Functions of the Vestibular Apparatus

Functions of the Vestibular Apparatus

Control of Muscle Function by the Spinal Cord and Brain Stem

Motor Signals

•Cause the muscle contraction, secretory function, other motor effects throughout the body.

Sensory Information

•Integrated at all levels of the nervous system.

•Causes appropriate motor responses.

•Cerebrum –where the most complicated responses are controlled.

Responses:

•Begin in the spinal cord with simple reflexes.

•Extend into the brain stem with still more complicated responses.

•Extend to the cerebrum.

Motor Functions of the Spinal Cord

•The cord grey matter is the integrative area for the cord reflexes and other motor functions

•Sensory signals enter the cord through the posterior (sensory) roots.

Signals travel to two separate destinations.

•One branch of the sensory nerve terminates in the grey matter of the cord and elicits local segmental reflexes and other effects.

•Another branch transmits signals to higher levels of the nerve system –to higher levels in the cord itself- to the brain stem, or even to the cerebral cortex.

Neurons

Several million neurons are in the grey matter of the spinal cord between one spinal nerve and the next.

•Sensory relay neurons

•Anterior motor neurons

•Interneurons

The anterior Motor Neurons

•Located on the anterior roots of the cord gray matter

•Several thousand of them

•Fifty to 100% larger than the others

•Give rise to the nerve fibers that leave the cord via the anterior roots and innervate the skeletal muscle fiber

The neurons are of two types

(1) Alpha motor neurons

•Give rise to Aαnerve fibers that innervate the large skeletal muscle fibers

•Motor Units- Stimulation of a single nerve fiber excites three to several hundred skeletal muscle fibers.

(2) The Gamma Motor Neurons

•Smaller

•Transmit impulses through the Aγfibers to very small intrafusal fibers

Intrafusal Fibers

•Are part of the muscle spindle

•Help control muscle contraction

Interneurons

•Located in all area of the cord gray matter (dorsal, anterior, and in between)

•More numerous than anterior motor neurons

•Small and highly excitable firing 1500 times/second

•Have interconnections one with the other

•Innervate the anterior motor neurons

Interneurons

•Interconnections between interneurons and anterior motor neurons are responsible for the integrative functions of the spinal cord.

Types of Neuronal Circuits

Neuronal circuits found in the interneuron pool of cells of the spinal cord include:

–Diverging circuits

–Converging circuits

–Repetitive-discharge circuits

Multisegmental Connections in the Spinal Cord

The propriospinal fibers:

•Ascending and descending nerve fibers in the spinal cord

•Fibers that run from one segment of the cord to another

•Provide pathways for the multisegmental reflexes (reflexes that coordinate simultaneous movements in the forelimbs and hindlimbs)

Sensory receptors

•(1) Muscle spindles- distributed throughout the belly of the muscle; send information to the nervous system about either the muscle length or rate of change of its length.

•(2) Golgi tendon organs- located in the muscle tendons; transmit info about tendon tension.

Receptor Function of the Muscle Spindle

•Each spindle is built of around 3 to 12 intrafusal (small skeletal) muscle fibers pointed at their ends and attached to the glycocalyx of the extrafusal skeletal muscle fibers.

•Central portion doesn’t contract as the ends do (few actin and myosin filaments); It functions as sensory receptor.

•End portions (contract)- excited by small γmotor nerve fibers (originating from the γmotor neurons) in the anterior horns of the spinal cord.

Sensory Innervation of the Muscle Spindle

Receptor area of the spindle has two types of sensory nerve endings:

•The Primary Ending- A large type Ia sensory fiber innervates the center of the spindle receptor

•The Secondary Ending- type II nerve fibers innervate the receptor to one side of the primary ending (The fiber spirals around the intrafusal fiber; fiber stretches stimulating nerve ending)

Static Response of Primary and Secondary Endings

•Receptor portion of the muscle spindle is stretched slowly, the number of impulses transmitted increases directly proportional to the degree of stretch, and the endings transmit the impulses for many minutes.

•The receptor responds to a change in length of the spindle and also continues to transmit its signal for a prolonged period of time.

Dynamic Response of the Primary Ending

•The primary (but not the secondary) ending responds to rapid changes in length transmitting increased/decreased no. of impulses into the Ia fiber while the length is increasing/decreasing

•Primary ending sends strong signals to the spinal cord to apprise it of any changes in length of the spindle receptor area.

Intrafusal/Extrafusal Muscle Length

Two ways in which the spindle can be stimulated:

•(1) By stretching the whole muscle

•(2) By contracting the intrafusal muscle fiber while the extrafusal fiber remains at the normal length

The Muscle Spindle is the Comparator

Length of the two types of muscle fibers is compared:

•Length of extrafusal fibers > length of intrafusal fibers → Spindle excited

•Length of extrafusal fibers < length of intrafusal fibers → Spindle inhibited

The Stretch (Myotatic) Reflexhas two components

The Dynamic Stretch Reflex

•Rapid stretch of the muscle causes potent signal while the length of the muscle is increased.

•Signal goes directly to the anterior motor neurons without passing through interneurons.

•This sudden stretch of the muscle causes reflex contraction of the same muscle opposing further stretch of the muscle.

The Static Stretch Reflex

•After the muscle has been stretched to its new length (dynamic), a weaker (static) stretch reflex continues for a long period of time (elicited by static receptor signals excessive length transmitted from primary and secondary endings of muscle spindles).

It continues to cause muscle contraction.

The Dumping Function of the Stretch Reflex

•Signals from other parts of the nervous system are unevenly transmitted to a muscle in an increasing then decreasing fashion followed by a change to another intensity level.

The Load Reflex

•Biceps is contracted and forearm is horizontal to the earth.

•Five lb. weight is put in the hand.

•Degree of activity of static muscle fiber reflex determines the extent of the hand drop.

•Very active static reflex causes strong contraction of the skeletal muscle fiber in the biceps limiting the degree of fall and maintaining the forearm in a horizontal position.

The Gamma Efferent System

•The gamma efferent stimulation of the spindle muscle fibers tenses these fibers and enhances the excitability of the muscle spindles.

•The bulboreticular facilitatory region and its allied areas of the brain stem transmit excitatory signals through the gamma nerve fibers of the muscle spindles; this shortens the ends of the spindles and stretches the central receptor regions increasing their signal output.

The Muscle Spindle System Stabilizes Body Position

•Spindles on both sides of each joint are activated at the same time increasing reflex excitation of the skeletal muscle on both sides of the joint.

•Joint becomes stabilized and force tending to move the joint is opposed by sensitized stretch reflex.

•Movements of the fingers require intricate motor procedures.

The Golgi Tendon Reflex

•Is an encapsulated sensory receptor through which a small bundle of muscle tendon fibers pass.

•Ten to 15 muscle fibers are connected in series with each Golgi tendon organ and the organ is stimulated by the tension produced by the small bundle of muscle fibers.

•While the spindle detects changes in the muscle length, the tendon organ detects changes in muscle tension.

The Muscle Tension

•Like the primary receptor of the muscle spindle, the tendon organ has both dynamic and static responses responding when tension increases (dynamic response) and settling down to a lower level of steady-state firing (static response).

•When the Golgi organs of a muscle are stimulated by increased muscle tension, signals are transmitted into the spinal cord causing reflex inhibition in the muscle and preventing the development of too much tension in the muscle.

•Fibers that exert excess tension become inhibited while those that exert too little tension become more excited (absence of reflex inhibition), spreading the muscle load over all the fibers and preventing muscle damage.

Neural Circuit of the Tendon Reflex

•The dorsal spinocerebelar tracts carry instantaneous information from the muscle spindles and the Glogi tendon organs to the cerebellum (120m/s).

•Additional pathways transmit similar info into the reticular regions of the brain stem and to the motor areas of the cerebral cortex.

Flexor Reflex

•Also called nociceptor/pain reflex

•Painful stimulus is applied to the hand and flexor muscle of the upper arm becomes excited.

•Flexor reflex eliciting pathway: first into the interneuron pool and then to the anterior motor neurons.

Signals traverse many types of circuits

•Diverging circuits to spread the reflex to the necessary muscle for withdrawal

•Circuits to inhibit the antagonist muscle (reciprocal inhibition circuits)

•Circuits to cause a prolonged repetitive afterdischarge (after the stimulus is over)

Reciprocal Inhibition

•Excitation of a group of muscles is associated with inhibition of another group.

•When a stretch reflex excites one muscle, it inhibits the antagonist muscle at the same time.

•Reciprocal innervation refers to the neural circuit that causes the reciprocal relationship.

•Reciprocal relationships exist between the two sides of the cord (ex. flexor and extensor reflexes).

The Positive Supportive Reaction:

•The pressure on the foot pads of an animal whose spinal cord was transected will by reflex stiffen the limbs sufficiently to support the weight of the body.

•Complex circuit exists in the interneurons.

•The locus of the pressure on the foot pad determines the direction in which the limb is extended.

The Rhythmical Stepping Reflex

•When the lower portion is separated from the remainder of the spinal cord and a longitudinal section is made down the center of the cord to block neuronal connections between the two limbs, each hind limb can still perform individual stepping functions.

•Cycle of forward reflection of the limb followed by backward extension takes place.

•Every time stepping occurs forward in a limb, the opposite limb steps backward (reciprocal innervation between the two limbs).

The “Mark Time Reflex”

•Stepping reflex that involves all four limbs occurs diagonally between the forelimbs and hindlimbs (reciprocal innervation occuring the entire distance up and down the cord between the fore- and hind limbs).

The Galloping Reflex

•Both forelimbs move backward while both hindlimbs move forward.

•If stretch or pressure stimuli are applied (1) equally to opposite limbs at the same time- a galloping reflex will result.

(2) unequally on one side- diagonal walking reflex is elicited.

Abdominal Spasm in Peritonitis

•Is the type of local muscle spasm caused by a cord reflex resulting from irritation of the parietal peritoneum by peritonitis.

•Same type of spasm occurs during surgical operation. Pain impulses from the parietal peritoneum cause the abdominal muscle to contact and squeeze the abdomen and extrude the intestines through the surgical wound.

•Deep anesthesia is required for intra-abdominal operation.

Muscle Cramp

•A small amount of initial irritation causes more and more contraction until a muscle cramp ensues.

•Severe cold, lack of blood flow to the muscle, or exercise of the muscle elicit sensory impulses (pain) that are transmitted from the muscle to the spinal cord causing reflex muscle contraction.

•Contraction stimulates sensory receptors more and a positive feedback mechanism occurs.

Spinal Shock

•Spinal cord is transected in the neck and all cord functions (including cord reflexes) are blocked.

•Normal activity of the cord neurons depend on continual signals from higher centers transmitted through:

-The vestibulospinal tract

-The reticulospinal tract

-The corticospinal tract

Spinal Functions Affected during Spinal Shock:

•The arterial blood pressure falls immediately (40 mmHg) illustrating that activity in the sympathetic nerves to the blood vessels and heart is blocked.

•All skeletal muscle reflexes are blocked.

•The sacral reflexes for control of bladder and colon emptying are suppressed.

Brain Stem Anatomy

- Medulla

- Pons

- Mesencephalon

It is an extension of the spinal cord upward into the cranial cavity because it contains motor and sensory nuclei that perform motor and sensory functions for the face and head regions in the same way the anterior and posterior gray horns of the spinal cord do (from the neck down).

Brain stem provides special control functions:

-Control of respiration

-Control of cardiovascular system

-Control of gastrointestinal function of the body

-Control of stereotyped movements

-Control of equilibrium

-Control of eye movement