Muscles & Motor Units

Question 1

What are the 3 types of motor behaviour movements?

Answer 1

Reflexes - involuntary muscle contraction elicited by stimulus, some overridden by conscious control

Rhythmic - controlled primarily by spinal cord circuits using paired muscle sets which occur suddenly/voluntarily

Voluntary - self-initiated under conscious control

Question 2

Describe the 3 basic muscle types

Answer 2

Skeletal (0.3m, multi-nucleated, elongated/striated, voluntary control by SNS, overall body motility)

Cardiac (50-100um, single nucleus, branched/striated, involuntary control by ANS, rhythmic heart rate/blood flow)

Smooth (30-20um, single-centre nucleus, spindle/non-striated, lines Res/CVS/dig/reproductive tracts, involuntary control by ANS, force substances into internal channels/valves/arrector pili muscles/pupils

Question 3

Describe 3 general functions of muscles

Answer 3

Maintain posture (skeletal muscles)
Stabilise joints
Produce heat (during contraction to maintain constant body temp.)

Question 4

Describe skeletal muscle anatomy

Answer 4

Nerve stimulates each muscle fibre (made of myofibrils)

Muscle fibres wrapped in endomysium membrane form bundles (fascicles) which are wrapped in perimysium.
Fascicle bundles wrapped in epimysium to form muscles.

These tough CT sheet sheaths transfer force from muscles to tendons/skeletal fixation points

Vast net-like capillary structure supplies nutrients/O2, removes CO2/metabolic waste, allows changes in shape

Question 5

Describe the sarcomere

Answer 5

Repeated myofibril pattern (contractile unit of myofilaments/proteins) bounded at each end by Z disc

Thick myosin filaments represent Dark A band region
Thin actin filaments represent Light I band region
Elastic titin filaments provides elasticity

Question 6

How does neural stimulation cause muscle contraction?

Answer 6

Motor neurons release ACh at synapse of neuromuscular junction causing local depolarisation of sarcolemma resulting in action potential along cell/T-tubules in middle

Action potential causes endoplasmic reticulum to release Ca2+ ions triggering myofibrils to contract

Question 7

How do Ca2+ ions unblock the myosin-binding site?

Answer 7

Sarcoplasmic reticulum releases Ca2+ which binds to troponin on actin to chang cell/tropomyosin (blocker) shape exposing myosin binding site

Question 8

Explain the Sliding Filament model of the actin/myosin contraction cycle

Answer 8

ATP moves myosin from relaxed to cocked (if unavailable myosin can’t detach; permanent contraction/rigor mortis)

1. Cross bridge forms - Ca2+ allow energised (cocked) myosin to bind to actin; ADP/phosphates still on myosin.
2. Power stroke - ADP/Pi released from myosin which pulls actin filament towards M line
3. Cross bridge detaches - myosin hydrolyses ATP forming ADP/Pi, energy released returns myosin to high-energy cocked state for next cycle

Question 9

Describe cardiac muscle anatomy

Answer 9

Striated, involuntary muscle contraction initiated by pacemaker cells in atrial node

Electric connection between neighbouring cells, contraction waves spread across heart, regulated by ANS

Atria contract forcing blood into ventricles, electric signal via atrio-ventricular node/purkinje cells to ventricle apex contracting to expel blood in repeating cycle

Question 10

Describe smooth muscle anatomy

Answer 10

Walls of hollow visceral organs (gut/stomach/BVs/airways)
Involuntary muscles indicated by pacemaker cells/ANS nerves controlling iris dilation

Direct electric connection between gap junctions, spontaneous contraction upon stretch

Longitudinal and circumferential layer arrangement allows peristalsis

Question 11

Explain 3 production pathways for the energy source ATP

Answer 11

1. Direction phosphorylation - 1 ATP per creatine phosphate from CP + ADP
2. Anaerobic pathway - 2 ATP per glucose + lactic acid from glycogen breakdown
3. Aerobic pathway - 32 ATP per glucose + CO2 + H2O from glycogen/pyruvic acid/fatty acids/amino acids using O2

Question 12

What are the rules of skeletal muscle activity?

Answer 12

All skeletal muscles cross at least 1 joint with it’s bulk lying proximal to the joint
All skeletal muscles have at least 2 attachments (insertion moving towards origin during contraction)
Skeletal muscles only pull, never push

Question 13

Explain the lever effect

Answer 13

effort x distance effort applied = load x distance load is moved

= work done on either side of fulcrum, shorter distance means larger force applied

Question 14

Describe the 3 classifications of levers

Answer 14

1st Class: Load > Fulcrum > Effort (can be adv or disadv) tilts head up

2nd Class: Fulcrum > Load > Effort (adv for small, powerful movement) stands on tiptoes

3rd Class: Load > Effort > Fulcrum (disadv for fast, large movement) curling weight

Question 15

Describe agonists, antagonists and synergists

Answer 15

Agonist: muscles responsible for specific movement

Antagonist: opposes/reverses specific movement

Synergist: acts with agonists/antagonists stabilising joints while the other 2 act

Question 16

What factors affect force of muscle contraction?

Answer 16

Degree of muscle stretch
Frequency of stimulation
No. of muscle fibres recruited
Size of muscle fibres

Question 17

Explain muscle twitch

Answer 17

Latent period
Contraction period
Relaxation period

Rapid brief contraction (eye muscles)

Slow sustained (gastrocnemius/soleus in calf)

Question 18

Describe 4 frequencies of stimulation

Answer 18

Individual distinct twitches
Temporal summation: twitches overlap for partial relaxation
Infused tetanus: max tension with some relaxation
Fused tetanus: high tension plateau with no relaxation

Question 19

Describe muscle tension due to fibre recruitment

Answer 19

More motor units excited as stimulus increases to hit threshold stimulus where contract begins

Question 20

Explain size principle of fibre recruitment in muscles

Answer 20

Smallest fibres activated first for small movement/fine control

Larger fibre motor units excite = increased contraction strength

Question 21

Which factors increase muscle force?

Answer 21

High stimulation frequency
More muscle fibres recruited
Larger muscle fibres
Muscle/sarcomere stretched beyond 100% of resting length

Question 22

Describe 3 types of muscle fibre

Answer 22

Red Type I: slow twitch, small

Pink Type IIA: fast, mid-size

White Type IIB: very fast, large

Question 23

Describe 3 types of motor units

Answer 23

Type S (slow): 50-110ms twitch, few Type I fibres, small force, oxidative metabolism/many mitochondria increase FR, recruited 1st in contraction

Type FR (fast fatigue resistant): 25-45ms, (myosin isoform) intermediate no. Type IIA fibres/force, OxMetab increase FR, recruited after S

Type FF (fast fatiguable): <10ms, (myosin isoform) lot of Type IIB fibres/force, anaerobic metabolism of glycogen store rapidly used

Question 24

Explain muscle performance during exercise like running

Answer 24

Increased O2 demand, increase capillaries around muscle fibres which synthesise myoglobin (stores O2 in fibres)

Mitochondria/ATP increase for efficient metabolism and endurance against fatigue

Question 25

Explain muscle performance during lifting

Answer 25

Effect mainly type IIB fibres (fast, glycolytic), switch to these from type IIA (fast oxidative)