Download Chapter 12 () - Austin Community College

Anatomy Lecture Notes
Chapter 12
A. functions of nervous system
1. monitors changes in internal and external environment
2. processes and interprets sensory input
3. controls effector organs (muscles and glands)
B. organization of nervous system (NS)
1. components
CNS
brain
spinal cord
PNS
cranial nerves
spinal nerves
2. organization
afferent/sensory = carrying information toward the CNS
efferent/motor = carrying information away from the CNS
somatic - skeletal muscles, bones and skin
visceral - cardiac muscle, smooth muscle and glands
CNS
PNS
sensory
somatic
visceral
receptors
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motor
somatic
visceral
effectors
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Anatomy Lecture Notes
Chapter 12
C. nerve tissue
1. neurons = specialized to transmit electrical signals
 extreme longevity
 do not divide
 high metabolic rate
2. neuroglia (supporting cells, glial cells)
 non-excitable
 function is to support the neurons
 make up 50% of brain mass
 ratio in brain is about 10 glial cells per neuron
 can divide to replace themselves
D. neurons
1. structure
a. cell body/soma/perikaryon
 nucleus
 chromatophilic bodies (rER and ribosomes)
 neurofibrils
b. processes/fibers - extensions of the cell body
dendrites:
 most neurons have many dendrites
 enlarge the surface area of the cell
 receive signals from other neurons and send it to the cell body
axon:

one per neuron

arises from axon hillock in most neurons

axonal transport delivers materials from cell body to distant
parts of cell

branches are called collaterals

axons end in smaller branches called telodendria or terminal
branches

at the ends of the telodendria are small enlargements called
axon terminals or synaptic knobs; they contain
neurotransmitters

axons carry signals away from the cell body
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Anatomy Lecture Notes
Chapter 12
2. structural classification - number of processes:
a. multipolar - most common (>99%)
b. bipolar - occur in sensory pathways
c. (pseudo)unipolar - sensory neurons
3. functional classification - direction of transmission
a. afferent/sensory
 mostly pseudounipolar, some are bipolar
 cell bodies are in sensory ganglia of spinal or cranial nerves
 distal end is either a sensory receptor or synapses with a sensory
receptor cell
b. efferent/motor
 multipolar
 cell bodies in CNS or motor ganglia
 synapse with effector cells
c. interneurons
 entirely within CNS
 99.98% of all neurons
 most are multipolar
CNS
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Anatomy Lecture Notes
Chapter 12
E. glial cells / neuroglia
1. astrocytes (CNS)
 most abundant type
 processes wrap neurons or capillaries
 control ion levels near neurons
 capture and recycle neurotransmitters
2. microglia (CNS)
 smallest and least abundant
 derived from monocytes during development
 phagocytize pathogens and dead cells
3. ependymal cells (CNS)
 simple epithelium
 lines brain ventricles and spinal cord central canal
 physical barrier between cerebrospinal fluid (CSF) and brain tissue fluid
 help make and circulate CSF
4. oligodendrocytes (CNS)
 cell processes produce myelin in CNS
5. satellite cells (PNS)
 surround sensory neuron cell bodies in sensory ganglia
6. Schwann cells (PNS)
 form myelin in PNS
F. myelin sheath
 made of lipoprotein
 surrounds some axons
 many layers of cell membrane wrapped around axon
 electrically insulates membrane of axon
1. PNS (Schwann cells)
 incomplete at birth
 formation continues for first year
a. neurilemma - Schwann cell outer membrane and cytoplasm external to myelin
sheath
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Anatomy Lecture Notes
Chapter 12
b. gaps between adjacent Schwann cells are nodes of Ranvier (1mm apart)
 no myelin or neurilemma at node
 nerve signals "jump" from node to node
 myelin speeds signal transmission
2. CNS (oligodendrocytes)
 one oligodendrocyte's processes wrap several axons
 nodes are farther apart than in PNS
G. chemical synapses
synapse = point of communication between two neurons
1. presynaptic neuron axon terminals contain neurotransmitter in synaptic vesicles

synaptic cleft or gap = space between presynaptic and postsynaptic neurons
2. postsynaptic neuron membrane contains:
 receptors for neurotransmitter
 ion channels controlled by the receptors
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Anatomy Lecture Notes
Chapter 12
H. basic multi-neuronal structures
1. CNS
a. gray (not grey) matter consists mostly of cell bodies
 cortex is a thin layer of neuron cell bodies (gray matter) covering the
outside of parts of the cerebrum and cerebellum
 a nucleus is a clump of neuron cell bodies (gray matter) located in the
brain
b. white matter consists mostly of myelinated axons
 a tract is a bundle of axons (myelinated) in the brain or spinal cord
CNS
2. PNS
a. nerve = bundle of axons surrounded by c.t.
 each axon is surrounded by an endoneurium made of loose c.t.
 axons are grouped into fascicles by the perineurium
 the epineurium surrounds all fascicles and is made of tough fibrous c.t.
b. ganglion/ganglia - clump of neuron cell bodies in PNS, surrounded by c.t.
 can be either sensory or motor
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Anatomy Lecture Notes
Chapter 12
I. reflex arc
reflex = rapid, automatic, unlearned, predictable motor response to a stimulus
reflexes are motor responses caused by a hard-wired neural circuit
reflex arc = pathway for a reflex
1. receptor - detects changes in environment
2. afferent neuron - carries signals from receptor to CNS
3. integration center - one or more synapses in CNS
incoming sensory signals elicit outgoing motor signals
4. efferent neuron - carries signals from CNS to effectors
5. effector - organ that produces reflex response (muscle or gland)
J. damaged neurons cannot be replaced
1. neuroblasts stop being produced during fetal development
2. new cells generated
a. in hippocampus from ependymal cells
b. in olfactory epithelium
3. damaged nerve processes can regenerate if the cell body is intact and there is a
path of Schwann cells for it to follow (this happens in PNS only)
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