Motor System Spinal Reflexes
Georgia Bishop, Ph.D.
Professor and Vice Chair
Department of Neuroscience
bishop.9@osu.edu
OBJECTIVES
At the end of the module
you will learn to:
Describe the peripheral receptors and pathways that regulate spinal
reflexes.
1.
2.
3.
4.
5.
6.
7.
Define the terms proprioception and proprioceptor.
Define the terms motor unit and recruitment.
Explain how motor units function to increase muscle tension is increased
Describe muscle spindles and Golgi tendon organs.
Differentiate between alpha and gamma motor neurons.
Describe the term Gamma Bias and explain its functional role.
Differentiate the role of the muscle spindle and the Golgi tendon organ in
proprioception.
8. Describe the neural correlates of spinal reflexes including the stretch reflex and
flexor
9. withdrawal with crossed extension reflexes.
10. Describe the clinical significance of hyperactive or absent reflexes.
2
REFLEX CIRCUITS
Reflex circuits in the spinal cord produce automated responses adaptive for
typical situations. When a specific kind of sensory input consistently elicits a
particular response, we call this a reflex.
Spinal or reflex circuits govern many muscle recruitment patterns within and
between limbs, including standing and walking.
Reflex circuits require, at a minimum, 2 components:
A sensory input
A motor output
In the circuit, the sensory neuron synapses on the motor neuron which then elicits
a muscle contraction.
Many reflex circuits also contain interneurons that may either excite or inhibit
populations of motor neurons.
Spinal Nerve
Intervertebral
Foramen
Vertebral Canal
Dorsal
Root
DRG
Cell
Dorsal
Horn
Periphery
Dorsal Root
Ganglion
Dorsal
Sensory
Ventral
Motor
Spinal
Nerve
Ventral
Horn
Motor
Neuron
Ventral
Root
Efferent
Axon of
Motor
Neuron
Peripheral
Process
ALPHA MOTOR NEURONS
40 m
Innervate extrafusal muscle fibers: Regular
contractile portion of muscle
SKELETAL MUSCLE
MUSCLE FIBER
Skeletal muscle is made
up of bundles of
cylindrical muscle fibers.
Each muscle fiber is a
single, multinucleated
cell
MOTOR UNIT
Defined as a single alpha motor neuron and ALL THE MUSCLE FIBERS it innervates
Each muscle fiber receives input from a single motorneuron which synapses at the single motor
end plate. However, a motor axon may innervate more than one muscle fiber).
MOTOR UNIT
The size of motor units varies from small (10 – 100 fibers/motor neuron) to large
(100 – several thousand fibers/motor neuron).
Small motor units provide more precise control of motor activity. These would be found in
muscles that control individual digits or muscles that control movements of the eye.
MOTOR UNIT
Motor End-Plate – Site where axons make synaptic contact with muscle fiber
MUSCLE CONTRACTION
MUSCLE TENSION
1 TWITCH (100 msec)
The force of contraction of individual muscle fibers is determined by the firing
frequency of the motor neuron
Total force of contraction of a muscle is determined by number of alpha motor
neurons that are active.
MUSCLE CONTRACTION
Tetany = a sustained muscular contraction caused by a series of stimuli repeated
so rapidly that the individual muscular responses are fused. Maximal force a
muscle can generate. Temporal summation.
TETANIC CONTRACTION
TETANUS
Clostridium tetani is a grampositive rod-shaped bacterium that
is found worldwide in soil; it is
usually in its dormant form, spores,
and becomes the rod-shaped
bacterium when it multiplies.
1.
2.
3.
4.
5.
Binds to peripheral nerve terminals and transported within the axon to CNS.
Binds to proteins at presynaptic inhibitory motor nerve endings and taken up into the neurons.
Effect is to block release of inhibitory neurotransmitters (GABA, glycine).
Results in uncontrolled firing of motor neurons resulting in muscular spasms.
Acts by selective cleavage of a protein required for neurotransmitter release, synaptobrevin II
PROPRIOCEPTION AND PROPRIOCEPTORS
PROPRIOCEPTION include awareness of the body’s position in space,
sensation of forces acting on the body, and sensation of body
movements underway
Some define proprioception as position awareness and kinesthesia as
awareness of movement. For this module, proprioception is used
inclusively.
A PROPRIOCEPTOR is a sensory receptor that is principally used for
proprioception
SENSORY INPUTS INFLUENCE RECRUITMENT
Two main kinds of proprioceptors influence
recruitment levels in the motor pools:
Muscle spindles - muscle length, and velocity of
muscle length changes
Golgi Tendon Organs (GTOs) - muscle tension (force)
information
Cutaneous receptors, receptors in joints, and pain receptors also influence
recruitment
MUSCLE SPINDLES
Muscle Spindle
Muscle spindles detect muscle length, position, velocity, and
acceleration
Extrafusal fibers are the muscle fibers we have been talking
about. They make up the muscles we see.
Muscle spindles are miniature, long, thin stretch receptors
that are present in all striated muscle. They are made up
of specialized intrafusal muscle fibers surrounded by a
capsule of connective tissue. They are scattered
throughout the muscle and are aligned in parallel with the
extrafusal fibers.
SPINDLE
EF
The primary sensory afferent from the muscle spindle is called the Group Ia afferent (Ia)
MUSCLE SPINDLES HAVE 3 MAIN COMPONENTS
Intrafusal muscle fibers provide regulation of
muscle spindle stiffness, for variable sensitivity to
stretch
Primary sensory axons (Ia and II) terminate on
central region to sense stretch
The type II afferents only detect length of a
muscle whereas type Ia detect length and rate of
change in length.
Gamma motor axons synapse on intrafusal fibers
to regulate their tension (stiffness)
16
TYPES OF MOTOR NEURONS
Alpha motor neurons (medium – large neurons)
Project to extrafusal (skeletal muscle) fibers
Responsible for generating the muscle forces used to control movement
Gamma motor neurons (very small neurons)
Only present to control muscle spindles by synapsing on intrafusal muscle fibers
Cannot produce any appreciable muscle force
ALPHA-GAMMA CO-ACTIVATION (GAMMA BIAS)


The CNS independently regulates gamma motor neurons
GOLGI TENDON ORGANS
Receptors found at the junction of the muscle
fibers with the collagenous tendon. Unlike the
muscle spindles that are arranged in parallel to
the muscle fibers, the receptors in the GTO
intertwine with the collagen fibers of the tendon.
“Ib afferents” coming from the GTOs convey
force data to the spinal cord. Detect muscle
tension.
19
CIRCUITRY OF GOLGI TENDON ORGAN
The circuit of the Ib afferent
is:
Activated Ib afferent
synapses on
inhibitory interneuron in the
spinal cord.
Ib excitatory
interneuron
This inhibitory interneuron
inhibits the motor neuron that
projects to the contracting
muscle to decrease tension
and prevent damage to the
tendon.
STRETCH REFLEX WITH RECIPROCAL
INHIBITION
As described previously, when a muscle is quickly stretched, Ia
afferent synapses excite that muscle’s alpha motoneurons
The is the stretch reflex
Muscle length homeostasis
The Ia afferent also synapses on the Ia inhibitory
interneuron, which inhibits antagonist moto-neurons,
relaxing that muscle. Provides a complementary
functional effect.
21
STRETCH REFLEX
Muscle spindles respond to weight of mug and
activate the biceps to keep it upright. At same
time, inhibitory interneuron blocks activity in the
triceps.
Increasing weight causes passive stretch of the
muscle and increased activation of Ia afferent
which activates more motor neurons to produce
stronger contraction. May recruit more motor
units.
Increased recruitment of more motor units brings
arm back to level position and overcomes
increased resistance.
FLEXOR WITHDRAWAL WITH
CROSSED EXTENSION
Results from activation of a
nociceptor.
Primary afferents branch into collaterals that activate
2nd order sensory neurons in the dorsal horn
These then project to another set of interneurons in the
spinal cord which have multiple effects. They project
to:
23
1.
Inhibitory interneurons that inhibit alpha motor
neurons that project to the quadriceps (stance
muscle) and to excitatory interneurons that activate
flexor muscles (e.g., hamstrings).
2.
Excitatory interneurons on the contralateral side of
the spinal cord that activate the contralateral
quadriceps and inhibitory interneurons that inhibit
the contralateral flexor muscles.
EFFECTS FROM JOINT RECEPTORS
Joint receptors contribute some proprioceptive awareness, especially at end of the
joint range and under high force or strain conditions
Joint receptors can promote stability or safety
Rapid reaction to high joint force is increased muscle tone around the joint
Severe, acute pain from trauma causes extreme recruitment to splint the joint
Long term pain and swelling produces inhibition of muscles to protect the joint
CLINICAL APPLICATIONS
Tendon jerk reflexes reveal CNS excitability state.
When cerebral cortex is damaged, the default state of
gamma motoneurons is hyperactive, and stretch reflexes
are exaggerated.
Absence of a reflex indicates there is damage at the level of the spinal
cord. This may result from damage to a sensory neuron or the Ia afferent
(e.g., diabetes) or damage to the motor neuron or pools of interneurons.
Stretching exercises should be performed slowly less likely to elicit a stretch reflex.
Smooth, coordinated movement requires good
proprioception
SUMMARY
1.
Sensation and control of reflexes is required for normal, coordinated movements
2.
Proprioceptors are sensory receptors dedicated to monitoring movements and forces.
3.
Muscle spindles (intrafusal muscle) are miniature muscle fibers that are aligned in parallel with
the extrafusal muscle and detect length and rate of movement of a muscle
4.
Golgi Tendon Organs are proprioceptors that are entwined with the collagen of a tendon and
detect tension in the muscle.
5.
Alpha motor neuron innervate extrafusal muscle and gamma motor neurons innervate intrafusal
muscle fibers. For proper function, both are co-activated.
6.
Gamma motor neurons determine the sensitivity of the muscle to small changes in length.
7.
Spinal cord reflexes may be elicited by proprioceptors or by cutaneous (nociceptors) receptors.
8.
The stretch reflex elicits muscle contraction following stretching of a muscle. In addition to
activation of the stretched muscle, the antagonist is inhibited via interneurons in the spinal cord.
9.
The flexor withdrawal reflex with crossed extension involves pools of interneurons that induce
removal of a limb from a painful stimulus and extension of the contralateral limb to maintain
stability.
10. Absent reflexes may indicate spinal cord or motor neuron damage. Abnormal reflexes (e.g.,
hyperactive) are a sign of damage to higher parts of the nervous system.
Thank you for completing this module
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Bishop.9@osu.edu
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