Physiology Ch 54 p655-666
Motor Functions of the Spinal Cord; the Cord Reflexes
Organization of the Spinal Cord for Motor Functions – gray matter is the area for reflexes;
sensory signals enter cord almost entirely through posterior roots
-after entering the cord, each sensory signal travels to two locations:
1. one branch terminates immediately in gray matter and elicits local reflex
2. another branch transmits signals to higher levels of nervous system
-each segment of spinal cord has millions of neurons in gray matter of type types: anterior
motor neurons and interneurons
Anterior Motor Neurons – in ventral horn of gray matter are several thousand neurons larger
than others called anterior motor neurons, which give rise to nerve fibers that leave cord by
way of anterior roots to directly innervate skeletal muscle
-two types: alpha motor neurons and gamma motor neurons
1. Alpha Motor Neurons – give rise to large type A alpha (Aα) motor nerve fibers; enter muscles
and innervate skeletal muscle. Stimulation of 1 alpha fiber excites 3-100 skeletal muscle fibers,
called the motor unit
2. Gamma Motor Neurons – smaller gamma motor neurons are in anterior horns and transmit
impulses through smaller type Aγ fibers which go to small, special skeletal muscle fibers called
intrafusal fibers inside the muscle spindle to help control muscle tone
3. Interneurons – present in all areas of gray matter and are 30x more numerous than other
neurons. Highly excitable and have spontaneous activity; found in diverging, converging, and
repetitive discharge circuits to perform specific reflex acts by spinal cord
-most signals first terminate in interneurons for processing before going on to anterior
motor neurons
Renshaw Cells Transmit Inhibitory Signals to Surrounding Motor Neurons – in the anterior
horns of spinal cord, there are a small number of Renshaw cells in association with motor
neurons
-almost immediately after anterior motor neuron axon leaves neuron, collateral branches pass
to Renshaw cells, which act to inhibit surrounding motor neurons
-thus, stimulation of a motor neuron tends to inhibit adjacent motor neurons, called lateral
inhibition
-motor system uses lateral inhibition to focus/sharpen signal
Multisegmental Connections from One Spinal Cord Level to other Levels – Propriospinal Fibers
-more than 50% of all fibers ascending/descending in cord are propriospinal fibers, which run
from one segment of cord to another
-as sensory fibers enter cord from posterior roots, they bifurcate and branch both up
and down spinal cord, transmitting signals to one or more segments for reflexes
Muscle Sensory Receptors- Muscle Spindles and Golgi Tendon Organs – sensory information
such as length of the muscle, instantaneous tension, and rate of length/tension change is
transmitted by two receptors: (1) Muscle spindle (scattered throughout muscle and signal
muscle length or rate of change of length) and (2) golgi tendon organ, located in tendons and
transmit information about tension and rate of change of tension
Receptor Function of Muscle Spindle
Structure and Motor Innervation of Muscle Spindle – spindles are 3-10mm long and build
around 3-13 intrafusal muscle fibers pointed at ends and attached to glycocalyx of extrafusal
muscle fibers
-each intrafusal fiber is a tiny skeletal muscle fiber, with the central region having NO
actin/myosin filaments; central region functions as receptor
-end portions that do contract are excited by gamma motor nerve fibers originating from type
Aγ motor neurons in anterior horns of spinal cord (gamma efferents) as opposed to alpha
efferents (Type Aα) fibers that innervate skeletal muscle
Sensory Innervtion of Muscle Spindle – receptor portion of muscle spindle is in the center,
where there is no actin/myosin in the intrafusal fibers; stimulated by stretching of midportion of
spindle. Can be excited in 2 ways:
1. lengthening of whole muscle stretches midportion of receptor to excite it
2. contraction of spindle’s intrafusal fibers without length change excites receptor
-two types of sensory endings are found in central receptor area: primary and secondary
1. Primary Ending – in center of receptor area, a large sensory nerve fiber encircles central
portion of each intrafusal fiber, forming the primary ending or annulospiral ending
-it is a type Ia fiber transmitting fast sensory signals (70-120m/s)
2. Secondary Ending – one or two small sensory nerve fibers – type II fibers innervate receptor
region on 1 or both sides of primary ending; can circle the intrafusal fibers, but usually spreads
Division of Intrafusal Fibers into Nuclear Bag and Chain Fibers – Dynamic and Static Response
of Muscle Spindle – there are 2 types of muscle spindle intrafusal fibers: (1) nuclear bag fibers,
where muscle nuclei are congregated in expanded bags in central receptor area, and (2) nuclear
chain fibers which are half as large and have nuclei aligned in a long chain throughout receptor
-primary sensory nerve ending is excited by both types of fibers
-secondary sensory ending is ONLY innvervated by nuclear chain fibers
Response of Both Primary + Secondary Endings to Length of Receptor – “Static Response” –
when receptor is stretched SLOWLY, # of impulses increases proportionally in both primary and
secondary endings, called the static response, where both endings transmit for several minutes
if spindle remains stretched
Response of Primary Ending to Rate of Change of Receptor Length – “Dynamic” Response –
when the length of spindle increases SUDDNELY, only the primary ending (NOT secondary) is
stimulated powerfully, called the dynamic response, where primary ending response very
actively to a rapid rate of change in spindle length
-primary receptor transmits ONLY while length is actually increasing, regardless of how small
-soon as the length stops increasing, impulse discharge returns to static response
-when spindle receptor SHORTENS, opposite happens from primary ending; thus the primary
ending can send both positive and negative signals
Control of Intensity of Static/Dynamic Responses by γ-Motor Nerves – γ motor nerves are
divided into two types: gamma-dynamic (gamma-d) and gamma-static (gamma-s)
-the gamma-dynamic fibers excite mainly nuclear bag intrafusal fibers
-the gamma-static fibers excite mainly nuclear chain intrafusal fibers
-when gamma-d fibers excite nuclear bags, muscle spindle dynamic response is enhanced while
static is not affected, and when gamma-s fibers are stimulated, they excite nuclear chain fibers
to enhance static response without affecting dynamic response
Continuous Discharge of Muscle Spindles Under Normal Conditions – normally, muscle spindles
emit sensory nerve impulses continuously; stretching spindle increases rate of firing, and
shortening decreases rate of firing; thus either positive or negative signals can be sent to cord
Muscle Stretch Reflex – simple example of muscle spindle function; when muscle is stretched
suddenly, excitation of spindles causes reflex contraction of large skeletal fiber
Neuronal Circuitry of Stretch Reflex – a type Ia proprioceptor nerve fiber in muscle spindle
enters dorsal root; branch then synapses on anterior motor neuron in ventral horn of gray
matter, which sends fiber back to same muscle; called monosynaptic pathway that allows reflex
signal to return with the shortest time delay back to muscle after spindle excitation
-most type II fibers from spindle terminate on interneurons in cord gray matter, which signal to
anterior motor neurons
Dynamic Stretch Reflex and Static Stretch Reflexes – stretch reflex can be divided into these 2:
1. Dynamic Stretch Reflex – elicited by potent dynamic signal transmitted from primary
sensory endings of muscle spindles caused by rapid stretch or unstretch
a. Reflex acts to oppose sudden changes in muscle length
b. Dynamic reflex is over within fraction of a second after muscle has been
stretched/unstretched to its new length, after which a weaker static stretch
reflex continues for a prolonged period of time
2. Static reflex is elicited by both primary and secondary endings, and is important for
maintaining degree of muscle contraction
Damping Function of Dynamic and Static Stretch Reflexes – an important function of stretch
reflex is the ability to prevent oscillation or jerkiness of body movements (called dampening)
Mechanism of Damping in Smoothing Muscle Contraction – signals from cord often transmitted
in unsmooth form (increasing and decreasing in intensity variably)
-when muscle spindle is dysfunctional, muscle contraction is jerky during course of each signal,
so muscle spindle acts to average signal
Role of Muscle Spindle in Voluntary Motor Activity – 31% of all motor fibers to muscle are
small, type Aγ fibers
-when signals transmitted from brain to alpha-motor neurons, the gamma neurons are
stimulated simultaneously, called coactivation, causing both extrafusal fibers and intrafusal
fibers to contract
-purpose of simultaneous contraction is because it keeps the length of receptor portion of
muscle spindle from changing during muscle contraction (to avoid opposing signals), and it also
maintains proper dampening function regardless of any change in muscle length
Brain Areas for Control of Gamma Motor System – gamma efferent system is excited by
bulboreticular facilitatory region of brainstem and impulses transmitted here from the
cerebrellum, basal ganglia, and cerebral cortex; important for dampening mvements of different
body parts during walking/running
Muscle Spindle Stabilizes Body Position During Tense Action – most important function of
muscle spindle is to stabilize body position during tense motor action
-bulboreticular facilitatory region transmit excitatory signals through γ nerve fibers to intrafusal
muscle fibers of spindles to shorten ends of spindles and stretch central receptor to increase
signal output
-if spindles on both sides of each joint activated at the same time, reflex excitation of skeletal
muscles on both sides of joint also increases, producing tight, tense muscles opposing each
other at the joint
-net effect is that position of joint becomes stabilized, and any force that moves joint
from current position is opposed by sensitized stretch reflezes on both sides of a joint
Clinical Applications of Stretch Reflex – Knee jerk and other jerks can be used to assess
sensitivity of stretch reflexes
-knee jerk is elicited by striking patellar tendon with reflex hammer, which instantaneously
stretches quadriceps muscle + excites dynamic stretch reflex causing lower leg to jerk forward
-similar reflexes can be obtained from any muscle by striking the tendon or muscle belly
-when large numbers of facilitatory impulses are transmitted from upper regions of CNS, the
muscle jerks are greatly exaggerated
-when facilitatory impulses are depressed, muscle jerks are weakened or absent
-large lesions in the motor areas of cerebral cortex, but not in lower motor control areas, cause
greatly exaggerated muscle jerks on opposite side of the body
Clonus-Oscillation of Muscle Jerks – muscle jerks can oscillate, called clonus
-if person standing on tip ends of feet suddenly drops his body down and stretches
gastrocnemius muscle, the stretch reflex impulses are transmitted to spinal cord from muscle
spindles to excite the stretched muscle and cause it to lift the body up again
-fraction of a second later the reflex dies out and body falls again; exciting fibers again to
oscillate for long periods, called a clonus
-ONLY OCCURS when stretch reflex is highly sensitized by facilitatory impulses from brain
Golgi Tendon Reflex (muscle tension) – golgi tendon organ is an encapsulated receptor through
which muscle tendon passes (10-15 muscle fibers to each golgi tendon organ)
-golgi tendon organ detects muscle tension as reflected by tension in itself
-golgi tendon organ has both dynamic response and a static response; reacting intensely when
muscle tension suddenly increases (dynamic response) and settling down within a fraction of a
second to a lower level of steady firing directly proportional to muscle tension
Transmission of Impulses from Tendon Organ into CNS – golgi tendon organ signals transmitted
through large, rapid type Ib fibers (16um in diameter) to local areas of cord, and after synapsing,
transmitted to long fiber pathways like spinocerebellar tracts into cerebellum and others to
cerebral cortex
-local cord excites a single inhibitory interneuron that inhibits anterior motor neuron, which
inhibits individual muscle without affecting adjacent muscles
Inhibitory Nature of Tendon Reflex – stimulation of golgi tendon organ by increased tension in
muscle, signals are transmitted to spinal cord to cause reflex effects in muscle (inhibitory)
-this reflex provides negative feedback that prevents too much tension from developing
-when tension is extreme, the inhibitory effect can be so great that it leads to sudden reaction in
spinal cord to cause instantaneous relaxation of whole muscle
-this is called the lengthening reaction, a protective mechanism to prevent muscle tearing
-role of tendon reflex to equalize contractile force among muscle fibers – golgi tendon reflex
functions to equalize contractile forces of separate muscle fibers; those fibers that exert excess
tension become inhibited by reflex, whereas those exerting too little tension are excited
Function of Muscle Spindles/Golgi Tendons in Conjunction w/ Motor Control from higher
brain – both organs apprise higher motor control centers of instantaneous changes taking place
in muscles
-dorsal spinocerebellar tracts carry both spindle and golgi tendon info to cerebellum at 120m/s,
the most rapid of anywhere in the body
Flexor Reflex and Withdrawal Reflexes – any cutaneous sensory stimulus from limb can cause
flexor muscles of limb to contract, withdrawing limb from stimulating object (flexor reflex)
-flexor reflexes are triggered by stimulation of pain endings, also called nociceptive or pain
reflex
-if part of both other than limbs is painfully stimulated, part will be withdrawn from stimulus,
called withdrawal reflex
Neuronal Mechanism of Flexor Reflex – painful stimulus applied to hand; flexor msucles of
upper arm become excited to withdraw hand from painful stimulus
-pathways for flexor reflex to not pass directly to anterior motor neuron but instead pass first
into the spinal interneuron pool and THEN to motor neurons
-shortest route is a 3-4 neuron pathway, but may be more and involve these circuits:
1. Diverging circuits to spread reflex to necessary muscles for withdrawal
2. Circuits to inhibit antagonist muscles (reciprocal inhibition)
3. Circuits to cause afterdischarge lasting may fractions of a second
-after first few seconds of flexor response, the reflex begins to fatique (characteristic of all
reflexes of spinal cord)
-after stimulus is over, contraction of the muscle returns to baseline, but can take a while due to
afterdischarge (time of afterdischarge proportional to intensity of pain stimulus eliciting reflex)
-afterdischarge results from repetitive firing of excited interneurons, and also after
strong pain stimuli, afterdischarge results from recurrent pathways that initiate
oscillation in reverberating interneuron circuits to signal anterior motor neurons
Pattern of Withdrawal – depends on which sensory nerve is stimulated; pain on inward side of
arm elicits contraction of flexor muscles but ALSO abductor muscles to pull arm outward
Crossed Extensor Reflex – after 0.2-0.5s of a stimulus eliciting flexor reflex in one limb, the
opposite limb begins to extend (can push entire body away from object causing stimulus)
Mechanism of Crosses Extensor Reflex – signals from sensory nerves cross to opposite side of
cord to excite extensor muscles
-because it takes 200-500ms for extensors to contract, many interneurons are involved
-after removal of stimulus, there is an even longer afterdischarge period than flexor,
resulting from reverberating circuits in interneurons
Reciprocal Inhibition and Reciprocal Innervation – excitation of one muscle group inhibits other
muscle groups; when stretch reflex excites one muscle, it often simultaneously inhibits the
antagonist muscles, called reciprocal inhibition, and nerve circuit is called reciprocal
innervation
Reflexes of Posture and Locomotion
Postural and Locomotive Reflexes of Cord
Positive Supportive Reaction – pressure on footpad of decerebrate animal causes limb to
extend against pressure applied to foot; if spinal cord transected for long time, reflexes can be
so exaggerated that the reflex can stiffen limbs to support weight of body
-involves interneuron circuit similar to flexor/extensor circuit; locus of pressure on foot
determines direction in which limb will extend
-pressure on one side causes extension in that direction, called magnet reaction
Cord “Righting” Reflexes – when a spinal animal is laid on its side, it will make uncoordinated
movements to try to raise itself to standing position (cord righting reflex)
Stepping and Walking Movements
Rhythmical Stepping Movements of Single Limb – rhythmical stepping movements are
frequently seen in limbs of spinal animals; forward flexion of one limb is followed after a second
by backward extension, and then flexion occurs again
-oscillation of flexion and extension can occur even after sensory nerves have been cut, and
seems to result from mutually reciprocal inhibition circuits within matrix of the cord
-sensory signals from footpads and from position sensors around joints play a role in controlling
foot pressure and frequency of stepping when foot is allowed to walk along surface
-stumble reflex – when foot hits obstruction, it is quickly raised over obstruction
Reciprocal Stepping of Opposite Limbs – if lumbar cord is not split down center, every time
stepping occurs in forward direction of one limb, the opposite limb moves backward
Diagonal Stepping of All 4 Limbs – “Mark Time” Reflex – if animal is held up and legs allowed to
dangle, stretch on limbs elicits stepping reflexes in all 4 limbs; stepping occurs diagonally
between forelimbs and hindlimbs due to reciprocal innervtion
Galloping Reflex – both forelimbs move backward in unison while both hindlimbs move forward
Scratch Reflex – initiated by itch or tickle sensation and involves 2 functions: position sense to
allow paw to find point of irritation, and a to-and-fro scratching movement
-position sense is highly developed and utilizes many muscles
-to-and-fro movement – involves reciprocal innervation circuits causing oscillation
Spinal Cord Reflexes Causing Muscle Spasm – localized pain is most common cause of spasm
-Muscle Spasm Resulting from Broken Bone – results from pain impulses initiated from broken
edges of bone, which cause muscles surrounding the area to contract tonically
-Abdominal Muscle Spasm in Peritonitis – local spasm caused by cord reflexes, results from
irritation of parietal peritoneum by peritonitis; relief of pain allows spastic muscle to relax
-Muscle Cramps – any local irritating factor or metabolic abnormality of muscle (cold, lack of
blood, overexercise) can elicit pain or other sensory signals transmitted from muscle to cord,
which causes reflex feedback contraction
Autonomic Reflexes in Spinal Cord – changes in vascular tone from changes in local skin heat,
sweating (localized heat on surface), intestinointestinal reflexes controlling motor function of
gut, peritoneointestinal reflexes inhibiting GI mobility in response to irritation, evacuation
reflexes for emptying bladder or colon
-all reflexes can be elicited simultaneously in the form of a MASS REFLEX
Mass Reflex – if spinal cord becomes excessively active, massive discharge in large portions of
cord occurs caused usually by strong pain to skin or excessive filling of a viscus (overdistention of
bladder), can cause mass reflex
-involves large portions of cord to cause major skeletal muscles to spasm, colon and bladder
evacuation, and arterial pressure rises to maximum values, large areas of body breaks out into
sweating
Spinal Cord Transection and Spinal Shock – when spinal cord is transected in upper neck, all
cord functions, including reflexes are depressed to silence, called spinal shock
-normal cord activity depends on continual tonic excitation by discharge of nerve fibers entering
cord from higher centers (reticulospinal tracts, vestibulospinal tracts, corticospinal tracts)
-after few hours – weeks, spinal neurons regain excitability by increasing their own natural
degree of excitability to make up for loss above
-During spinal shock, the following functions are affected:
1. Arterial pressure falls instantly and drastically; sympathetic nervous system blocked almost
to extinction, but returns normal after a few days
2. All skeletal muscle reflexes in spinal cord are blocked in the initial stages of shock, returning
to normal in humans after several weeks to months; reflexes may eventually become
hyperexcitable especially if a few facilitatory pathways remain intact
a. First reflexes to return are stretch reflexes followed by flexor, postural gravity, and
stepping reflexes
3. Sacral reflexes for bladd and colon evacuation are suppressed for a few weeks but do return