Skeletal System Physiology

Question 1

Muscle Morphology

Answer 1

• Tendon -> muscle -> tendon
• Muscle fiber = single myocyte surrounded by a membrane (sarcolemma)
• Myofibrils = individual filaments within a fiber (separated by sarcoplasmic reticulum)
• Filament (thick & thin) = collection of contractile proteins

Question 2

Contractile Proteins

Answer 2

Myosin
Actin
Tropomyosin
Troponin (3 subunits: I, T, and C)

Question 3

Muscle Filaments

Answer 3

“Striated”- regular position of filaments gives a striped appearance

• Thick filament: myosin proteins = H band
• Thin filament: actin, troponin, & tropomysin = I band
• Thin-thick overlap = A band (constant)

*Sarcomere = functional unit (Z to Z line)

Question 4

Process of Muscle Contraction

Answer 4

1. ATP attaches to myosin head, dislodging it from actin
2. Myosin hydrolyzes ATP to ADP, “cocking” its head into a high energy position
3. Ca2+ enters 7 binds to troponin C
4. Ca2+ & troponin C complex form tropomyosin
5. Altered tropomyosin reveals actin binding site
6. Myosin-ADP complex binds to actin
7. Phosphorus leaves, changing configuration of the myosin head (= power stroke) -> slides actin filaments toward each other

Question 5

Slow Twitch (Type I) Skeletal Muscle Fibers

Answer 5

• Long contraction-relaxation cycle (100 ms)
• Strong, gross, posture-maintaining, sustained movements
• “Red muscle”-contains high density of type 1 fibers
  `“Dark meat”
• Highly oxidative metabolism (aerobic), more mitochondria, capillaries, & myoglobin

EX. Erector spinae, gastrocnemius

Question 6

Fast Twitch (Type II) Skeletal Muscle Fibers

Answer 6

• Quick contraction-relaxation cycle (8 ms)
• Fine, rapid, precise movements
• “White muscle”-contains high density of type 2 fibers
  `“White meat”
• Highly glycolytic metabolism (anaerobic), less mitochondria, capillaries, & myoglobin

EX. Extraocular muscles

Question 7

Types of Contraction

Answer 7

1. Isometric

2. Isotonic

Question 8

Isometric Contraction

Answer 8

• No change in muscle length
• Shortened contractile elements, but no muscle shortening
• “Tensing the muscle before motion begins”

Question 9

Isotonic Contraction

Answer 9

• Same load bilaterally
• Shortened contractile elements leads to shortened muscle
• Shortened muscle works against a load

Question 10

Concentric Movement

Answer 10

Isotonic contraction occurring as muscle shortens
- less force needed = less injury possibilities

Ex. lifting an object against gravity

Question 11

Eccentric Movement

Answer 11

Isotonic contraction occurring as a muscle lengthens
- more force needed = more injury possibilities

Ex. trying to stop slipping on ice by spreading legs -> hip abduction -> “groin pull”

Question 12

Skeletal Muscle Metabolism

Answer 12

• Rest & light exercise: free fatty acids
• Increased exercise intensity: glucose
  free glucose in bloodstream liver & skeletal glycogen stores
• Glycolysis
  aerobic: citric acid cycle anaerobic: tricarboxylic acid cycle (makes lactic acid)

Question 13

Muscle Disorder Basics

Answer 13

Denervation: loss of motor nerve input

• d/t nerve injury
• leads to distal weakness & muscle atrophy
• sxs: sensory neuropathy (numbness, paresthesias, dysesthesias)
• Ex. carpal tunnel syndrome

Myopathy: pathology of the muscle, not due to the supplying motor neurons

• leads to proximal weakness & fibrosed fibers
• sxs: myalgias
• Ex. muscular dystrophy

Question 14

Alpha-Motor Neurons

Answer 14

• Attach to & activate muscles*
• reside in the ventral spinal cord
• send signals via ventral root -> aMNs

Question 15

Muscle Spindles

Answer 15

~10 muscle fibers enclosed in a connective tissue capsule (= intrafusal fibers)

• 2 types: nuclear bag & nuclear chain fibers
• End spindle capsules are continuous w/ the muscle’s tendon
• Sensing nerves = primary Ia afferents
• Motor neurons = y (gamma) motor efferents
• Function: regulate muscle length

Question 16

Primary Ia Afferent Nerves

Answer 16

• sensory nerves in muscle spindles*
• transmit afferent activity from the intrafusal fibers -> spinal cord -> a-MN
• a-MN then sends efferent activity down through the neuromuscular junction -> extrafusal fibers (contract)

= monosynaptic reflex arc

Question 17

Muscle Tone

Answer 17

= resistance of muscle to stretch

Hypotonic–decreased resistance to muscle stretch or flaccid muscle

Hypertonic/spastic–increased resistance to muscle stretch

Question 18

Inverse Muscle Stretch Reflex

Answer 18

Too much muscle tension -> sudden contraction cessation to prevent injury

• 2 synaptic reflex arc
• negative feedback loop (autogenic inhibition)
• sensing structure = golgi tendon organ

Question 19

Golgi Tendon Organ

Answer 19

= Network of nerve endings in the tendons

• function: regulate muscle force
• sensing nerves = primary Ib afferents
• synapse w/ interneurons on a-MNs to decrease their activity (stop contraction)

Ex. Sudden stop in contraction while lifting empty gallon of milk presumed to be full

Question 20

Withdrawal Reflex

Answer 20

• Polysynaptic reflex arc
• Peripheral noxious stimuli (pain or temperature) sensory organs send afferent information to the spinal cord & synapses on a-MNs
  activates ipsilateral flexors inhibits ipsilateral extensors
  `activates contralateral extensors

Question 21

Neuropraxia

Answer 21

Temporary damage to Schwann’s cells (myelin)–“my leg fell asleep”

• demyelination
• best prognosis, probable full recovery

Question 22

Axonotmesis

Answer 22

Injured axon +/- injury to the myelin, endoneurium, and/or perineurium

Question 23

Neurotmesis

Answer 23

Injury to all neuronal layers

• Nerve transection
• Worst prognosis, usually partial to no recovery