Transcript 6-3 Nervous Tissue

Nervous Tissue
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Nervous Tissue

Consists of two main cell types:
- neurons - sensory control, and
regulation
- neuroglia - support, protection, and
homeostasis
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Parts of a Neuron
cell body
 dendrites
 axon
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Cell Body
contains nucleus, surrounded by
granular cytoplasm
 contains organelles (i.e., lysosomes,
mitochondria, Golgi complex, etc.)
 Nissl bodies (chromatophilic substance)
- arrangement of rough endoplasmic
reticulum (site of protein synthesis)
 neurofibril
- provides support and shape for cell
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Two Kinds of Nerve Fibers
dendrites
- neurons usually contain many
 axons
- neurons contain only one
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Dendrites
dendro = tree
highly branched processes
 not myelinated
 extend out from cell body
 receive stimuli
 carry a nerve impulse toward the cell
body
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Axon
long, thin, cylindrical projection
 may or may not be myelinated
 contains mitochondria and neurofibrils but
no rough ER; no protein synthesis
 cytoplasm - axoplasm
 surrounded by axolemma
(lemma = sheath or husk)
 carries a nerve impulse away to other
cells i.e., (nerves, muscles, glands)
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Parts of an Axon

long nerve cell fibers are covered with
myelin
- fatty substance
- protects and insulates fibers
- increases transmission rate of
- nerve impulses
- forms myelin sheath
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Parts of an Axon
(cont.)
neurolemma sheath surrounds myelin
 Schwann cells are surrounded by
neurolemma
- Schwann cells are also called
neurolemmocytes
 narrow gaps in myelin sheath between
adjacent Schwann cells are called nodes of
Ranvier
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Parts of an Axon
(cont.)
axon hillock
- cone-shaped elevation where axon
joins cell body
 initial segment
- first portion of axon
 trigger zone
- junction of the hillock and initial
segment where nerve impulses arise
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Parts of an Axon
(cont.)
axon collateral
- side branch along length of axon
 axon terminals
- fine branched end portions of an axon
and its collaterals
- when impulse reaches axon terminal,
neurotransmitter is released
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Parts of an Axon

(cont.)
neurons are close to each other but
separated by synaptic gap
- this junction is called a synapse
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Parts of an Axon
(cont.)
synaptic end-bulbs
- bulb-shaped structures
- tips of axons
 synaptic vesicles
- membrane-enclosed sacs within synaptic
end-bulbs
- store neurotransmitters (different
neurons have different neurotransmitters)
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Neuroglia
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Neuroglia
constitute one-half of volume of CNS
 outnumber neurons 5-50 times
 can multiply and divide, unlike neurons
 Functions:
- nerve glue
- supports
- insulates
- protects
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Types of Neuroglial Cells
astrocytes
 oliogodendrocytes
 microglia
 ependymal cells
 Schwann cells
 satellite cells
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Astrocytes
(astro = star)
star-shaped
 many processes
 participate in metabolism of
neurotransmitters
 maintain potassium (K+) balance for
generation of nerve impulses
 participate in brain development
 aid in formation of blood-brain barrier
 link between neurons and blood vessels
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Oliogodendrocytes
(oligo = few, dendro = tree)
few processes
 smaller than astrocytes
 form supporting network by twining
around neurons
 produce a lipid and protein wrapping
called a myelin sheath
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Microglia
(micro = small, glia = glue)
small
 protect CNS from disease
 phagocytic
- engulf microbes and clear away debris
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Ependymal Cells
(ependyma = upper garment)
shape ranges from squamous to
columnar
 may be ciliated
 line brain ventricles and central canal of
spinal cord

ventricles are spaces that produce and
circulate cerebrospinal fluid
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Schwann Cells
(Neurolemmocytes)
found in peripheral nervous system
 form myelin sheath
 support, protect, and nourish neurons
 plasma membrane of neurolemmocytes
forms myelin sheath around axon in
PNS
 myelin sheath insulates speed of
impulse transmission
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Satellite Cells
found in peripheral nervous system
 support neurons in ganglia (clusters of
neuron cell bodies) of PNS
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Neuron Classification
structural classification
- based on number of processes
extending from cell body
 functional classification
- based on direction in which a neuron
transmits a nerve impulse
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Structural Classification
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unipolar (pseudounipolar)
- have a single process extending from
cell body
- always are sensory neurons
- originate in embryo as bipolar
- during development, axon and dendrite
fuse together into a single process
- single process divides into two
branches a short distance from cell body
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Structural Classification
bipolar
- have one dendrite and one axon
- found in retina, internal ear, and
olfactory area of brain
 multipolar
- several dendrites and one axon
- most neurons in brain and spinal cord
are of this type
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Functional Classification
based on the direction in which a
neuron transmits a nerve impulse
 Two types of neurons in PNS:
- afferent (toward CNS)
- efferent (away from CNS)
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Afferent
carry impulses (sensory information)
toward CNS
 also known as sensory neurons

Efferent
carry impulses away from CNS
 affect activity of muscles or glands
 also known as motor neurons
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Sensory = Afferent
Motor = Efferent
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Afferent or Sensory Neurons
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Four categories:
- general somatic
- special somatic
- general visceral
- special visceral
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Afferent or Sensory Neurons

(cont.)
general somatic
- carry impulses from skin, skeletal
muscles, and joints
(pain, temperature, touch, vibration, and
pressure)
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Afferent or Sensory Neurons

(cont.)
special somatic
- carry or relay impulses from retina and
internal ear
(vision, hearing, and balance)
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Afferent or Sensory Neurons (cont.)

general visceral
- carry impulses from internal organs
and receptors of body fluids
(distension of internal organs and
chemical conditions in body)
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Afferent or Sensory Neurons

(cont.)
special visceral
- carry impulses from tongue and
olfactory mucosa
(taste and smell)
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Efferent or Motor Neurons
general somatic
- carry impulses to most skeletal
muscles
 general visceral
- carry impulses to smooth muscle,
cardiac muscle, and glands
 special visceral
- carry impulses to skeletal muscles that
control facial expression
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Associated Neurons
located in CNS
 carry impulses from one neuron to
another inside brain and spinal cord
 establish interrelationship with other
neurons forming neuronal circuits
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