spinal cord.ppt

U N I T XI

Textbook of Medical Physiology, 11th Edition

GUYTON & HALL

Copyright © 2006 by Elsevier, Inc.

Chapter 54:Motor Functions of the Spinal Cord

DR Sabeen Haq


The Spinal Cord is More Than Just a Conduit for Nerve Fibers

• Neuronal circuits for walking and various reflexes are contained within the spinal cord.

• Higher brain centers activate and command these circuits.– walking– maintaining equilibrium– Analytical and command signals


FunctionsSPINE Brain

•Posture•Support•Walking

•Commands•Sequences•Lean forward during walk•Walk to jump change•Monitor continuously•Control equilibrium


Spinal Animal

Decerebrate Animal

Initial Spinal ShockLater most intrisic funtions return

Initial increased toneVery excitable reflexes

All help to study Spinal Funtions


Motor Organization of the Spinal Cord

• Sensory fibers enter the cord and are transmitted to higher centers, or they synapse locally to elicit motor reflexes.

• Motor neurons are located in the anterior portion of the cord.– motor neurons are 50 - 100 % bigger than other

neurons


Anterior Motor Neurons

• Alpha motor neurons– give rise to large type A alpha fibers (~14 microns).– stimulation can excite 3 - 100 extrafusal muscle fibers

collectively called a motor unit

• Gamma motor neurons– give rise to smaller type A gamma fibers (~5 microns)– stimulation excites intrafusal fibers, a special type of

sensory receptor


Interneurons and Propriospinal Fibers

• Interneurons– 30 times as many as anterior motor neurons– small and very excitable– comprise the neural circuitry for the motor reflexes

• Renshaw cell inhibitory system– Receive collaterals from anterior motor neurons– Send inhibitory signals back to motor neurons– Recurrent inhibition– Lateral inhibition to avoid unnecessary spread


• Propriospinal fibers– travel up and down the cord for 1 - 2 segments– provide pathways for multisegmental reflexes


Sensory Receptors of the Muscle

• Muscle Spindle– sense muscle length and change in length– Located in the belly of musles

• Golgi Tendon Organ– sense tendon tension and change in tension– located in muscle tendons


Muscle spindle

• 3 to 10mm long• 3 to 12 small intrafusal muscle fibers• Attached to glycocalyx of extrafusal fibers• No or few actin and myosin filaments in

midway


Functional Anatomy of Muscle Spindles

• E/ spindle has 3-10 intrafusal muscle spindles– ¼ of the size of lg extrafusal muscle fibers (effector

fibers)– Central region lacks myofilaments– Wrapped by 2 types of nerve fibers

• Primary sensory endings of type Ia fibers– Innervate spindle center– Stimulated by rate and degree of stretch

• Gamma efferent fibers– Innervate contractile regions– Arise from ventral horn and maintain spindle sensitivity

• Also alpha efferent fibers – Stimulate extrafusal muscle fibers to contract






The Muscle Spindle


Annulospiralprimary

Flower-Spraysecondary

1-3/spindle

3-9/spindle

Dynamic Response

Static Response

a


Static Response of the Muscle Spindle

• When the center of spindle is stretched slowly - the number of impulses generated by the primary and secondary endings increases in proportion to the degree of stretch.

• This is the ‘static response’.• Function of the static nuclear bag and

nuclear chain fibers.


Dynamic Response of the Muscle Spindle

• When the center of the spindle is stretched rapidly - the number of impulses generated by the primary endings increases in proportion to the rate of change of the length.

• This is the ‘dynamic response’.

• Function of the dynamic nuclear bag fiber.


Physiologic Function of the Muscle Spindle

• Comparator of length between the intrafusal and extrafusal muscle fiber.

• Opposes a change in length of the muscle.• When the muscle is stretched the spindle returns it

to its original length.• Leads to the stretch reflex.• Positive signals indicating stretch ,increse firing• Negative signals decrease signals, unstretch


Function of the Muscle Spindle

Whenever skeletal muscle is stretched the muscle spindles are stimulated. They detect:

The rate of change at which the muscles are lengthening.

Changes in the length of muscle fibers.



Response characteristics of the stretch receptor


Muscle Spindle Animation


Function of the Gamma System

- Controls the intensity of the stretch reflex.

- Spindle is normally tonically active as a result of input from higher brain centers.

- Performs a damping function by adjusting sensitivity.


Muscle Spindle


Review of reflex arc.


The Stretch Reflex • Muscle spindle stretched in 1 of 2 ways:

– Applying extra force that lengthens whole muscle• What would be an example?

– Activating gamma motor neurons distal ends of intrafusal fibers stretches middle of spindle (internal stretch)

• Once spindles are activated, sensory neurons send AP at higher frequency



Muscle Stretch Reflex


The Stretch Reflex

• Sensory neurons alpha motor neurons excite extrafusal muscle fiber of stretched muscle– Efferent impulse also sent to antagonistic

muscles• Inhibits stretch

– Called reciprocal inhibition



The Stretch Reflex• Also helped by gamma motor neuron reflex arc• As muscle shortens, spindle firing rate decreases,

therefore decreased AP generation by alpha motor neurons

• If stretch reflex alone, jerky movements– Regulate how intrafusal muscle fibers respond



The intrafusal muscle fibers are tiny muscle fibers attached to either end of the stretch receptor. They are innervated by gamma efferent neurons and are not part of the stretch reflex arc.


• This has been a brief description of the muscle stretch reflex arc and how it operates.

• The next series of slides will demonstrate how the muscle stretch reflex arc, the intrafusal muscle fibers, and the gamma efferent neurons are used in local control of muscle fibers.


D—extrafusal muscle fiberE—stretch receptorF—intrafusal muscle fiber


A—stretch receptor afferent neuron


B—alpha efferent neuronThis completes the stretch reflex arc.


Review: what happens when the stretch receptor is stretched? Notice the change in frequency of action potentials.


What events will stretch the stretch receptor?

• Stretch of the entire muscle (tap the patellar tendon, watch the foot jerk!)




OR




OR• Contraction of the intrafusal muscle fiber!





– What would make this happen?


Reminder:The intrafusal muscle fibers are innervated by gamma efferent neurons.





– This will occur any time the gamma efferent fiber stimulates the intrafusal muscle fiber.


C—gamma efferent neuron to intrafusal muscle fiber


What if we could stimulate C? Work through the sequence: C →


What if we could stimulate C? Work through the sequence: C → F


What if we could stimulate C? Work through the sequence: C → F →E


What if we could stimulate C? Work through the sequence: C → F →E →A


What if we could stimulate C? Work through the sequence: C → F →E →A →B


What if we could stimulate C? Work through the sequence: C → F →E →A →B →D


• This seems like a stupid thing to do because it would be easier just to stimulate the extrafusal muscle fiber with the alpha efferent neuron.

• But look what actually happens:


Descending neurons (pyramidal tracts!) stimulate both alpha and gamma neurons.


Both the intrafusal fibers AND the extrafusal fibers contract to the same extent. The stretch receptor doesn’t feel a thing!


What if the load is too big for the muscle to lift? What happens to the stretch receptor?




It becomes stretched (the muscle doesn’t shorten, remember?) and increases the frequency of action potentials along the stretch receptor afferent…..


…which stimulates only the alpha efferent, causing the extrafusal muscle fiber to generate greater amounts of tension.


• This combined stimulation of the alpha and gamma efferent neurons sets up a situation in which there will automatically be an increase in tension of the muscle if the load is too heavy. This combined stimulation of the two neuron types is called alpha-gamma co-activation.


Control of the Gamma Motor System (Fusimotor System)

• Gamma signal excited by the bulboreticular facilatory area of the brain stem.

• Secondarily by areas that send impulses to this area.– cerebellum, basal ganglia, cortex

• Little is known about the precise control of this system.


Clinical Application of the Stretch Reflex( myotatic reflex)

• Knee jerk reflex– striking the patellar tendon with a hammer stretches the

quadriceps muscle.– this initiates a stretch reflex which shortens the muscle

and causes the knee to move forward.

• Can be done with almost any muscle.• Index of the facilitation of the gamma

efferents.• Cortical lesions usually increase muscle

stretch reflexes.



Golgi Tendon Reflex

• Mediated by the golgi tendon organ receptor located in the tendon.

• This receptor responds to tension.• When the tension becomes too great the

reflex inhibits the motor fibers attached to the tendon.

• Function is to equalize force among muscle fibers.


Golgi Tendon Reflex

• Opposite effect of stretch reflex– Muscle relaxation and lengthening in response to

contraction• Polysynaptic• Muscle tension increases during contraction

– Golgi tendon organs activates afferent impulse spinal cord cerebellum

• At same time motor neurons in cord supplying contracting muscle inhibited– Antagonistic muscles activated

• Reciprocal activation– Contracting muscle relaxed, antagonistic activated



Transmission of Stretch Information to Higher Centers

• Muscle spindle and golgi tendon signals are transmitted to higher centers.

• This informs the brain of the tension and stretch of the muscle.

• Information is transmitted at 120 m/sec.• Important for feedback control of motor

activity.


The Withdrawal Reflexes

• A painful stimulus causes the limb to automatically withdraw from the stimulus.

• Neural pathways for reflex:– nociceptor activation transmitted to the spinal cord– synapses with pool of interneurons that diverge the to

the muscles for withdrawal, inhibit antagonist muscles, and activate reverberating circuits to prolong muscle contraction

– duration of the afterdischarge depends on strength of the stimulus


Withdrawal Reflex


Crossed Extensor Reflex

• Painful stimulus elicits a flexor reflex in affected limb and an extensor reflex in the opposite limb.

• Extensor reflex begins 0.2 - 0.5 seconds after the painful stimulus.

• Serves to push body away from the stimulus, also to shift weight to the opposite limb.


14 June 2007 Reflexes.ppt 73

Examples of Reflexes: Crossed extensor reflex

1. Stimulus sensed on one side2. sensory neuron to spinal cord,3. synapse with association neuron; synapse with motor neuron (to

withdraw on same side) & impulse crosses to opposite side of cord, synapse with motor neuron (to extend opposite appendage)

4. Motor neurons to muscle cells/ motor units



Crossed Extensor Reflexes


Neuronal Circuits for Withdrawal

and CrossedExtensor Reflex


The Stretch Reflex


Other Reflexes for Posture and Locomotion

• Positive support reactionPressure on the bottom of the feet cause

extensor reflex.• Magnet reaction• Cord righting reflexesRighting from a lying posture


Stepping and walking movements

Basic walking reflexes reside in the spinal cord.•Rhythmical stepping movements of single limb•Reciprocal stepping of opposite limb•Mark time reflexDiagonal stepping of all four limbs•Galloping reflex


Scratch reflex

• Initiated by itch and tickle sensation• InvolvesPosition senseTo and fro scratching movements


Reflexes that Cause Muscle Spasm

• Pain signals can cause reflex activation and spasm of local muscles

• Muscles around a broken bone.• Inflammation of peritoneum can cause abdominal

muscle spasm.• Muscle cramps caused by painful stimulus in

muscle:– can be due to cold, ischemia, of overactivity– reflex contraction increases painful stimulus and causes more

muscle contraction


Autonomic reflexes in spinal cord• Vascular tone changes in response to temprature• Sweating• Intestino-Intestinal reflexes• Peritoneo-intestinal reflexes• Evacuation reflexes• Mass reflex Spontaneus discharge from large cord segments Flexor spasm of body Colon and bladder likely to evacuate Raised arterial pressure Profuse sweating in large areas of body


Spinal shock

Loss of all spinal functions in response to sudden transaction of cord•Mechanism

Loss of tonic excitation fromReticulospinal tractsVestibulospinal tractsCorticospinal tracts•Spinal functions affected areDecreased arterial blood pressuresBlocked reflex integeration of musclesLoss of sacral reflexes

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spinal cord.ppt