Phys Chapter 54
Motor Functions of Spinal Cord
 There is no neuronal circuit for the motions of the legs for walking in the brain – the brain simply sends
command signals to spinal cord to set into motion the walking process
o Brain is required to give directions that control sequential cord movements
 Cord gray matter is integrative area for cord reflexes
 Sensory signals come in through posterior roots – then signal goes 2 separate directions
o One branch of sensory nerve terminates almost immediately in gray matter of cord and elicits local
segmental cord reflexes and other local effects
o Another branch transmits signals to higher levels of nervous system (higher levels of cord, brain stem, or
cerebral cortex)
 Anterior motor neurons – located in each segment of anterior horns of cord gray matter
o Give rise to nerve fibers that leave cord by way of anterior roots and directly innervate skeletal muscle
fibers
o Alpha motor neurons – give rise to large Type Aα motor nerve fibers that branch many times after they
enter muscle and innervate large skeletal muscle fibers
 Stimulation of single alpha nerve fiber excites anywhere from 3 to several hundred skeletal
muscle fibers of its motor unit
o Gamma motor neurons – about half as many as alpha motor neurons – located in spinal cord anterior
horns
 Transmit impulses through smaller type Aγ motor nerve fibers, which go to intrafusal fibers that
constitute the middle of the muscle spindle, controlling basic muscle tone
o Interneurons – present in all areas of cord gray matter – about 30x as numerous as anterior motor
neurons
 Small and highly excitable, often exhibiting spontaneous activity
 Responsible for most of integrative functions of spinal cord
 Almost all types of neuronal circuits are found in interneuron pool of cells
 Renshaw cells – small neurons in anterior horns of spinal cord in close association with motor neurons
o Almost immediately after anterior motor neuron axon leaves body of neuron, collateral branches from
axon pass to adjacent Renshaw cells – inhibitory cells that transmit inhibitory signals to surrounding
motor neurons
o Stimulation of each motor neuron tends to inhibit adjacent motor neurons (lateral inhibition)
o Important because it sharpens the focus of the outgoing signal
 Propriospinal fibers – more than half of all nerve fibers that ascend and descend into spinal cord – run from one
segment of cord to another
o As sensory fibers enter cord from posterior cord roots, they bifurcate and branch both up and down
spinal cord – some of the branches transmit signals to only a segment or two, while others transmit
signals to many segments
o Ascending and descending propriospinal fibers of cord provide pathways for multisegmental reflexes,
including reflexes that coordinate simultaneous movements in forelimbs and hindlimbs
Muscle Sensory Receptors and their Roles in Muscle Control
 Proper control of muscle function requires continuous feedback of sensory information from each muscle to
spinal cord, indicating the functional status of each muscle at each instant
 Muscles and their tendons are supplied abundantly with sensory receptors
o Muscle spindles – distributed throughout belly of muscle and send information to nervous system about
muscle length or rate of change length
o Golgi tendon organs – located in muscle tendons and transmit information about tendon tension or rate
of change of tension
 Sensory receptors transmit their information to spinal cord, cerebellum, and cerebral cortex, helping each
portion of CNS function to control muscle contraction
Receptor Function of Muscle Spindle
 Receptor portion of intrafusal muscle fiber is central portion – has no myosin and actin contractile elements
here – sensory fibers originate in this area
o
Stimulated by stretching the midportion of the spindle – thus can be stimulated by lengthening whole
muscle or contraction of ends, stretching center
 Primary ending (annulospiral ending) – large sensory nerve fiber encircling central portion of each intrafusal
fiber – transmits sensory signals to spinal cord as rapidly as any nerve fiber in entire body
o Excited by both nuclear bag intrafusal fibers and nuclear chain fibers
 Secondary ending – usually one but sometimes two smaller sensory nerve fibers that innervate receptor region
on one or both sides of primary ending – sometimes encircles intrafusal fibers in same way type Ia (primary)
fiber does, but often spreads like branches on a bush
o Only excited by nuclear chain fibers
 Static response – when receptor portion is stretched slowly, number of impulses transmitted from both primary
and secondary endings increases almost directly in proportion to degree of stretching and endings continue to
transmit impulses for several minutes
 Dynamic response – when length of spindle receptor increases suddenly, primary ending (not secondary) is
stimulated powerfully – only transmitted while receptor is actively being stretched, and as soon as it stops,
dynamic response stops and static response takes over
 Exact opposite reactions are sent if muscle is shortened (same intensity and type, just positive or negative
depending on stretching or shortening)
 Gamma motor nerves to muscle spindle can be divided into
o Gamma-dynamic (gamma-d) – excites mainly nuclear bag intrafusal fibers
 When they excite intrafusal fibers, dynamic response of muscle spindle becomes tremendously
enhanced and static response is hardly affected
o Gamma-static (gamma-s) – excites mainly nuclear chain intrafusal fibers
 Excitation of nuclear chain fibers by these enhances static response while having little influence
on dynamic response
 Normally, particularly with gamma nerve excitation, muscle spindles emit sensory nerve impulses continuously –
stretching them increases rate of firing, and shortening them decreases rate of firing (decreased rate = negative
signal, increased rate = positive signal)
Muscle Stretch Reflex
 Muscle stretch reflex – whenever a muscle is stretched suddenly, excitation of spindles causes reflex contraction
of large skeletal muscle fibers of stretched muscle and closely allied synergistic muscles
o Monosynaptic pathway
o Type Ia proprioreceptor nerve fiber
o normal originating in muscle spindle and entering dorsal root of spinal cord – branch of this fiber goes
directly to anterior horn of cord gray matter and synapses with anterior motor neurons that send motor
nerve fibers back to same muscle from which muscle spindle fiber originated
 Stretch reflex can be either
o Dynamic stretch reflex – elicited by potent dynamic signal transmitted from primary sensory endings of
muscle spindles, cause by rapid stretch or shortening – causes strong reaction reflex
o Static stretch reflex – continues over prolonged period after initial dynamic stretch reflex – much weaker
– elicited by continuous static receptor signals transmitted by both primary and secondary endings –
causes the degree of muscle contraction to remain reasonably constant, except when person’s nervous
system specifically wills otherwise
 Damping function of stretch reflex – allows for smoothness of motion without jerkiness or oscillation
o When muscle spindle apparatus not functioning satisfactorily, muscle contraction is jerky during course
of (normal) unsteady signal
o Normal effect of muscle spindle reflex is signal averaging, which smoothes out motion
Role of Muscle Spindle in Voluntary Motor Activity
 Whenever signals transmitted from motor cortex or any other are of brain to alpha motor neurons, in most
instances gamma motor neurons are stimulated simultaneously (coactivation)
o Causes both extrafusal skeletal muscle fibers and muscle spindle intrafusal muscle fibers to contract at
same time
 Purpose for coactivation
o
Keeps length of receptor portion of muscle spindle from changing during course of whole muscle
contraction, and thus causing a reflex that would oppose the muscle contraction
o Maintains proper damping function of muscle spindle, regardless of any change in muscle length (so
receptor portion doesn’t flail or become overstretched)
Brain Areas for Control of Gamma Motor System
 Bulboreticular facilitatory region of brain stem – excites gamma efferent system
 Gamma efferent system also stimulated by impulses transmitted into bulboreticular area from cerebellum, basal
ganglia, and cerebral cortex
 Bulboreticular facilitatory area concerned with antigravity contractions
 Antigravity contractions have a high density of muscle spindles
Muscle Spindle System Stabilization during Tense Action
 Bulboreticular facilitatory region and its allied areas of brain stem transmit excitatory signals through gamma
nerve fibers to intrafusal muscle fibers of muscle spindles, shortening ends of spindles and stretching central
receptor regions, increasing their signal output
 If spindles on both sides of each joint are 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 position of joint becomes strongly stabilized, and any force that tends to move joint from its current
position is opposed by highly sensitized stretch reflexes operating on both sides of joint
 This is used whenever someone wants to do a task involving delicate work with exact positioning – stabilizing of
the major joints so they don’t move during the fine-tune work
Clinical Applications of Stretch Reflex
 During physical exams, testing stretch reflexes determines how much background excitation (tone) the brain is
sending to the spinal cord
 Can clinically test these reflexes by knee jerk and other such reactions
o Striking the tendon with a reflex hammer causes stretching of the attached muscle and subsequent
stretch reflex contraction of said muscle
o Some reflexes are caused by striking the belly of the muscle
 When large numbers of facilitatory impulses are transmitted from upper regions of CNS into cord, muscle jerks
are greatly exaggerated
 If facilitatory impulses are depressed or abrogated, muscle jerks are considerably weakened or absent
 Reflexes are used most frequently in determining presence or absence of muscle spasticity caused by lesions in
motor areas of brain or disease that excite bulboreticular facilitatory area of brain stem
 Ordinarily, large lesions in motor areas of cerebral cortex but not in lower motor control areas (especially lesions
caused by strokes or brain tumors) cause greatly exaggerated muscle jerks in muscles on opposite side of body
Clonus-Oscillation of Muscle Jerks
 Clonus – muscle jerks oscillating
 Example of how this works – person standing on tiptoe to reach something from top shelf drops down fast,
which stretches gastrocnemius, which then contracts, sending person partially back on tiptoe, who then
bounces back down, etc.
 Ordinarily occurs only when stretch reflex is highly sensitized by facilitatory impulses from brain – tested for
by suddenly stretching a muscle and applying a steady stretching force to it – if clonus occurs, degree of
facilitation is high
Golgi Tendon Reflex
 Golgi tendon organ – encapsulate sensory receptor through which muscle tendon fibers pass
 About 10-15 muscle fibers usually connected to each Golgi tendon organ, and organ is stimulated when this
small bundle of muscle fibers is tensed by contracting or stretching the muscle, so excitation is reflected by
tension itself, not change in tension
 Has both dynamic response and static response, reacting intensely when muscle tendon suddenly increases but
settling down within a fraction of a second to lower level of steady-state firing that is almost directly
proportional to muscle tension
 Provide nervous system with instantaneous information on degree of tension in small segments of muscle

Signals from tendon organ transmitted through large, rapidly conducting type Ib nerve fibers – transmit signals
both into local areas of cord and, after synapsing in dorsal horn of cord, through long fiber pathways such as
spinocerebellar tracts into cerebellum and through other tracts to cerebral cortex
 Local cord signal excites single inhibitory interneuron that inhibits anterior motor neuron – directly inhibits
individual muscle without affecting adjacent muscles
 When Golgi tendon organs of muscle tendon are stimulated by increased tension in connecting muscle, signals
are transmitted to spinal cord to cause reflex effects in respective muscle – ENTIRELY INHIBITORY, providing
negative feedback mechanism that prevents development of too much tension on muscle
 When tension on tendon becomes extreme, inhibitory effect from tendon organ can lead to sudden reaction in
spinal cord that causes instantaneous relaxation of entire muscle (lengthening reaction)
 Serves as a protective mechanism to prevent tearing of muscle or avulsion of tendon from attachments on bone
 Can also equalize contractile forces of separate muscle fibers (those fibers that exert excess tension become
inhibited by the reflex, whereas those that exert too little tension become more excited because of absence of
reflex inhibition) – spreads muscle load over all fibers and prevents damage in isolated areas of muscle where
small numbers of fibers may be overloaded
Function of Muscle Spindles and Golgi Tendons in Conjunction with Motor Control from Higher Levels
 Motor spindles and Golgi tendon organs apprise higher motor control centers of instantaneous changes taking
place in muscles
 Information from these receptors is crucial for feedback control of motor signals that originate in higher areas
Flexor Reflex
 Flexor reflex – flexor muscles in limb contract, withdrawing limb from the stimulating object
 Can be stimulated by powerful stimulation of pain endings, such as pinprick, heat, or wound
 Also called nociceptive reflex or pain reflex
 Pathways do not pass directly to anterior motor neurons but instead pass first into spinal cord interneuron pool
of neurons and secondarily to motor neurons
o Shortest possible circuit is 3-4 neuron pathway, and most of signals of reflex transverse many more
neurons
 Types of circuits
o Diverging circuits spread reflex to necessary muscles for withdrawal
o Circuits to inhibit antagonist muscle (reciprocal inhibition circuits)
o Circuits to cause afterdischarge lasting many fractions of a second after stimulus is over
 Duration of afterdischarge depends on intensity of sensory stimulus that elicited reflex
 Immediate afterdischarge results from repetitive firing of excited interneurons themselves
o Prolonged afterdischarge occurs after strong pain stimuli, resulting from recurrent pathways that initiate
oscillation in reverberating interneuron circuits, that in turn transmit impulses to anterior motor
neurons, sometimes for several seconds after incoming sensory signal is over
 During afterdischarge, other reflexes and actions of CNS can move entire body away from painful stimulus
Withdrawal Reflex
 If some part of body other than one of limbs is painfully stimulated, that part will be withdrawn from stimulus,
but reflex may not be confined to flexor muscles
 Pattern of withdrawal depends on which sensory nerve is stimulated
o Pain stimulus on inward side of arm elicits not only contraction of flexor muscles of arm but contraction
of abductor muscles to pull arm outward
o Integrative centers of cord cause muscles to contract that can most effectively remove pained part of
body away from object causing pain
Crossed Extensor Reflex
 About 0.2-0.5 seconds after stimulus elicits flexor reflex of one limb, opposite limb begins to extend
 Extension of opposite limb can push entire body away from object causing painful stimulus in withdrawn limb
 Many interneurons involved in circuit between incoming sensory neuron and motor neurons of opposite side of
cord responsible for crossed extension
 After painful stimulus removed, crossed extensor reflex has even longer period of afterdischarge than flexor
reflex
o
o
Prolonged afterdischarge results from reverberating circuits among interneuronal cells
Of benefit for holding body away from painful object until other nervous reactions cause entire body to
move away
Reciprocal Inhibition and Reciprocal Innervation
 Reciprocal inhibition – stimulation of agonist simultaneously inhibits antagonist
 Reciprocal innervation – neuronal circuit that causes reciprocal inhibition
 Stronger reflex stimulus on one side of the body can override a reflex stimulus on the other side of the body
Reflexes of Posture and Locomotion
 Positive supportive reaction – pressure on footpad causes limb to extend against pressure applied to foot
(makes leg rigid to support body weight)
o Involves complex circuit in interneurons
o Locus of pressure on pad of foot determines direction in which limb will extend – pressure on one side
will cause extension in that direction (magnet reaction), which helps keep someone from falling to that
side
 Cord righting reflex – making uncoordinated movements to keep oneself in a standing/upright position – in
some animals with exaggerated reflexes, they can stand up and start walking away based solely on these
reflexes
 Rhythmical stepping movements – forward flexion of lower limb followed second or so later by backward
extension, and this cycle repeats
o Results from mutually reciprocal inhibition circuits within matrix of cord itself, oscillating between
neurons controlling agonist and antagonist muscles
o Sensory signals from footpads and position sensors around joints play strong role in controlling foot
pressure and frequency of stepping
 Stumble reflex – if the top of a foot encounters an obstruction during forward thrust, the forward thrust will
stop temporarily, then in rapid sequence, the foot will be lifted higher and proceed forward to be placed over
the obstruction
 Reciprocal stepping of opposite limbs – every time stepping occurs in forward direction in one limb, opposite
limb ordinarily moves backward, resulting from reciprocal innervation between the 2 limbs
 Scratch reflex – initiated by itch or tickle sensation that involves
o Position sense that allows hand to find exact position of irritation on surface of body – highly developed
function because it can locate a moving stimulus and if stimulus crosses midline, first limb stops
reaching for it and opposite limb begins scratching motion
o To-and-fro scratching movement – involves reciprocal innervation circuits that cause oscillation
 Muscle spasm resulting from broken bone – localized pain is usually the cause of this
o Pain impulses initiated from broken edges of bone cause muscles that surround the area to contract
tonically – for bad reactions, some form of anesthetic must be used to stop spasm so two ends of bone
can be set back into their appropriate positions
 Abdominal muscle spasm in peritonitis – abdominal spasm caused by irritation from peritonitis – relief from pain
allows spastic muscle to relax – same type of spasm often occurs during surgical operations, so deep anesthesia
is usually necessarily for intra-abdominal operations
 Muscle cramps – any local irritating factor or metabolic abnormality of muscle, such as severe cold, lack of blood
flow, or overexercise, can elicit pain or other sensory signals transmitted from muscle to spinal cord, which in
turn cause reflex feedback muscle contraction
o Reflex contraction stimulates same sensory receptors even more, which causes spinal cord to increase
intensity of contraction, eventually turning into muscle cramp
Automatic Reflexes in Spinal Cord
 Segmental autonomic reflexes include
o Changes in vascular tone, resulting from changes in local skin heat
o Sweating, which results from localized heat on surface of body
o Intestinointestinal reflexes that control some motor functions of gut
o Peritoneointestinal reflexes that inhibit gastrointestinal motility in response to peritoneal irritation
o Evacuation reflexes for emptying full bladder or colon

All segmental reflexes can at times be elucidated simultaneously by mass reflex, which involves large portions to
all of cord
o Happens if spinal cord suddenly becomes excessively active, causing massive discharge in large portions
of cord – usual stimulus causes this is strong pain stimulus to skin or excessive filling of viscus, such as
overdistention of bladder or gut
o Effects are
 Major portion of body’s skeletal muscles goes into strong flexor spasm
 Colon and bladder are likely to evacuate
 Arterial pressure often rises to maximal values, sometimes to systolic pressure of over 200
 Large areas of body break out into profuse sweating
o Because mass reflex can last for minutes, it results from activation of great numbers of reverberating
circuits that excite large areas of cord at once – similar mechanism of epileptic seizures, which involve
reverberating circuits that occur in brain instead of cord
Spinal Cord Transection and Spinal Shock
 When spinal cord is suddenly transected in upper neck, at first, essentially all cord functions, including cord
reflexes immediately become depressed to the point of total silence (spinal shock)
o Reason is normal activity of cord neurons depends on continual tonic excitation by discharge of nerve
fibers entering cord from higher centers
 After a few hours to weeks, spinal neurons gradually regain excitability by increasing their own natural degree of
excitability to make up at least partially for the loss
 Some spinal functions specifically affected during or after spinal shock include
o At onset of spinal shock, arterial blood pressure falls instantly and drastically, sometimes as low as 40
mm Hg, showing SNS blocked almost to extinction – pressure returns to normal in a few days
o All skeletal muscle reflexes integrated in spinal cord blocked during initial stages of shock – anywhere
from 2 weeks to a few months is required for reflexes to return to normal
 Some reflexes may become hyperexcitable, particularly if a few facilitatory pathways remain
intact between brain and cord while remainder of spinal cord is transected
 First reflexes to return are stretch reflexes, followed in order by progressively more complex
reflexes (flexor, postural antigravity, and remnants of stepping reflexes)
o Sacral reflexes for control of bladder and colon evacuation are suppressed during first few weeks after
cord transection, but in most cases they eventually return