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Chapter 13
The Spinal Cord and Spinal
Nerves
Functions of the nervous system
• Sensory (input):
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–
–
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–
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–
Light
Sound
Touch
Temperature
Taste
Smell
Internal Chemical
Pressure
Stretch
Functions of the nervous system (cont’d)
• Integration:
– Integration means making sense of
sensory input. Analyzing stimuli based on
experience, learning, emotion & instinct
and reacting in a useful way (you hope).
• Motor (output):
– The response to the sensory input and
subsequent integration. Sending signals to
the muscles and other organs of the body
instructing them how to respond to the
stimuli.
Nervous System Organization
Overview of Ch 13 & Ch 14
The Spinal Cord & Nerves
• The spinal cord is part of the Central
Nervous System.
• The spinal nerves are part of the
Peripheral Nervous System.
• The lowest level of integration occurs in
the spinal cord and peripheral ganglia.
Spinal cord:
gross
anatomy
Spinal cord
anatomy:
the meninges
Spinal cord
and
associated
structures
Functional arrangement of the spinal
cord tissues
Cross section of the spinal
cord
White Matter in the Spinal Cord
• Fibers run in three directions –
ascending, descending, and
transversely
• Divided into three funiculi (columns) –
posterior, lateral, and anterior
• Each funiculus contains several fiber
tracks
– Fiber tract names reveal their origin and
destination
– Fiber tracts are composed of axons with
similar functions
Spinal nerves
• Thirty-one pairs of mixed nerves arise
from the spinal cord and supply all parts
of the body except the head
• They are named according to their point
of issue
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8 cervical (C1-C8)
12 thoracic (T1-T12)
5 Lumbar (L1-L5)
5 Sacral (S1-S5)
1 Coccygeal (C0)
Spinal
nerve
structure
Spinal Nerve structure
• Each axon is also called a “nerve fiber”
– These are covered by an endoneurium
– The endoneurium is made of areolar c.t.
• Fibers are bundled into “fascicles”
– These are covered by perineurium
– This is an extension of the outer layer of
collagen fibers
• Nerves are bundles of fascicles
– Each nerve is covered by dense irregular
tissue of collagen fibers
Components of the Peripheral
Nervous System
• Motor pathways
– Leave the spinal cord via the ventral nerve
roots.
– Have multipolar cell bodies found in the
gray matter.
– Carry motor impulses to skeletal muscle
(somatic pathways) and to glands, smooth
muscle, the heart, organs, etc (autonomic
pathways).
– Are also called efferent pathways.
Motor pathways
Components of the PNS
• Sensory Pathways
– Enter the cord via the dorsal roots.
– Have unipolar cell bodies found in the dorsal
root ganglia.
– Carry sensory inputs into the CNS via the
central processes of their axons. They begin
at the general sensory receptors of the skin
(somatic sensory) and internal organs
(visceral sensory).
– Are also known as afferent pathways.
– Special sense will be covered in Chapter 17
Sensory pathways
What’s a
damn
dermatome?
Nerve
plexuses
Nerve plexuses
• Fibers travel to the periphery via several
different routes
• Each muscle receives a nerve supply
from more than one spinal nerve
• Damage to one spinal segment cannot
completely paralyze a muscle
Cervical plexus
Summary: Cervical Plexus
Table 13-1
Brachial
plexus
The brachial plexus
Fig. 13.13a
Some common injuries to the brachial
plexus
Study question: Which nerves are affected here?
Summary: Brachial Plexus
Table 13–2 (1 of 2)
Summary: Brachial Plexus
Table 13–2 (2 of 2)
Lumbar and sacral plexuses
Summary: Lumbar
and Sacral Plexuses
Table 13-3 (1 of 2)
Summary: Lumbar
and Sacral Plexuses
Table 13-3 (2 of 2)
Lumbar &
sacral plexus
nerves
Neural circuits
Neural Circuits
• Divergent – spread information from one
source to several destinations.
– Examples: visual input being processed at
a conscious level (the horizon is tilting) and
a subconscious level (I adjust my body so
that I don’t fall over).
• Convergent – multiple sources of input
into one neuron.
– Examples: conscious – I contract my rectus
femoris to step over a pile of dog poop.
Unconscious – my rectus femoris
automatically contracts as the bus moves
Neural Circuits
• Serial – a series of neurons in a sequence.
– Example: Pain pathways
• Parallel – Divergence followed by serial.
– Example – reflexes that result in a complex
series of responses simultaneously
• Reverberation – positive feedback loops
– Examples – many of the complex processes of
the brain.
Reflexes
• Rapid, automatic responses to stimuli.
• Can be visceral (e.g. swallowing) or
somatic (“knee-jerk”).
• Have little variability
Components of a reflex arc
1. Stimulus activates a receptor.
2. Impulse travels along a sensory
pathway.
3. Integration occurs in an integration
center (most often in the CNS)
4. Impulse then travels by a motor
pathway.
5. An effector responds.
4 Classifications of Reflexes
1.
2.
3.
4.
By early development
By type of motor response
By complexity of neural circuit
By site of information processing
Development
• How reflex was developed:
– innate reflexes:
• basic neural reflexes
• formed before birth
– acquired reflexes:
• rapid, automatic
• learned motor patterns
Response
• Nature of resulting motor response:
– somatic reflexes:
• involuntary control of nervous system
– superficial reflexes of skin, mucous membranes
– stretch reflexes (deep tendon reflexes) e.g.,
patellar reflex
– visceral reflexes (autonomic reflexes):
• control systems other than muscular system e.g.,
glands smooth muscle and cardiac muscle
Complexity
• Complexity of neural circuit:
– monosynaptic reflex:
• sensory neuron synapses directly onto
motor neuron
– polysynaptic reflex:
• at least 1 interneuron between sensory
neuron and motor neuron
Processing
• Site of information processing:
– spinal reflexes:
• occurs in spinal cord
– cranial reflexes:
• occurs in brain
Monosynaptic: Stretch reflex
Monosynaptic Reflexes
• Have least delay between sensory input
and motor output:
– e.g., stretch reflex (such as patellar reflex)
• Completed in 20–40 msec
Muscle Spindles
• The receptors in stretch reflexes
• Bundles of small, specialized intrafusal
muscle fibers:
– innervated by sensory and motor neurons
• Surrounded by extrafusal muscle fibers:
– which maintain tone and contract muscle
Muscle spindles
Postural Reflexes
• Postural reflexes:
– stretch reflexes
– maintain normal upright posture
• Stretched muscle responds by
contracting:
– automatically maintain balance
Polysynaptic Reflexes
• More complicated than monosynaptic
reflexes
• Interneurons control more than 1
muscle group
• Produce either EPSPs or IPSPs
Polysynaptic: Flexor
The Tendon Reflex
• Prevents skeletal muscles from:
– developing too much tension
– tearing or breaking tendons
• Sensory receptors unlike muscle
spindles or proprioceptors
Withdrawal Reflexes
• Move body part away from stimulus
(pain or pressure):
– e.g., flexor reflex:
• pulls hand away from hot stove
• Strength and extent of response:
– depends on intensity and location of
stimulus
Reflex Arcs
• Ipsilateral reflex arcs:
– occur on same side of body as
stimulus
– stretch, tendon, and withdrawal
reflexes
• Crossed extensor reflexes:
– involves a contralateral reflex arc
– occurs on side opposite stimulus
The
Crossed
Extensor
Reflex
Figure 13–18
5 General Characteristics
of Polysynaptic Reflexes
1.
2.
3.
4.
Involve pools of neurons
Are intersegmental in distribution
Involve reciprocal inhibition
Have reverberating circuits:
–
which prolong reflexive motor response
5. Several reflexes cooperate:
–
to produce coordinated, controlled
response
• Normal in infants
• May indicate CNS damage in adults
The Babinski Reflexes
Figure 13–19
Spinal Cord Trauma: Paralysis
• Paralysis – loss of motor function
• Flaccid paralysis – severe damage to
the ventral root or anterior horn cells
– Lower motor neurons are damaged and
impulses do not reach muscles
– There is no voluntary or involuntary control
of muscles
Spinal Cord Trauma: Paralysis
• Spastic paralysis – only upper motor
neurons of the primary motor cortex are
damaged
– Spinal neurons remain intact and muscles
are stimulated irregularly
– There is no voluntary control of muscles
Spinal Cord Trauma:
Transection
• Cross sectioning of the spinal cord at
any level results in total motor and
sensory loss in regions inferior to the
cut
• Paraplegia – transection between T1
and L1
• Quadriplegia – transection in the
cervical region
Spinal cord transection
Poliomyelitis
• Destruction of the anterior horn motor
neurons by the poliovirus
• Early symptoms – fever, headache,
muscle pain and weakness, and loss of
somatic reflexes
• Vaccines are available and can prevent
infection
Some effects of
Polio
Amyotrophic Lateral Sclerosis
(ALS)
• Lou Gehrig’s disease – neuromuscular
condition involving destruction of
anterior horn motor neurons and fibers
of the pyramidal tract
• Symptoms – loss of the ability to speak,
swallow, and breathe
• Death often occurs within five years
• Linked to malfunctioning genes for
glutamate transporter and/or superoxide
dismutase
Some Famous Victims of ALS
Lou Gehrig
Steven Hawking,
renowned physicist
Axonal degeneration of motor neurons evident in lateral
corticospinal (pyramidal) pathways, especially in the loss of
myelinated fibers of the corticospinal tracts
SUMMARY (1 of 7)
• General organization of nervous
system:
– CNS, PNS
• Gross anatomy of spinal cord:
– enlargements, dorsal and ventral roots,
filum terminale, conus medullaris
SUMMARY (2 of 7)
• Afferent (sensory) and efferent (motor)
fibers
• Structures and functions of spinal
meninges
• Gray matter and horns of spinal cord
SUMMARY (3 of 7)
• White matter and columns (tracts) of
spinal cord
• 3 layers in spinal nerves
• Distribution (rami) of spinal nerves:
– white, gray, dorsal, ventral
SUMMARY (4 of 7)
• 4 major nerve plexuses:
– cervical, brachial, lumbar, sacral
• Neuronal pools and neural circuit
patterns:
– divergence, convergence, serial, parallel,
reverberation
SUMMARY (5 of 7)
• Reflexes and reflex arcs
• Classifications of reflexes:
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innate vs. acquired
somatic vs. visceral
cranial vs. spinal
monosynaptic, polysynaptic, or
intersegmental
SUMMARY (6 of 7)
• Characteristics of monosynaptic
reflexes:
– stretch reflex, postural reflex, muscle
spindles
• Characteristics of polysynaptic reflexes:
– tendon, withdrawal, flexor, and crossed
extensor reflexes
End
Chapter 13