Muscle Physiology and Reflexes

Resting membrane potential

Plasma membrane is more permeable to potassium than sodium, the cells resting membrane potential is more negative inside than outside.

Depolarization

sudden influx of sodium into cell

Repolarization

Potassium moves out of the cell

action potential

rapid depolarization and repolarization of the membrane that is propagated along the entire membrane from the point of stimulation is

motor unit

one finger moving=one motor unit

subthreshold stimulus

can feel it but fingers don’t move

maximal stimulus

weakest stimulus at which all muscle fibers are being stimulated

muscle twitch

single contraction of skeletal muscle

temporal summation

(incomplete tetanus) muscle is already in a partially contracted state when subsequent stimuli are delivered

complete tetanus

maximum force generated by a skeletal muscle

latent period

is the interval from stimulus application until the muscle begins to shorten

period of contraction

muscle fibers shorten; tracing shows an increasingly higher needle deflection and the tracing peaks

period of relaxation

downward curve of the tracing, the muscle fibers relax and lengthen

threshold stimulus

voltage at which the first perceptible contractile response is obtained (when we first see movement)

Identify and describe the function of each of the five elements of a reflex arc

1. receptor: site of stimulus action

2. sensory neuron: transmits afferent impulses to the CNS

3. Integration center: sensory neuron synapses with one or more other neurons to process the information and generate a response

4. motor neuron: conducts efferent impulses from the integration center to an effector organ

5. effector: muscle fiber or gland cell responds to efferent impulse by contracting or secreting.

Explain why reflex testing is an important part of every physical examination.

evaluates the health of the nervous system, particularly the motor pathways involving the brain and spinal cord.

patellar reflex and importance

tap patellar tendon which stretches the quadriceps muscle which should extend the leg. Helps body maintain posture and helps when you walk or trip. Can determine if the nervous system is working properly and hyporeflexia can suggest a problem with the PNS and hyperflexia can suggest upper motor neuron lesion in CNS

stretch reflex

assesses L2-L4 level of the spinal cord

calcaneal or ankle-jerk reflex and importance

tests first 2 sacral segments of the spinal cord

stretch reflex

tap calcaneal tendon with foot dorsiflexed. Foot should quickly jerk down with gastrocnemius muscle is contracting in response to the stretch. Important for balance and walking. Absent response could suggest problem with nerve pathway from spinal cord to the calf muscle.

plantar reflex and importance

superficial reflex- depend on both functional upper-motor pathways and on the spinal cord-level reflex arc

stimulate cutaneous receptors in the sole of the foot by stroking lateral border of the subjects sole

Causes the toes to flex and move closer together. Damage to the primary motor cortex or the corticospinal tract produces a Babinski sign, an abnormal response in which the great toe moves in an upward direction, and the smaller toes fan out. In a newborn infant, it is normal to see the Babinski response because myelination of the nervous system is incomplete.

ciliospinal reflex and importance

autonomic reflex

gently stroke the mastoid process under subjects ear. Same side of stroke that same pupil should dilate. IPSILATERAL

sympathetic reaction which can help maintain alertness. Horner syndrome: the reflex can be used to diagnose this by accentuating the difference in pupil size between the 2 eyes.