Download 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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