Physiologic anatomy of skeletal muscle. Excitation – contraction coupling in skeletal muscles. Mechanism of muscle contraction. Types of skeletal muscle contractions.

Question 1

skeletal muscle - structure 1

Answer 1

MYOCYTE = basic unit of muscle cell

• long + cylindrical
• sacroplasm is cytoplasm of muscle cell, containing glycogen + myoglobin
• many nucleus peripherally under sacrolemma

Question 2

skeletal muscle - structure 2

Answer 2

sarcolemma

• membrane of muscle cell
• role in conduction of electrical signals

Question 3

skeletal muscle - structure 3

Answer 3

MYOFIBRILS

rod like structure running over the myocyte

made of repeating units called sarcomeres outlined by z-dics

Question 4

skeletal muscle - structure 4

Answer 4

thick filaments = myosin
- contains a tail + x2 heads which form a cross bridge structure with actin during muscle contraction

Question 5

skeletal muscle - structure 5

Answer 5

thin filaments = actin
- g-actin molecules form f-actin molecules
- contains binding site for tropomyosin + troponin which regulate interaction of actin-myosin

Question 6

skeletal muscles - structure 6

Answer 6

A-line
= region where actin-myosin overlap
- causing striation appearance

Question 7

skeletal muscles - structure 7

Answer 7

I-band
= area containing only actin filaments
= located on either side of Z-line

Question 8

skeletal muscles - structure 8

Answer 8

SACROPLASMIC RETICULUM

= endoplasmic reticulum of muscle cells
stores and releases Ca2+ for muscle contraction

Question 9

skeletal muscle - structure 9

Answer 9

TRANSVERSE TUBULES

• invaginations into sacrolemma (muscle cell membrane)
• function for rapid transmission of action potential between sacrolemma to myocyte

Question 10

skeletal muscles - structure 10

Answer 10

NEUROMUSCULAR junction = communication site between motor neurone and muscle fibres

Question 11

skeletal muscle - structure 11

Answer 11

CONNECTIVE TISSUE LAYERS

endomysium = inner layer
perimysium = median layer
epimysium = outer layer

Question 12

skeletal muscle - structure 12

Answer 12

INNERVATION

all skeletal muscles innervated by motor neurone forming motor unit

all skeletal muscle innervated by sensory neurones providing feedback on muscle tension

Question 13

mechanism of contraction - step 1

Answer 13

resting membrane potential

• muscle cell is at rest due to no ions going in/ out
• resting membrane potential is -90mv
• unequal distribution of ions inside/ outside of membrane
• selective permeability of these ions

Question 14

mechanism - step 2

Answer 14

ARRIVAL of ACTION POTENTIAL

• AP travels along neurone down axon
• reach pre-synaptic membranes where motor neurones releases Ach -> synaptic cleft -> post synaptic membrane N receptors

Question 15

mechanism - step 3

Answer 15

EXCITATION-COUPLING MECHANISM

• AP reaches post-synaptic membrane of sacrolemma
• wants to reach myocytes cells inside via T-tubules
• activates the DHPR receptors of T-tubules
• activating RyR receptors of SR

ca2+ influx from SR into cytoplasm

Question 16

muscle contraction - mechanism

Answer 16

1 - calcium binds to troponin C = conformational changes

2 - tropomyosin caused to move away from binding site on actin, leaving it exposed

3 - myosin head binds to exposed site contining ADP + Pi

4 - ADP + Pi pull actin towards centre of sacrolemma = muscle shortens = power stroke = cross bridge formation

5 - ATP renergises another myosin whilst actin removed off myosin

Question 17

muscle relaxation - mechanism

Answer 17

no AP action

• remaining Ach in synaptic cleft is destroyed by AChE
• remaining Ca2+ is actively pumped into SR by Ca2+-ATPase pump + dissociates from troponin
• tropomyosin moves back into myosin binding sites to block ATP+Pi binding
• myocyte returns to original shape

Question 18

types of skeletal muscle contraction - isotonic

Answer 18

muscle changes length whilst maintaining tension

• concentric = muscle length decreases (lifting weight)
• eccentric = muscle length increases (lowering weight)

Question 19

types of skeletal muscle contraction - isometric

Answer 19

muscle produces tension without changing length
- posture
- stability

Question 20

types of skeletal muscle contraction - twitch

Answer 20

twitch
= single rapid contraction followed by relaxation forming only x1 AP

Question 21

types of skeletal muscle contraction - tetanus

Answer 21

tetanus = many AP’s = sustained contraction = maximum force produced