SPINAL MECHANISMS OF MOTOR CONTROL
Gross
Anatomy
of the
Spinal
Cord
Figure 13-2
Spinal cord is much like the CNS!
• Like spinal cord but
with another layer of
gray outside the white
– Called cortex
– Cerebrum and
cerebellum have
• Inner gray: “brain
nuclei” (not cell nuclei)
– Clusters of cell bodies
Remember, in PNS
clusters of cell bodies
were called “ganglia”
More words: brains stem is caudal (toward tail)
to the more rostral (noseward) cerebrum
Spinal
Meninges
Dura mater:
outer
layer
Arachnoid
mater:
middle
layer
Pia mater:
inner
layer
Figure 13–3
Sagittal section through spinal cord
1. Intervertebral
disc
2. Vertebral body
3. Dura mater
4. Extradural or
epidural space
5. Spinal cord
6. Subdural space
Major fiber tracts in white matter of spinal cord
sensory
Damage: to motor areas – paralysis
to sensory areas - paresthesias
motor
Some
Descending
Pathways
Synapse with ventral (anterior)
horn interneurons
Pyramidal tracts:
Lateral corticospinal – cross in
pyramids of medulla; voluntary motor
to limb muscles
Ventral (anterior) corticospinal –
cross at spinal cord; voluntary to
axial muscles
“Extrapyramidal” tracts: one example
Sectional Anatomy of
Spinal Cord
• Four zones are evident within the gray
matter – somatic sensory (SS), visceral
sensory (VS), visceral motor (VM), and
somatic motor (SM)
Figure 13–5a
Sectional Anatomy
of the Spinal Cord
Figure 13–5b
Dorsal Root Ganglia
Dendrites
Nature of Reflexes
• reflexes - quick, involuntary, stereotyped reactions
of glands or muscle to stimulation
– automatic responses to sensory input that occur without
our intent or often even our awareness
• four important properties of a reflex
– reflexes require stimulation
• not spontaneous actions, but responses to sensory input
– reflexes are quick
• involve few if any interneurons and minimum synaptic delay
– reflexes are involuntary
• occur without intent and difficult to suppress
• automatic response
– reflexes are stereotyped
• occur essentially the same
way every time
13-13
Nature of Reflexes
• pathway of reflex arc
– somatic receptors
• in skin, muscles, or tendons
– afferent nerve fibers
• carry information from receptors to posterior horn of spinal
cord or the brainstem
– integrating center
• a point of synaptic contact between neurons in the gray matter
of the spinal cord or brainstem
• determines whether the efferent neurons issue a signal to the
muscles
– efferent nerve fibers
• carry motor impulses to skeletal muscle
– skeletal muscles
13-14
• the somatic effectors carry out the response
The Flexor (Withdrawal) Reflexes
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2 Sensory neuron
activates multiple
interneurons
+
+
+
+
+
+
+
+
5
Contralateral
motor neurons
to extensor
excited
3 Ipsilateral motor
neurons to flexor
excited
• flexor reflex – the quick
contraction of flexor
muscles resulting in the
withdrawal of a limb from
an injurious stimulus
• requires contraction of the
flexors and relaxation of the
extensors in that limb
4 Ipsilateral flexor
contracts
+
+
6 Contralateral
extensor
contracts
1 Stepping on glass
stimulates pain receptors
in right foot
ithdrawal of right leg
(flexor reflex)
Extension of left leg
(crossed extension reflex)
• polysynaptic reflex arc –
pathway in which signals
travel over many synapses
on their way back to the
muscle
Stretch Reflex
To brain
6 Primary afferent
neuron stimulates
inhibitory interneuron
4 Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
7 Interneuron inhibits
alpha motor neuron
to flexor muscle
3 Primary afferent
neuron excited
5 Alpha motor neuron
stimulates extensor
muscle to contract
2 Muscle spindle
stimulated
1
Extensor muscle
stretched
13-16
8 Flexor muscle
(antagonist) relaxes
Stretch Reflexes
Stretch Reflex Maintains Posture
Spinal Reflexes
Gamma Motor Neurons
•Muscle spindle
•Intrafusal fibers: gamma
•Extrafusal fibers: alpha
•Gamma feedback loop provides
more control
Cortical/Brain Stem Modulation
of Reflexes
Reflexes can also chained together
Reflexes can also be conditioned
Classical conditioning – the
animal responds to the
environment – learning results
from the environment
“Give
me a dozen healthy infants, wellformed, and my own specified world to
bring them up in, and I'll guarantee to
take any one at random and train him to
become any type of specialist I might
select–a doctor, lawyer,artist, merchantchief, and, yes, even into a beggar-man
and thief, regardless of his talents,
penchants, tendencies, abilities,
vocations and race of his ancestors.
[Watson, 1924,]
But not all movements are
reactive….
e.g. Operant conditioning – the
animal operates on the
environment – the animal
performs arbitrary behaviors and
if a behavior is rewarded it will
occur again
In fact most movements are
planned
Generation of Rhythmic Motor
Patterns
Central pattern generator (CPG) a neural
circuit capable of producing repetitive activity
in the absence of any sensory input. Although
a CPG doesn't need sensory input in order to
produce oscillatory output, its activity can be
affected by sensory input
Preparations used for studying the neural control of stepping
+ dorsal root section
or curare
KSJ, Fig. 37-1
Central Pattern Generators
• Can be generated in single neurons, e.g.
pacemaker neurons in the heart
• Can be generated synaptically involving
networks
Two reciprocally inhibitory neurons that fatigue and
show postinhibitory rebound will oscillate:
Central Pattern Generators for
Locomotion
• Half-center Model
– alternating activity in flexor & extensor
• Each limb has own pattern generator
Half-center Model
Flexor
+
Tonic
input
+
a
+
+
+
a
+
+
Extensor
+
Feedback from Golgi tendon organs and muscle spindles in
extensor muscles control walking