Chapter 13:
The Spinal Cord, Spinal Nerves,
and Spinal Reflexes
1
General Organization
of the Nervous System
• Highly organized, very efficient
2
Figure 13–1
Nervous System
• Organization:
–
–
CNS = brain and spinal cord
PNS = all other neural tissue
• Structures in the PNS
1. Ganglia = collection of somas together in one place
2. Nerves = bundles of axons
3
Nervous System
• Structures in the CNS:
1. Center = collection of somas with a common function
2. Nucleus = a center with a visible boundary
3. Neural cortex = gray matter (somas) covering the brain
4. Tracts = bundles of axons with common origins,
destinations and function
5. Columns/funiculi = large tracts in the spinal cord
6. Pathways = centers and tracts that link brain w/ body
• Sensory pathways: receptor  CNS
4
• Motor pathways: CNS  effector
Gross Anatomy of the Spinal Cord
• 45cm (18”) from
brain to L2
• Inside vertebral
canal
– Stacked vertebral
foramen
• Surrounded by CT:
– Spinal Meninges
• Support spinal cord
• Protect spinal cord
5
Figure 13-2
Spinal Meninges
Three Layers:
1. Dura Mater
2. Arachnoid
3. Pia Mater
Functions:
– Protect/support spinal cord
– carry blood supply
All three layers are continuous
with cranial meninges of the
same name and the
connective tissue around the
spinal nerves.
6
Figure 13–3
Spinal Meninges
1. Dura Mater: Outermost
-
Dense collagen fibers
Attaches to periosteum of occipital bone and coccyx by
coccygeal ligament
2. Arachnoid: Middle
-
Two layers
1. Arachnoid membrane:
- Simple squamous epithelium, contacts dura mater
2. Arachnoid trabeculae:
- Collagen and elastin fibers that bind to pia mater
- Fibers pass through the subarachoid space which
contains cerebrospinal fluid (CSF)
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Spinal Meninges
3. Pia Mater: Innermost
-
Fine mesh of collagen and elastin fibers
bound to neural tissue
Attached to arachnoid trabeculae
Dentriculate ligaments
-
extend through arachnoid to dura mater to
prevent lateral movement of the cord
8
Anatomy of the Spinal Cord
9
Figure 13–5a
Anatomy of the Spinal Cord
• Posterior median sulcus
– Posterior surface bears a shallow longitudinal groove
• Anterior median fissure
– Deeper groove along the anterior surface
• Central canal: contains CSF for diffusion
• Gray matter: somas, neuroglia, unmyelinated axons
– Posterior gray horn:
• somatic and visceral sensory nuclei
– Anterior gray horn:
• somatic motor nuclei
– Lateral gray horn:
• throacic and lumbar only, visceral motor nuclei (ANS)
– Gray commissure:
• axons cross from on side of the cord to the opposite side
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Anatomy of the Spinal Cord
• White Matter: myelinated axons
–
–
–
–
Posterior white column/funiculus
Anterior white column/funiculus
Lateral white column/funiculus
All 6 column contains tracts:
• Ascending tracts: sensory to brain
• Descending tracts: motor from brain
• Transverse tracts: decussation
• Axons exit as spinal roots (dorsal +ventral = spinal
nerve)
– Dorsal root: sensory axons from receptor to CNS
– Dorsal root ganglion: somas of the sensory neurons
– Ventral root: motor axons from CNS to effectors
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KEY CONCEPT
• Spinal cord has a narrow central canal
– surrounded by gray matter that contains sensory and
motor nuclei
• Sensory nuclei are dorsal
• Motor nuclei are ventral
• Gray matter:
– is covered by a thick layer of white matter
• White matter:
– consists of ascending and descending axons
– organized in columns
– containing axon bundles with specific functions
12
Anatomy of the Spinal Cord
• Spinal roots exit vertebral canal
through intervertebral foramen
• Dorsal and ventral roots combine to
form spinal nerve
13
Damage to which root of a spinal nerve
would interfere with motor function?
1.
2.
3.
4.
posterior root
anterior root
dorsal root
ventral root
14
Where is the cerebrospinal fluid that
surrounds the spinal cord located?
1.
2.
3.
4.
epidural space
subarachnoid space
above the dura mater
between the pia mater and the
nervous tissue of the brain
15
Spinal Nerves
• 31 pairs
• Exit via intervertebral
or sacral foramen
• Name for location of
exit on spin beginning
between skull and C1
• Nerves:
–
–
–
–
–
C1-C8
T1-T12
L1-L5
S1-S5
Co1
16
Spinal Nerves
• Cord and column grow
together until age 4
• Column continues to grow but
cord does not
• Adult: cord ends at L1-L2
• “stretched” spinal roots after
L2 = cauda equina
• Lumbar puncture:
– “spinal tap” at L3-L4 draw
CSF from subarachnoid
space
17
Intervertebral foramen
Maintained by
intervertebral
Discs between
vertebrae
18
• Herniated disc:
– Nucleus pulposus ruptures through anulus
fibrosis
– Compresses nerves in intervertebral
foramen and/or spinal cord in vertebral
canal
• Slipped disc:
– Intervertebral disc distorted or displaced
– Causes pressure
19
Nerve Structure
•
Dorsal root + ventral root +
blood vessels + connective
tissue
1. Epineurium
-
Outermost layer
Dense collagen fibers
2. Perineurium
-
Partitions that extend inward
from epineurium and divide axons
into bundles called fascicles
3. Endoneurium
-
Innermost layer
Thin collagen fibers that surround
each axon
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Figure 13–6
Nerve Structure
• Axons repair if cut, it will follow original path
• Severed nerves do not usually repair
– Axons do not line up correctly
• Spinal nerves branch off cord near to what they
innervate
• Cervical and lumbar enlargements of cord house
cell bodies of motor neurons for muscles of
appendages
• Dermatome: Region of skin surface
– Innervate by one pair spinal nerves
21
Dermatomes
22
Figure 13–8
Nerve Plexus
• Most spinal nerves do not go directly to
target:
– Axons from multiple nerves intermingle in
a nerve plexus
23
Spinal Nerve Plexus
• Nerve plexus:
– Interwoven network of
nerves
1. Cervical plexus:
– Nerves C1-C5
– Innervate muscles of
neck and diaphragm
2. Brachial plexus:
– Nerve C5-T1
– Innervate pectoral girdle
and upper limbs
3. Lumbar plexus:
– Nerves T12-L4
– Innervate pelvic girdle
and lower limbs
4. Sacral plexus:
– Nerves L4-S4
– Innervate lower limbs
24
Trauma and Disorders
• Often result form damage or pressure
1. Paralysis:
– Loss of motor function
– Disorder of ventral root or anterior gray horn
2. Paresthesia:
– Sensory loss
– Disorder of dorsal root or posterior gray horn
– Complete cut results in loss of both motor and sensory below injury
3. Paraplegia:
– Sever between T1 and L4
– Loss of lower limb function
4. Quadriplegia:
– Sever in cervical
– Loss of all limb function (above C5 can kill)
25
Organization of Neural Pathways
• 10 million sensory neurons
– receptor to CNS
• 500 thousand motor neurons
– CNS to effector
• 20 billion interneurons
– coordinate sensory and motor, they are between
sensory and motor neurons located in the CNS
• Interneurons organized into neuronal pools
– functional groups with limited input sources (sensory)
and output locations (motor)
• Spread of info organized into neural circuits
26
5 Neural Circuits
27
Reflexes
• Rapid, automatic nerve responses triggered
by specific stimuli
• Used to maintain homeostasis
• Simple reflex:
– Sensory perception in, motor response out
28
Reflex Arcs = Single Reflex
• Negative Feedback
– Action opposes stimulus as form of defense
– Fast response, but not always coordinated
29
5 Steps in a Neural Reflex
30
Figure 13–14
5 Steps in a Neural Reflex
• Step 1: Stimulus Activates Receptor
– Receptor = specialized cell or dendrite of sensory
neuron
– Receptor responds to a particular type of stimuli
• physical or chemical changes
• Step 2: Sensory Neuron is Activated
– Stimulation causes action potential on axon of
sensory neuron
– Nerve impulse travels into spinal cord via dorsal
root
31
5 Steps in a Neural Reflex
• Step 3: Information Processing
– Simple case: sensory neuron synapses on motor neuron
– More complex: sensory neuron synapses on an interneuron
• Step 4: Motor Neuron is Activated
– Motor neuron is stimulated to threshold
– Action potential travels down motor neuron axon to the
effector tissue
• Step 5: Peripheral effector responds
– Neurotransmitters released by motor neuron trigger events
in effector
– E.g. muscle contraction
32
Reflex Classification: 4 Ways to Classify
33
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
34
Response
• Nature of resulting motor response:
– somatic reflexes:
• involuntary control of nervous system
– superficial somatic reflexes
» Stimuli originate at skin or mucous membranes
– stretch reflexes (deep tendon reflexes)
» Stimuli from overstretched tendon
» e.g., patellar reflex
– visceral reflexes (autonomic reflexes):
• control systems other than muscular system
35
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
• Response delayed by each synapse but capable of
more complex output
36
Examples of Common Spinal Reflexes
1. Patellar Reflex
2. Withdrawal Reflex
37
1. Patellar Reflex
• Monosynaptic stretch reflex
• Carried on type A fibers: largest myelinated axons
– Carries action potential at the fastest rate
• Sudden stretch of patellar ligament activates muscle
spindle  signal quadriceps group to contract
38
Figure 13–15
Stretch Reflex
• Muscle spindles = sensory receptors
involved in the stretch reflex
– Consist of a bundle of small, specialized
skeletal muscle fibers called intrafusal muscle
fibers
• Innervated by sensory and motor neurons
– Surrounded by a larger extrafusal muscle fiber
responsible for
• The resting muscle tone
• The contraction of the entire muscle, when greater
levels of stimulation are present
39
A Muscle Spindle
Axons of motor
neurons innervating
intrafusal fibers
40
Figure 13–16
Muscle Spindle:
Specialized Muscle Fiber
• Constantly signal CNS
• Relaxed = signal less
• Stretched = signal more 
– threshold, trigger reflex arc
• Prevent overstretching of muscle and
tendons
• Aid in maintaining upright position
41
2. Withdrawal Reflexes
• Complex polysynaptic spinal reflex
• Consists of three parts:
1. Flexor reflex: flex to withdraw
2. Reciprocal Inhibition: inhibit extensors
3. Crossed extensor reflex: maintain balance
42
Withdrawal Reflexes
• Pain  flexor muscles pull limb away
 extensors same limb inhibited
to prevent opposition to flexion
 limbs on opposite side extend
to provide balance for sudden
flexion
43
A Flexor Reflex
44
Figure 13–17
45
Reflexes
• Reflexes automatic but can be impacted
by higher brain centers:
– Take cues from reflex for coordinated
voluntary movements
– Facilitate or inhibit reflexes
• Reflexes serve as diagnostic tool to assess
health and function of spinal cord and
brain
46
Integration and Control
of Spinal Reflexes
• Though reflex behaviors are automatic:
– processing centers in brain can facilitate
or inhibit reflex motor patterns based in
spinal cord
47
KEY CONCEPT
• Reflexes:
– rapid, automatic responses to stimuli
– “buy time” to plan and execute complex,
conscious responses
• Somatic motor reflexes:
– fastest reflexes
– involve myelinated axons
– involve only 1 spinal cord segment or brain
nucleus
– are monosynaptic
48
SUMMARY
• General organization of nervous system:
– CNS, PNS
• Afferent (sensory) and efferent (motor)
fibers
• Structures and functions of spinal meninges
• Gray matter and horns of spinal cord
• 4 major nerve plexuses:
– cervical, brachial, lumbar, sacral
• Neuronal pools and neural circuit patterns:
– divergence, convergence, serial, parallel,
reverberation
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SUMMARY
• Reflexes and reflex arcs
• Classifications of reflexes:
–
–
–
–
innate vs. acquired
somatic vs. visceral
cranial vs. spinal
monosynaptic, polysynaptic, or intersegmental
• Characteristics of monosynaptic reflexes:
– stretch reflex, postural reflex, muscle spindles
• Characteristics of polysynaptic reflexes:
– tendon, withdrawal, flexor, and crossed extensor
reflexes
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