Transcript Lab 5: Nervous System I

Nervous System I Lab 5
( rev 3-11)
• The CNS consists of the brain and spinal cord
– is the integrating and command center of the
nervous system
– it interprets incoming sensory information and
produces motor responses
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• The peripheral nervous system is the part of the
nervous system outside the brain and spinal cord
(the CNS)
– it contains the communication lines that link all
parts of the body to the CNS
– consists mainly of nerves that extend from the
brain and spinal cord
• the Peripheral NS has 2 functional subdivisions
– the sensory or afferent division carries
impulses TO the CNS and keeps the CNS
informed of events going on inside and outside
of the body
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Nervous System I Lab 5
– The motor or efferent division which carries
impulses FROM the CNS
• This division enables us to respond to stimuli
• The Motor Division can be further subdivided
into:
– the Somatic or Voluntary nervous system
• carries impulses from the CNS to skeletal muscles
thus allowing us to consciously control our skeletal
muscles
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– The Autonomic nervous system (ANS), is
also referred to as the involuntary nervous
system
• this division regulates the activity of smooth
muscles, cardiac muscles, and glands
– Is further sub-divided into the
• the SYMPATHETIC NERVOUS SYSTEM
which mobilizes body systems during emergency
situations
• the PARASYMPATHETIC NERVOUS
SYSTEM which primarily conserves our energy
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Composition of the Nervous System
• The Nervous System is composed of 2 types
of tissue
– Neurons are the conductive cells which
transmit messages
– Neuroglia are the supporting cells of the
nervous system
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Neurons
• Are the specialized cells which conduct nerve
impulses
• All neurons have
• dendrites (receive stimuli)
• a cell body (contains the nucleus)
• an axon (carries impulses)
– Some neurons have a myelin sheath around the
axon
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• the myelin sheath
– is a fatty wrapping around the axon which
provides insulation to the axon
– it speeds impulse transmission
– is formed from Schwann Cells in the
peripheral nervous system and
Oligodendrocytes in the CNS
• Between neighboring Schwann cells are short,
uninsulated gaps called Nodes of Ranvier
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Neurons can be classified based on structure
or function: Structural Classification
Unipolar Neurons
• have a single process which emerges from the cell
body
• this process divides into a proximal and distal
branch
– One branch behaves as an afferent branch and
the other behaves as an efferent branch
• All unipolar neurons are sensory
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Bipolar Neurons
• have 2 processes emerging from a round cell body
• processes extend from opposite sides of the cell body
• found only in some of the special sense organs where
they act as receptor cells (i.e. located in the retina
[eye], in the ear and olfactory mucosa)
Multipolar Neurons
• have 3 or more processes
• are the most common neuron type in humans and
major neuron type in the CNS
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Classification Systems: Structural Classification
Interneurons or Association Neurons
• are a multipolar neuron
• located in the CNS
• conduct impulses within the CNS
• are the connecting link between sensory and motor
neurons
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Functional Classification
• Sensory (Afferent) Neurons
– carry (sensory) information from receptors to
the CNS
– Most are unipolar; a small number are bipolar
• Motor (Efferent) Neurons
– carry messages away from the CNS to the
muscles and the glands
– are multipolar
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Nervous System I Lab 5
Neuroglial or Glial Cells
• are non-nervous tissue cells which provide a
supportive scaffolding, protect and nourish
neurons
– Astrocytes anchor the neurons to nearby capillaries
– Ependymal cells are ciliated cells that move
cerebrospinal fluid around the brain and spinal cord
– Microglia are phagocytic cells for the nervous system.
They destroy bacteria and damaged tissues near the
neurons.
– Oligodendrocytes form the myelin sheath in the CNS
– Schwann cells form the myelin sheath in the PNS
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Impulse Transmission
This is primarily a lecture topic; we will quickly go over this.
• An impulse arrives at the dendrite
• When the impulse is strong enough, it depolarizes the membrane
and the impulse is transmitted along the axon
• When the impulse reaches the axon terminals, the information
needs to be converted to another form of energy in order for the
information to be transmitted to its target (i.e. a muscle, a gland,
or another neuron)
• A chemical, called a neurotransmitter, is released which
allows the impulse to jump the synapse, or space, between the 2
cells
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The Brain and the Spinal Cord
The CNS controls and processes all information
received by the body.
• Protection of the CNS
– Bone protects it from physical injury
– the brain is covered by the skull
– the spinal cord is surrounded by the
vertebrae
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– Meninges--3 protective membranes of
connective tissue enclose the brain and spinal
cord
• Dura mater is the outermost and strongest
layer
• Arachnoid mater is the middle layer; it has
spidery extensions which secure it to the
innermost layer.
• Pia mater is the innermost layer and clings
tightly to the CNS.
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– Cerebrospinal fluid (CSF): is found within
and around the brain and spinal cord. It
• functions as a liquid shock absorber
• serves as a blood-brain barrier by isolating
the CNS from infections
• aids in providing nutrients for cells and
removing waste products from cells
• by floating the brain, the CSF effectively
reduces brain weight by 97% and prevents
the brain from crushing under its own weight
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CSF
Nervous System I Lab 5
• CSF is formed from blood plasma
– it is made in the choroid plexuses in the
ventricles of the brain
– much of the fluid is found in the subarachnoid
space
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Gross Anatomy of a Nerve
• Nerves consist of parallel bundles of axons wrapped by
various layers of connective tissue.
• From the inside to the outside,
– each axon is covered by endoneurium, a delicate layer
of connective tissue. It also covers the myelin sheath
and neurilemma.
– groups of axons are bound into bundles called fascicles.
– the coarse layer of connective tissue which wraps the
fascicles is the perineurium.
– the epineurium surrounds the fascicles. It is a tough,
fibrous covering.
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Spinal Cord
• The spinal cord is located in the vertebral column.
• It is approximately the width of the thumb except
at the cervical and lumbar areas.
– Cervical enlargement is where the nerves for
the shoulder and arms enter and exit the cord.
– Lumbar enlargement is where the nerves for
the legs enter and exit the cord.
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Spinal Cord
Nervous System I Lab 5
• The spinal cord does not extend to the end of the
vertebral column. It ends at the level of L2 (the area
between the first and second lumbar vertebrae).
• The cord ends in a tapering cone shape which is
called the conus medularis.
• The lumbar and sacral nerves angle sharply
downward and travel through the vertebral canal
before they exit through intervertebral foramina.
• The collection of these nerves is called the cauda
equina because they resemble a horse’s tail.
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The outer portion of the spinal cord consists primarily of
bundles of axons called nerve tracts.
• These axons are usually myelinated and so they have a
white appearance and are called “white matter.”
– White matter is made up of ascending (sensory) and
descending (motor) nerve tracts of myelinated axons
• Near the center of the cord is an area called the gray
matter. It is occupied primarily by cell bodies,
dendrites and axons of CNS neurons (which are not
myelinated).
– Is in the shape of an “H”; this is where all synapses
between sensory, association, and motor neurons takes
place.
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Spinal Cord Anatomy-Other Things to Know
• The Central Canal is where the CSF circulates.
• The Ventral (or anterior) Root is where all
MOTOR axons exit from the spinal cord.
• The Dorsal (or posterior) Root is where all
SENSORY axons enter the spinal cord.
• The Dorsal Root Ganglion is where all sensory
nerve cell bodies are located.
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Spinal Nerves
Nervous System I Lab 5
• Dorsal and ventral roots join to form Spinal
nerves. These are called mixed nerves because
they have axons of both sensory and motor nerves.
• Spinal nerves unite with other nerves to form a
plexus.
• There are 8 pairs of cervical nerves (C1-C8).
– Most go to the cervical plexus
– except C8 which goes to the brachial plexus.
– Major nerve emerging from the cervical plexus
is the phrenic nerve (innervates the diaphragm).
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Spinal Nerves
Nervous System I Lab 5
• The brachial plexus is formed from C8, T1 and T2.
– Major nerves to emerge are the radial and ulnar
nerves
– these innervate many of the muscles of the
forearm and hand
• The 12 pairs of thoracic nerves (T3 through T12)
do not go to a plexus.
– They innervate the intercostal muscles used for
breathing.
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Spinal Nerves
Nervous System I Lab 5
• There are 5 pairs of lumbar nerves, L1 through L5.
Most go to the lumbar plexus.
– Major nerve is the femoral nerve which
innervates the anterior thigh muscles and the
skin covering them.
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Spinal Nerves
Nervous System I Lab 5
• There are 5 pairs of sacral nerves, S1 through S5.
These go to the sacral plexus.
– Major nerve is the sciatic nerve which
innervates the posterior thigh muscles and its
skin covering.
– The sciatic nerve is the largest nerve in the
body.
• The coccygeal nerve does not go to a plexus.
– It innervates genitoanal structures.
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The Reflex Arc
Many of the body’s control systems belong to a
general category known as reflexes.
• A reflex is a rapid, predictable motor response to a
stimulus.
– It is automatic, involuntary, and protective.
– Both the spinal cord and the brain are reflex
centers.
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Reflex Arcs
Nervous System I Lab 5
Reflex arcs include the following components.
1. The receptor is the where the stimulus begins.
2. The sensory neuron transmits the afferent
impulses to the CNS.
3. An association neuron receives the information
and causes an instantaneous impulse to be
transmitted to a motor neuron.
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Reflex Arc
Nervous System I Lab 5
4. The motor neuron sends an impulse to the
effector organ (organ which will cause a
response).
5. The effector is the muscle or gland that responds
in a characteristic way.
This allows us to respond to a stimulus instantly and
without thinking.
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More Reflex Arcs
Nervous System I Lab 5
• The Babinski Reflex is an extremely important reflex.
• It is normal in newborns to 6-12 months; it is
abnormal in older children and adults.
• Presence of this reflex indicates damage to the
motor cortex of the CNS.
• To test for this reflex, stroke the lateral aspect of
the foot starting from the heel, up to and across the
ball of the foot and end at the big toe.
• An abnormal response is dorsiflexion of the big
toe with extension and fanning of the other toes.
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REMINDER, page 1 of 2:
1. There are slides on the desks relating to nerve
cells. We will not be using them today since you
have already seen examples of this in the earlier
labs when we were looking at cells. Focus on the
models around the room.
2. Learn the 3 types of neurons and structure of the
neuron on models and diagrams (no slides).
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REMINDER, page 2 of 2:
3. Learn the structure of the spinal cord, plexuses
and spinal nerves on models and diagrams.
4. Learn the parts of the reflex arc.
5. Do the patellar reflex (skip ankle and Babinski
reflexes). Do NOT hit the patella--feel for the
space under it and tap in this space.
Get permission from your partner prior to using
the reflex hammer.
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