What are the various arrangements of skeletal muscle?
Circular - Orbicularis Oris
Convergent - Pec Major
Parallel - Sartorius
Unipennate - Extensor digitorum longus
Fusiform (amorphous structureless fibres): Biceps brachii
Bipennate: Rectus femoris
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
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)
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.
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.
Describe Isotonic Contraction
Constant tension (constant force and equal tone), variable muscle length - the muscle changes length and moves the load
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)
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.
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)
Describe Type I muscle fibres
High myoglobin levels
Rich capillary supply
Endurance activities, posture
Describe Type IIa Muscle Fibres
High myoglobin levels
Red to pink colour
Rich capillary supply
Moderate fatigue resistance
Describe Type IIb Muscle Fibres
Low myoglobin levels
White (pale) colour
Poorer capillary supply
Short, intense movements
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.
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.
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
Describe Communication between Neurones and Muscle
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
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
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
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.
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.
How can force of contraction be increased?
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
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
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.
What is an EMG?
Used for diagnosis in neurology e.g. For motor neurone disease
Electrodes can be placed above or in muscles to record electrical activity.
Describe Relaxation of Muscle
Ca2+ is pumped back into the SR via Ca2+ pumps
Some Ca2+ can bind to calmodulin
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.
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)
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