Block 5 W3

Question 1

What are the 3 types of movements?

Answer 1

Reflex
Rhythmic
Voluntary

Question 2

What are alpha motor neurones?

Answer 2

Found in ventral horn of SC.
Thick and myelinated (AP 70-120 m/sec).
Release ACh at terminal.

Question 3

What is the function of alpha motor neurones?

Answer 3

Release of ACh -> contracts muscle fibres for movement.

Lower motor neurones directly activate muscles.

Question 4

What are the inputs of the alpha motor neurones?

Answer 4

Muscle spindles (Ia afferents - proprioception)
Golgi tendon organs (Ib afferents)
Cutaneous receptors
Spinal interneurones
Upper motor neurones

Question 5

Define motor neurone disease.

Answer 5

Amyotrophic lateral sclerosis (ALS).

Progressive degeneration of alpha motor neurones.

Question 6

What are the symptoms of ALS?

Answer 6

Muscle weakness, atrophy, twitching, abnormal reflexes, difficulty breathing.

Question 7

What is the treatment of ALS?

Answer 7

Riluzole - blocks glutaminergic neurotransmission.

Question 8

Differentiate between UMN and LMN.

Answer 8

UMN - neurones in cerebral cortex and brainstem whose axons remain in CNS and synapses on lower motor neurones directly or indirectly.

LMN - neurones in brainstem and spinal cord whose axons leave the CNS to synapse on muscle fibres. Final coming pathway.

Question 9

Describe a reflex arc pathway.

Answer 9

Stimulus activates sensory receptor, which sends info via afferent sensory neurones to integration centre (CNS), which sends info via efferent motor neurones to the effector organ to perform the response.

Question 10

Define reflex.

Answer 10

Involuntary movement that occurs as a result of sensory stimulation and involves impulses travelling through a reflex arc.
Protects body and coordinates muscle activity.

Question 11

What does the alpha motor neurones control?

Answer 11

Extrafusal muscle.

Question 12

What does the gamma motor neurones control?

Answer 12

Intrafusal muscle.

Question 13

What is a muscle spindle?

Answer 13

Runs in parallel with muscle
Consist of:
- intrafusal muscle
- Ia sensory neurones
- gamma MN
Stretching causes increase in Ia afferent activity.
For proprioception

Question 14

What are gamma motor neurones?

Answer 14

Maintains tautness of muscle spindle.
Adjusts muscle spindle length to match extrafusal muscle length.
Alpha and gamma - co-activated.

Question 15

Describe the myotactic reflex.

Answer 15

Single synapse between sensory fibre and alpha MN (monosynaptic).
Sensory fibre activation quickly activates the alpha MN, contracting muscle fibres.

Question 16

Describe the patella tendon tap (knee jerk) reflex.

Answer 16

Hammer taps patellar tendon.
Stretches quadriceps muscle and activates Ia afferents.
Impulses travel to SC and activate alpha motor neurones.
Motor neurones release ACh -> contrition of quadriceps = kick.

Question 17

Describe the reciprocal inhibition in stretch reflex.

Answer 17

Patellar tendon reflex involves 2 simultaneous actions:
1. monosynaptic stretch reflex contracts agonist muscle.
2. reciprocal inhibition of the antagonist muscle.
Allows contraction of quadriceps to proceed unopposed so antagonist muscle (hamstrings relax) inhibited via inhibitory interneurone.

Question 18

Define Golgi tendon organ and its location.

Answer 18

Made of collagen and neurones.
Location - junction of muscle and tendon, runs in series with extrafusal muscle.
Innervated by Ib afferents.

Question 19

Why is Ib afferents described as high threshold receptor?

Answer 19

Muscle stretch produces little change in neural activity.

Question 20

How does Ib afferents cause muscle contraction?

Answer 20

Force generated acts directly on tendon to increase tension of collagen fibrils in Golgi organ -> causes compression of the intertwined sensory receptors and increases Ib afferent activity.

Question 21

What is the function of Golgi tendon organ?

Answer 21

Encodes and regulates muscle tension -> prevents muscle overload (if weight too heavy - automatically drop)

Question 22

Describe the inverse stretch reflex.

Answer 22

Ib afferents from Golgi tendon organ synapse with inhibitory interneurones in SC.
Activation of Ib decreases activity of alpha MN.
When tension is too much, contraction suddenly ceases and muscle relaxes.

Question 23

Describe the flexor withdrawal/crossed extensor reflex.

Answer 23

1. stepping on pin stimulates nociceptive cutaneous receptors.
2. signals travel through afferent neurone to interneurones in lumbar SC.
3. interneurones synapse with alpha MN on both sides.
4. alpha MN flexes hamstring to bend knee withdrawing stimulated leg.
5. alpha MN flexes quadriceps on opposite side to extend the leg for compensatory postural support.

Question 24

Define central pattern generator.

Answer 24

Neural networks that produce oscillatory (rhythmic) patterned outputs without sensory feedback.
Present in SC and brainstem - consists of local oscillatory circuit of sensory, interneurones and motor neurones.

Question 25

What is the role of CPG?

Answer 25

Produces alternating flexion and extension (rhythm) of limbs during locomotion. Automatic control of rhythmic movements for survival.

Question 26

Why are there cervical and lumbar enlargements?

Answer 26

Have large ventral horn, reflecting large number of motor neurones for arms and legs.

Question 27

How are the muscles represented on the spinal cord?

Answer 27

Axial -> distal (medial -> lateral)

Question 28

Describe the layers of the muscle.

Answer 28

Skeletal muscle (epimysium) -> muscle fascicles (perimysium) -> muscle fibre (endomysium) -> myofibril (sarcoplasmic reticulum) -> sarcomere.

Question 29

What are satellite cells?

Answer 29

Precursors to skeletal muscle cells. Quiescent mononucleated myogenic cells.

Question 30

Describe the sarcomere.

Answer 30

I-band -> thin actin filaments A-band -> actin and myosin (dark) M-line -> connects myosin filament with sarcomere H-band -> only myosin Z-line -> separates sarcomere, contains alpha-actin Titin - anchors myosin to Z-line Nebulin - determines length

Question 31

Describe the structure of myosin.

Answer 31

2 heavy chains and 4 light chains. Heads - bind actin (cross-bridges). Hinge.

Question 32

Describe the structure of actin.

Answer 32

``` Made of troponin and tropomyosin. Folded in double-strand alpha helix. Troponin complex: - TnT binds tropomyosin - TnC binds Ca2+ ions - TnI binds actin ```

Question 33

Describe neuromuscular transmission.

Answer 33

1. depolarisation opens voltage-gated Ca+ channels -> Ca2+ influx -> CaMII binds to synaptic vesicles. 2. vesicle docks to presynaptic membrane and ACh exocytose. 3. ACh binds nicotinic ACh receptors on motor end plate. 4. opens ions channels for Na+ and K+ -> depolarisation/end plate potential. 5. above threshold, AP triggered.

Question 34

Describe the relaxation of muscle.

Answer 34

- pump Ca2+ out the cell - Ca2+ pump sequesters Ca2+ within the SR - Ca2+ bound in SR by calreticulin and calsequestrin.

Question 35

Describe excitation-contraction coupling.

Answer 35

1. excitation spreads along the muscle membrane and into transverse tubule invaginations 2. muscle membrane consists of voltage-activated L-type Ca2+ channels (DHP receptors) arranged in clusters. 3. clusters are adjacent to Ca2+ channels in SR membrane (ryanodine receptors). 4. arrival of AP in T-tubule opens DHP receptors and Ca2+ enter the muscle fibre. 5. Ca2+ entry causes conformational change in ryanodine receptors -> opens and releases Ca2+ into sarcoplasm. 6. Ca2+ binds troponin C, troponin I moves away from actin and troponin T displaces tropomyosin -> cross-bridge cycling.

Question 36

Describe the cross-bridge cycling.

Answer 36

1. ATP binds myosin head, disengaging from actin. 2. Hydrolysis of ATP causes change in angle (90) of myosin head. 3. Head forms cross-bridge with actin two positions along. 4. Phosphate releases and causes head to change angle (45), filaments slide past each other 5. ADP is released.

Question 37

Describe the length-tension relationship.

Answer 37

At a sarcomere length of about 2.0-2.4µm (which is very close to the resting length in most muscles), the zone of overlap in each sarcomere is optimal, and the muscle fibre can develop maximum tension. As the sarcomeres are stretched to a longer length, the zone of overlap shortens, and fewer myosin heads can make contact with thin filaments. Therefore, the tension the fibre can produce decreases. When a skeletal muscle fibre is stretched to 170% of its optimal length, there is no overlap between the thick and thin filaments. Because none of the myosin heads can bind to thin filaments, the muscle fibre cannot contract, and tension is zero. As sarcomere length becomes increasingly shorter than the optimum, the tension that can develop again decreases. This is because thick filaments crumple as they are compressed by the Z discs, resulting in fewer myosin heads making contact with thin filaments. Normally the resting muscle fibre length is held very close to the optimum by firm attachments of skeletal muscle to bones (via their tendons) and to other inelastic tissues.

Question 38

What is the role of intramuscular connective tissue framework?

Answer 38

Distributes the forces passively imposed on a muscle by stretching.

Question 39

Define rigor mortis.

Answer 39

State of muscular rigidity, begins 3-4 hours after death. Ca2+ diffuse out of SR and allows myosin head to bind actin. No ATP synthesis so cross-bridges form and muscles locked in contraction.

Question 40

Define muscle twitch.

Answer 40

Response to stimuli that causes AP in one or more muscle fibres. 3 phases - lag, contraction and relaxation.

Question 41

What is size principle?

Answer 41

Low stimulation, small motor neurones controlling small diameter muscle fibres recruited first. As stimulus increases, larger motor neurones progressively recruited.

Question 42

Define isometric contraction.

Answer 42

Muscle develops tension but doesn't shorten.

Question 43

Define isotonic contraction.

Answer 43

Muscle develops tension and causes movement of a load.