3/23/99

Physiology II

 

Muscle Spindles

·        Responds to stretch

·        Annular—wraps around the segment, makes contact when at rest; when stretched it affects the contact

·        Rate receptors—rate of change

·        Static function—steady state of the muscle—requires more input

·        Dynamic function—change is occurring w/in the muscle—little control mechanism

·        Nuclear bags—in the central portion is a accumulation of nuclei; serviced by IA 1° afferent fibers—dynamic—acceleration

·        Nuclear chains—have 1° and 2° afferent also; static, maintains the force

·        Static and dynamic are gamma motor neurons

·        Gamma dynamic—nuclear bags

·        Gamma static—nuclear chains

·        Muscle spindles adapt to some degree, not all the way or we would collapse (~50-60%)

·        Emit signals on a continuous basis

·        Gravity never goes away; if it is taken away we experience many disturbances

·        Motion sickness—difference have what you perceive and what your senses perceive

·        Circuit—input and output of the muscle

·        Reflex arc—simple circuit; cerebral function is not required

·        Cord righting reflex

·        Stretch—2 components

1. dynamic—initiating
2. static—new position maintained

·        negative stretch reflex—sudden shortening of the muscle; protective mechanism; spastic contraction; can counter act to prevent damage

·        smoothness of movement—dampening affect w/in this process; muscle spindles send jerky signals; the negative stretch reflex allows for smooth movement

·        loss of control—clonus; the continuation of the signal after striking the reflex point; disruption of the feedback mechanism

·        muscle spindles help control large skeletal muscle—maintain length, maintain feedback, prevent damage

·        cervoassist process—intra- and extra-fusal fibers have to contract at the same time; functions at the same time or muscle contractions do not function properly

1. allows conscious contraction against load w/o excessive amount of energy
2. take muscle to desired length even when load is changing
3. compensates for fatigue that can affect muscle response

·        patellar reflex—feedback pathway;

·        contraction—Golgi Tendon Organ

·        similar to encapsulated sensory receptors

·        responds to pull on the receptors when the muscle contracts

·        GTO doesn't move when the muscle contracts; it receives the stretch of the muscle

·        Dynamic change in tension and new position (static)

·        Serviced by large IB fibers

·        Similar to 1° nerve endings seen in muscle spindles

·        Local event in reflex arc w/3 neurons

·        Keeps muscle fiber from contracting excessively when the rest of the muscle is not

·        Unilaterally inhibitory

·        Against excessive contraction

·        Equalizes contractile forces

·        Send fibers to cortex, cerebellum (timing of action, coordination) along w/muscle spindle

·        Cord = reflexes

·        Brain stem = involuntary function

·        Cerebellum = timing of events ex:  hitting a baseball

·        Coordination—magnitude of w/draw sensation is related to the intensity of the sensation

·        Pathway of w/draw

·        Go to interneuronal pool

·        Go to motor processes

·        Spastic responses b/c can stimulate several reflex arc

·        Reciprocal circuit—counter balance

·        Pain elicits flexor reflexes

·        After discharge—stimulus have stopped but you still feel it

·        Then fatigue sets in

·        Play a role in posture; very strong reciprocal circuits

·        Elicit a response against gravity

·        Standing responds into pushing down on the feet—positive response

·        Interneuronal circuits—flexor and extensor circuits

·        Walking is controlled by the cord

·        Scratching—some sense to isolate and a circuit to scratch esp for a dog or cat

·        19 different muscles are enabled to allow the dog to scratch

·        peritonitis—reflex if flexion—lessen the intensity; cord reflexes can cause muscle spasm in response to the pain ex:  cramps

 

Brain Stem

·        any voluntary movement requires conscious input

·        start the command in the cortex

·        the contraction of the muscle is from brain stem

·        finger control are not reflex—cortical events

 

Cortex

·        3 areas premotor cortex, supplementary motor cortex and 1° motor cortex

·        1°--one on one functions for given body parts—specific muscles presentations

·        leg does not have a lot of representation—stimulate and the entire lower extremity may move

·        hand—each finger as its own area

·        face—vocalization has the biggest area—disrupt on muscle group and can greatly affect speech

·        1° is closest to the sulcus; in Broadman area 4

·        premotor—Broadman are 6—carry out of task, usage of groups of muscles ex:  cerebral palsy has disruption of muscle groups don't have the ability to carry out a task b/c can't put muscles in the proper position

·        supplemental—near the front; humunculus is different; semi-silent area; need strong stimulus to get a response; background processes to carry out a task ex:  opposable thumb and the ability to grip