The Nervous System
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Transcript The Nervous System
Pathways and Tracts
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Pathway
route followed by a nerve impulse as it
travels through the nervous system
a reflex arc is simplest of these
pathways
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Pathways
Include:
- nerve fibers in nerves; bundles called
fascicles
- nerve fibers inside the brain and
spinal cord; bundles called tracts
- bundles of nerve fibers linking the
two halves of the brain called
commissures
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Pathways
(cont.)
sensory - ascending
motor - descending
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Sensory Pathways
(ascending)
start at sensory receptors and end in
cerebral cortex of brain
Consist of a series of three neurons:
- first-order neurons
- second-order neurons
- third-order neurons
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First-Order Neurons
sensory neurons that convey impulses
from sensory receptors to CNS
extend up to medulla on same side of
body
axon terminals form synapses with
second-order neurons
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Second-Order Neurons
(associated neurons)
carry sensory impulses to the thalamus
(integrating center)
axon of the second-order neuron
crosses to the opposite side of the
medulla to thalamus
in thalamus, axon terminals of secondorder neurons synapse with third-order
neurons
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Third-Order Neurons
(associated neurons)
carry impulses from thalamus to
cerebral cortex (where conscious
sensation is produced)
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Pathway
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Motor Pathways
(descending)
start in brain and terminate at muscles
or glands
consist of upper and lower motor
neurons
Two basic pathways:
- direct or pyramidal
- indirect or extrapyramidal
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Direct Motor Pathways
(pyramidal)
carry impulses from cerebral cortex
directly to lower motor neurons
simplest pathway consists of two
neurons; upper and lower motor neurons
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Upper Motor neurons
(pyramidal)
in cortex
fibers pass through bulges called
pyramids on medulla oblongata - hence
the name
conduct impulses from motor cortex to
motor nuclei of the cerebral nerves or to
the ventral gray columns of spinal cord
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Lower Motor Neurons
(pyramidal)
peripheral neurons whose cell bodies lie
in the ventral gray column of spinal cord
and terminate in skeletal muscles
responses are semivoluntary and
automatic
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Direct Pathway Impulses
Channeled into three tracts:
- lateral corticospinal tracts
- anterior corticospinal tracts
- corticobulbar tracts
direct pathways result in precise voluntary
movements
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Indirect Motor Pathways
(extrapyramidal)
impulses follow complex polysynaptic
circuits
carry lower motor neurons through other
parts of brain
Pathways involve:
- motor cortex, basal ganglia, thalamus,
cerebellum, reticular formation, nuclei in
the brain stem
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Tracts
bundles of nerve fibers (axons) in CNS
Types of tracts:
- sensory or ascending contain nerve
fibers that carry impulses up the spinal
cord to the brain
- motor or descending contain nerve
fibers that carry impulses down SC
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Tracts
(cont.)
Tracts are named according to:
- location in spinal cord
- origin
- termination
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Ganglia and Nuclei
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Ganglia
(ganglion - singular)
group of neuron cell bodies
located outside central nervous system
in the peripheral nervous system
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Four Basic Types of Ganglia
posterior root ganglia
sympathetic trunk ganglia
prevertebral ganglia
terminal ganglia
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Posterior Root Ganglia
(dorsal root ganglia)
contain cell bodies of sensory nerves
located near spinal cord in posterior
(dorsal) roots of spinal nerves
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Sympathetic Trunk Ganglia
(sympathetic division)
form a chain of ganglia on each side of
the vertebral column
extend from neck to coccyx
contain cell bodies of postganglionic
sympathetic neurons
also known as paravertebral ganglia
and sympathetic chain ganglia
can be dissected out separately like a
string of pearls
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Prevertebral Ganglia
(sympathetic division)
located anterior to vertebral column
close to the abdominal arteries
contain cell bodies of postganglionic
sympathetic neurons
also called collateral ganglia
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Terminal Ganglia
(parasympathetic division)
located near or inside internal organs
(visceral effectors)
consist of clusters of cell bodies of
postganglionic parasympathetic
neurons
also known as intramural ganglia
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Shingles or Herpes Zoster
caused by same virus that causes
chicken pox (Herpes varicella-zoster)
chicken pox virus may survive in
dormant state in dorsal root ganglia
stress or advancing age may cause the
virus to become active
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Shingles or Herpes Zoster (cont.)
virus is present in sensory trunk but
damage caused by the virus is seen in
the skin over the affected nerve
Symptoms:
- painful raised red lesions
- follow course of nerve on skin external
to it
- no specific treatment
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Shingles or Herpes Zoster (cont.)
if 7th. cranial nerve is affected, Bell’s
palsy (facial paralysis) results
if optic nerve is affected blindness will
occur
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Spinal Nerve
Attached to spinal cord by:
- dorsal (posterior) root is composed of
sensory fibers
- ventral (anterior) root is composed of
motor fibers
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Dorsal Root
(posterior)
exhibits small enlargement called dorsal
root ganglion which contains cell bodies
of sensory neurons
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Dorsal and Ventral Roots
pass laterally from spinal cord
merge to form single mixed spinal nerve
pass through intervetebral foramen (IVF)
after passing IVF spinal nerve divides into
two main branches
- large ventral (anterior) ramus
- smaller dorsal (posterior) ramus
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Spinal Cord Showing Roots
and Ganglia
spinal cord
posterior root ganglion
(dorsal)
sympathetic trunk ganglion
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Spinal Cord Showing Roots
and Ganglia
spinal cord
posterior
root
posterior root
ganglion
spinal nerve
anterior
root
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sympathetic trunk
ganglion
vertebrae
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Nuclei
located in brain or spinal cord
cluster of neuron cell bodies
CNS nuclei are isolated regions of gray
matter
located within white matter of brain and
spinal cord
neurons in a given nucleus perform
specific functions
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Examples of Nuclei
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Basal Ganglia
(cerebral nuclei or basal nuclei)
several groups of nuclei
located within white matter of cerebral
hemispheres
they integrate semi-voluntary automatic
movements like walking, swimming, and
laughing
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Thalamus
consists of a pair of oval masses on each
side of 3rd ventricle in diencephalon
mostly gray matter
made up of many nuclei
Functions include:
- language, memory, emotion, integration
and relay of sensory impulses to the
cerebral cortex
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Ventricles
four cavities within brain
- two lateral ventricles
- third ventricle
- fourth ventricle
each ventricle contains capillary network
called choroid plexus which forms
cerebrospinal fluid from blood plasma
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Hypothalamus
in region of diencephalon
located below two halves of thalamus
consists of a variety of nuclei and nuclear
areas
- most important control area for internal
environment
Functions:
- thirst, hunger, hormone production, and
fear and rage reactions
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Brain Stem
nuclei for most of the cranial nerves are
located in brain stem
other nuclei located in brain stem control
breathing, the force and rate of heart
contractions, and blood vessel diameter
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Cerebellum
cerebellar nuclei are regions of gray
matter located deep within cerebellum
Concerned with:
- balance, proprioception, (self-awareness),
and the planning and coordination of
complex muscular activities
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Nuclei
3rd. Ventricle
head of caudate
nucleus
2 divisions of
lenticular nucleus
tail of
caudate
nucleus
posterior lateral
nucleus of
thalamus
choroid
plexus
medial pulvinar
nucleus of
thalamus
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Reflex
(stimulus)
fast, predictable, automatic, unconscious
response to change in the environment that
helps to maintain homeostasis
occurs in gray matter
Change can be:
- external - outside the body
- internal - inside the body
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Homeostasis
internal environment of body is
maintained at a relatively constant level
blood pressure, plasma glucose, pH,
and body temperature are examples of
body conditions that must be
consistently maintained
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Homeostatic Mechanism
sequence of events that maintains a
consistent internal environment
homeostatic mechanisms are called
negative feedback control systems
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Negative Feedback
principle dictating most control systems
response in which a stimulus counteracts,
reverses or reduces original stimulus (back
to original value)
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Types of Reflexes
somatic reflexes:
- involve contraction of of skeletal
muscles
autonomic reflexes:
- involve the contraction of smooth
muscles, cardiac muscles, and glands
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Reflex Arc
basic structural and functional unit of
nervous system
begins with a receptor at end of a
sensory nerve fiber
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Sequence of Events in
Response to Stimulus
receptor
sensory pathway
integrating center
motor pathway
effector
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Receptor
specialized sensory nerve ending
detects environmental change
(stimulus)
responds by initiating a nerve impulse in
a sensory neuron
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Sensory Pathway
(afferent)
carries nerve impulse from receptor to
central nervous system
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Integrating Center
receives signals
determines responses
sends out instructions via motor neurons to
effector cells
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Motor Pathway
(efferent)
carries instructions from CNS to effector
(muscles or glands)
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Effector
muscle or gland that contracts, or
secretes
response is called a reflex
usually counteracts original stimulus
i.e., negative feedback effect
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