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Flashcards in Session 2 Deck (28):
0

What are the various arrangements of skeletal muscle?

Circular - Orbicularis Oris

Convergent - Pec Major

Parallel - Sartorius

Unipennate - Extensor digitorum longus

Multipennate: Deltoid

Fusiform (amorphous structureless fibres): Biceps brachii

Bipennate: Rectus femoris

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1

Describe the structure of skeletal muscle

Epimysium: tough outer connective tissue layer, continuous with tendon - distribute force onto tendon

Perimysium: surrounds fascicles (bundles of muscle fibres)

Endomysium: surrounds individual muscle fibres A fascicle is a functional unit

2

Describe the types of levers

First class levers: (skull is balanced on top of the cervical vertebrae) see - saw arrangement (most efficient)

Second class levers e.g. In the leg: not very efficient

Third class levers: least mechanically efficient - biceps brachii at the elbow joint (fulcrum)

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3

Define the muscle groups

Agonists: prime movers (main muscles responsible for a particular movement)

Antagonists oppose agonists. Agonist-antagonist allow control of fine movement.

Synergists: help agonists by producing the same movement or by reducing undesirable movements (neutralising extra motion).

Fixators: (specialised Synergists) - hold a bone still or stabilize the origin of a prime mover so all the tension can be used to move the insertion bone. The postural muscles that stabilize the vertebral column are fixators.

4

Discuss Compartment Syndrome

Each group of muscles in the limbs together with nearby neurovascular structures is contained within an enclosed space surrounded by fascia. The tough fascia do not easily stretch or expand easily so the pressure inside the compartment can easily increase if bleeding or swelling occurs.

Compartment Syndrome occurs when there is bleeding inside, increasing the pressure on the muscle therefore causing nerve damage due to decreased blood supply.

This can case paraesthesia, pain, swelling and movement restriction.

5

Describe Isotonic Contraction

Constant tension (constant force and equal tone), variable muscle length - the muscle changes length and moves the load

Two types:

Concentric ( muscle shortens e.g. Biceps shorten and flex the elbow joint when lifting load with the arm)

Eccentric (muscle exerts a force while being extended e.g. Walking downhill - can cause delayed onset muscle soreness)

6

Describe Isometric Contraction

Constant length, variable tension e.g. Hand grip. Muscle is exerting a force but length stays the same - muscles do not shorten.

7

What are the Muscle Fibre Types?

Type I (Slow Oxidative)

Type IIa (Fast Oxidative)

Type IIb (Fast glycolytic)

(2 types of fast twitch fibres and 1 type slow twitch)

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8

Describe Type I muscle fibres

Slow oxidative

Aerobic

High myoglobin levels

Red colour

Many mitochondria

Rich capillary supply

Fatigue resistant

Endurance activities, posture

9

Describe Type IIa Muscle Fibres

Fast oxidative

Aerobic

High myoglobin levels

Red to pink colour

Many mitochondria

Rich capillary supply

Moderate fatigue resistance

Walking, sprinting

10

Describe Type IIb Muscle Fibres

Fast Glycolytic

Anaerobic glycolysis

Low myoglobin levels

White (pale) colour

Few mitochondria

Poorer capillary supply

Rapidly fatigable

Short, intense movements

11

Describe the feedback control of movement

Muscle spindles are proprioreceptors (sensory receptors) within the belly of a muscle. They detect changes in the length of this muscle and convey this information via sensory neurones to the brain to determine the position of body parts.

The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motoneurones via the stretch reflex to resist muscle stretch.

12

Where and what are Muscle Spindles?

Found within the belly of muscles, embedded in extrafusal muscle fibres.

Muscle spindles are composed of 3-12 intrafusal muscle fibres.

Muscle spindles are also known as proprioreceptors.

13

What is a Motor Unit?

A motor neurone and the muscle fibres it innervates.

A single motor neurone will innervate a particular number of muscle fibres.

The fewer muscle fibres a motor neurone innervates, the finer the control of movement

14

Describe Communication between Neurones and Muscle

'Crosstalk'

Signalling molecules communicate between nerve and muscle.

Atrophy of nerve or muscle can lead to atrophy of the corresponding neurone or muscle.

Maintaining crosstalk is really important.

Examples: neurotrophins, cytokines, insulin-like growth factors

15

What is muscle tone?

Muscle never relaxes completely; muscle tone is baseline tone present in muscles at rest due to motor neurone activity and muscle elasticity (state of continuous partial contractions).

It is the result of different motor units scattered throughout the muscle, being stimulated by the nervous system in a systematic way.

Muscle tone enables the muscle to remain firm, healthy and constantly ready for action.

Control of muscle tone via: motor control centres in brain and afferent fibre signals originating in the muscle

16

What is Hypotonia and what could it be due to?

Decreased muscle tone could be due to:

Lesion (abnormality) of sensory afferents from the muscle spindles

Primary degeneration of the muscle (myopathies) Lesion of lower motor neurones e.g. Poly neuritis

Cerebral or Spinal Neural Shock

Lesions of the Cerebellum

Muscle relaxants - anaesthetics

Sarin gas - neuromuscular blocking agent

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17

Describe Excitation Contraction Coupling

1. AP reaches axon terminal of muscle neuron

2. Ca2+ channels open and Ca2+ enters the axon terminal 3.

Ca2+ entry causes some synaptic vesicles to release their contents e.g. ACh by exocytosis

4. ACh diffuses across the synaptic cleft and binds to receptors in the sarcolemma on the post-synaptic neurone (motor end plate)

5. Channels open that allow simultaneous influx of Na+ and efflux of K+ out of the muscle fibre. This causes depolarisation.

6. Depolarisation spreads along T tubules, causing a conformational change in the voltage sensor proteins. This stimulates the SR to release Ca2+ through gated channels in the adjacent terminal cisternae.

7. Ca2+ is rapidly released from the terminal cisternae into the sarcoplasm.

8. Ca2+ binds to the TnC subunit of troponin. Causes a conformational change which moves Tropomyosin away from actin's myosin binding sites.

9. Contraction cycle is initiated and Ca2+ is returned to the terminal cisternae of the SR.

18

Describe the Sliding Filament Theory

1. Attachment (myosin head tightly bound to actin molecule)

2. Release (ATP binds to the myosin head causing it to uncouple from the actin filament)

3. Bending (hydrolysis of the ATP causes the uncoupled myosin head to bend and advance 5nm)

4. Force generation (myosin head binds weakly to the actin filament causing release of inorganic phosphate which strengthens binding and causes the power stroke - myosin head returns to former position)

5. Reattachment (ATP binds to the myosin head causing detachment from actin. The myosin head will bind tightly again and cycle will repeat)

Muscle shortens as the thick and thin filaments slide past each other.

19

How can force of contraction be increased?

Recruitment

Summation

20

Explain Recruitment

Spatial summation

More motor neurones activated so more muscle fibres recruited to develop more force.

Reflex pathways from muscle spindles, joint receptors, Golgi tendon organs have a role in recruitment

21

Explain Summation

Temporal Summation: increased frequency of action potentials to muscle fibres cause summation tetanus

Wave Summation: increasing the frequency of stimulation of muscle fibres. Can be a form of incomplete or complete tetany

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22

What is Tetanus?

Incomplete (Unfused) Tetanus: muscle fibres are stimulated at a rate where they don't completely relax before the next stimulus; partial relaxation between twitches.

Complete (Fused) Tetanus: high rate of stimulation where the the muscle fibres do not relax between stimuli. Tetanus causes painful tightening of the muscles.

Twitch: a single rapid contraction and relaxation of a muscle fibre or a group of muscle fibres, due to a single AP of the motor neurone.

23

What is an EMG?

Electromyography

Used for diagnosis in neurology e.g. For motor neurone disease

Electrodes can be placed above or in muscles to record electrical activity.

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24

Describe Relaxation of Muscle

Ca2+ is pumped back into the SR via Ca2+ pumps

Some Ca2+ can bind to calmodulin

25

Describe the sources of energy for contraction

1. Short term stores of ATP in muscle fibre (a few seconds)

2. Phosphorylation of ADP by Creatine Phosphate lasts up to 15 seconds (100m sprint). Enzyme: Creatine kinase.

3. Anaerobic Glycolysis and Lactate formation (20 to 40 seconds; lactic acid causes muscle cramp)

4. Aerobic Respiration (Oxidative Phosphorylation) - prolonged aerobic muscular events.

26

What are the 2 types of Muscle Fatigue?

Central (in the brain)

Peripheral fatigue - depletion of muscle glycogen stores, occurs within 1 minute if blood flow interrupted, intermittent claudication (pain during exertion, stops at rest) is common in elderly people when they are walking due to narrowing or blockage of the femoral artery)

27

What is Contracture?

A state of continuous contraction

Occurs when ATP is depleted as the myosin cross bridges are unable to detach from actin filaments.

Comes on 3 hours after death - peaks at 12 hours, disappears after 72 hours