.
i.
Form the efferent division of the PNS (carry
instructions from the CNS to peripheral effectors)

Somatic motor neurons- (in SNS) innervate
skeletal muscle

Visceral motor neurons- (in ANS) innervate all
other peripheral effectors
ii.
~ ½ million in the body
iii.
Axons are known as efferent fibers
Interneurons (association neurons)
c.
i.
Responsible for distribution of sensory info &
coordination of motor activity
ii.
May be situated between sensory & motor neurons
iii.
Located entirely w/in brain & spinal cord
iv.
~20 billion in the body
III. Neuroglia
Neuroglia of the CNS:
A.
Ependymal Cells
1.
a.
Epithelial cells (cuboidal to columnar in shape)
b.
Line the ventricles of the brain and central canal
of the spinal cord (both are filled with
cerebrospinal fluid (CSF))
Astrocytes
2.
Largest, most numerous glial cells
Functions:
a.
b.
i.
Maintaining blood-brain barrier - isolates the CNS from
the general circulation
ii.
Creating a framework for the CNS (microfilaments)
iii.
Repairing damaged neural tissue-stabilize tissue &
prevent further damage
iv.
Guiding neuron development (in embryos)
v.
Controlling the interstitial environment
Oligodendrocytes
3.
a.
Myelinate CNS axons
(myelin= multilayered
membraneous sheath)
a.
Increases speed of communication
b.
Myelinated areas appear white, hence “white
matter”
c.
Tie clusters of axons together
Microglia
4.
a.
Remove cell debris, wastes, and pathogens by
phagocytosis
Neuroglia of the PNS:
B.
1.
Soma are clustered together in masses
called ganglia
2.
Soma & axons are completely insulated by
processes of glial cells:
a.
Satellite cells (amphicytes)- surround soma in
peripheral ganglia
b.
Schwann cells (neurilemmocytes)- form a
sheath around every peripheral axon (even
unmyelinated)
Demyelination
3.
a.
Progressive destruction of myelin sheaths in the
CNS & PNS
b.
Results in a gradual loss of sensation & motor
control that leaves affected areas numb &
paralyzed
i.
ii.
iii.
Heavy metal poisoning
Diphtheria
Multiple sclerosis (MS)
Neural Responses to Injury
Read page 407 in your text to learn how a
damaged axon is able to (sometimes)
repair itself.