Week 1 - A - Skeletal muscle physiology - motor unit, excitation contraction coupling, sarcomere, isotonic/metric, strecth reflex

Question 1

What are the four types of tissue in the human body? Which is the largest type?

Answer 1

Four types of tissue * Epithelial - covers the body surface and lines cavities * Connective - supports and protects body organs * Nervous - communication by transmitting electrical impulses * Muscle - for movement - this is the largest type of tissue in the body

Question 2

Which muscle is striated and which is voluntary? Which nervous system innervates each type of muscle?

Answer 2

Both cardiac and skeletal muscle are striated Skeletal muscle is voluntary however (usually) * Skeletal is innervated by the somatic nervous system (sensory and motor afferents) * Cardiac and smooth muscle are innervated by autonomic nervous system (sympathetic and parasympathetic)

Question 3

What is a motor unit?

Answer 3

A motor unit is a single alpha motor neurone and all the skeletal muscle fibres it innervates

Question 4

How does a motor neurone cause skeletal muscle to contract? (in simple terms)

Answer 4

A motor neurone fires a nerve impulse which causes the muscle fibres it connects to, to contract

Question 5

How does the number of muscle fibres per motor unit vary with the function of the muscles? Give an example of muscles where fine (precision) movement is more important than power Give an example of muscles where power is more important than precision

Answer 5

Muscles which serve fine movements have fewer muscles fibres per motor unit (smaller motor units) * Small motor units - muscles that control fine movement eg intrinsic hand muscles and extraoculuar muscles * Large motor units - weight bearing muscles with less precise movement eg thigh

Question 6

How many muscle cells equals a muscle fibre? What encompasses all the muscle fibres innervated by a single alpha motor neuron?

Answer 6

Each muscle cell equals one muscle fibres All the muscle fibres innervated by a single alpha motor neuron is equal to one motor unit

Question 7

What is the functional unit of muscles? What is the definition of a functional unit?

Answer 7

Functional unit of muscle is a sarcomere (shown in pic) A functional unit is the smallest component capable of performing all of the functions of that organ

Question 8

Where is calcium released from in skeletal and cardiac muscle during muscle contraction? What is the release of calcium in cardiac muscle known as?

Answer 8

In skeletal muscle, calcium is released from the sarcoplasmic reticulum In cardiac muscle, it is released from extracellular fluid (ECF) which triggers the release of large amounts of calcium from the sarcoplasmic reticulum - this is known as calcium induced calcium release

Question 9

Where is it in the skeletal muscle that the motor neuron is able to transmit a signal to the muscle, which results in contraction of the skeletal muscle?

Answer 9

The motor neurone releases a neurotransmitter at the neuromuscular junction (only present in skeletal muscle) This binds to receptors on the motor end plate of the neuromusuclar junction This becomes an action potential which stimulates muscle contraction

Question 10

What is the process through which an action potential transmitted by the alpha motor neurons stimulates the release of calcium resulting in contraction of the muscle fibres due to shortening of the sarcomere known as?

Answer 10

This is known as excitation contraction coupling

Question 11

What is the neurotransmitter released by the axon of the motor neurone at the neuromuscular junction?

Answer 11

This is acetylcholine

Question 12

Once the action potential is generated via the binding of acetylcholine to acetylcholine receptors on the post-synaptic end plate receptor, where does this action potential travel?

Answer 12

The action potential travels along the surface of the sarcomere and then down the transverse (T) tubules

Question 13

What does the action potential travelling down the transverse T tubules of the sarcoplasmic reticulum cause the release of? Where is this substance released from?

Answer 13

Calcium is released when the surface action potential spreads down the tralnsverse (T) tubules of the sarcoplasmic reticulum Calcium is released from the lateral sacs of the sarcoplasmic reticulum known as terminal cisternae

Question 14

What does calcium bind to once it is released from the sarcoplasmic reticulum and what does this cause?

Answer 14

Calcium released from the SR binds to the troponin pulling troponin-tropomysoin complexes aside exposing the active sites on the actin filaments This allows mysoin to bind to actin resulting in cross bridge binding inducing contraction - PROCESS OF EXCITATION CONTRACTION COUPLING explained

Question 15

Myosin cross bridges attach to actin and bend, pulling actin towards the centre of the sarcomere (contraction), what is needed for this force generation? When there is no longer an action potential what happens?

Answer 15

Force generation depends upon ATP-dependent interaction between thick (myosin) and thin (actin) filaments When there is no longer an action potential, the calcium is selectively taken up by the sarcoplasmic reticulum

Question 16

Do skeletal muscle fibres extend the entire length of the muscle? What attaches the muscles to the skeleton?

Answer 16

Sekeltal muscles DO fibres extend the entire length of the muscle Muscle is attached to the skeletal by tendons

Question 17

What is the predominant structure that makes up a skeletal muscle fibre (cell)?

Answer 17

Myofibrils - these are assemblies of the repeated structures called sarcomeres - it takes many many myofibrils to make a muscle fibre

Question 18

What is the connective tissue surrounding each muscle fibre known as? What is a bundle of muscle fibres known and as what is the connective tissue surrounding this known as? WHat is the connective tissue surrounding the whole muscle known as?

Answer 18

Each muscel fibres (muscle cell) is wrapped by endomysium (surrounds the sarcolemma) The muscles fibres are organised in bundles to form a fasicle - this is wrapped by the perimysium The whole muscle is wrapped by the epimysium

Question 19

What are the alternating segements of thick and thin filaments in myofibrils?

Answer 19

These are Mysoin (thick filaments) - dark appearance and Actin (thin filaments) - light appearance

Question 20

Sarcomeres are the functional unit of muscle What lines on the myofribil are sarcomeres found between?

Answer 20

Sarcomeres are found between two Z-lines (connect thin filaments of two adjoining sarcomeres)

Question 21

The four different zones of the sarcomere are * A band * I band * H zone * M line What are the A and I bands?

Answer 21

The A band spans the length of the thick filament - it consists of thicken filaments along witht he thin filament portion overlapping it The I bands consists of thin filament portions that do not project into the A band

Question 22

What is the H-zone? What runs vertically in the centre of the H-zone?

Answer 22

The H-zone is the ara within the middle of the A-band where thin filamets do no reach (H-zone is lighter than the A-band) The M-line is a vertical darker line that bisect the H-zone

Question 23

True or false ATP is required for both contraction and relaxation of muscle? State why it is needed

Answer 23

TRUE * ATP is needed during contraction to power the cross bridges * ATP is needed during muscle relaxation to pump calcium back into the sarcoplasmic reticulum

Question 24

If a muscle fibres is stimulated rapidly so it does not have an opportunity to relax, what is the maximum sustained contraction known as? What prevents this from happening to cardiac muscle?

Answer 24

Maximum sustained contraction of muscle is known as a tetanic contraction Cardiac muscle is prevented from becoming tetanised due to the long refractory period of the action potentials

Question 25

The muslce has to be its optimal length (Io) before onset of contraction to achieve maximal tetanic contraction What is the optimal length of skeletal muscle in the body?

Answer 25

The approximate optimal length of skeletal muscle in the body is its resting length

Question 26

What is the elastic component of muscle?

Answer 26

The elastic componetn of muscle is the connective tissue / tendon

Question 27

Muscle contractions can be categorised as isotonic or isomertic * What is the difference between isotonic and isometric contractions?

Answer 27

Isotonic contractions generate force by changing the length of the muscle Isometric contractions generate force without changing the length of the muscle.

Question 28

What are the two types of isotonic muscle contraction?

Answer 28

Isotonic contractions can be concentric or eccentric. A concentric contraction causes muscles to shorten, thereby generating force. An eccentric contraction causes muscles to elongate in response to a greater opposing force.

Question 29

How is the tension transmitted to bone in both isotonic (force generated by change in muscle length) and isometric (force generated without change in muscle length) contraction?

Answer 29

Tension is transmitted to bone via the elastic components of the muscle (connective tissue / tendon)

Question 30

What is the main source of ATP when oxygen is present? What is the main source of ATP when oxygen is not present? What is the immediate source of ATP?

Answer 30

Main source of ATP when oxygen is present is from oxidative phosphorylation Main source of ATP when oxygen is NOT present is glycloysis Immediate source of ATP is from the transer of high energy phosphate from creatine phosphate to ADP - forms ATP

Question 31

What are the three types of skeletal muscle fibres?

Answer 31

Type 1 - slow oxidative Type IIa - fast oxidative Type IIx = fasty glyolytic

Question 32

Which exercises are slow and fast oxidative used in? What exercise is fast glycolytic used in? State for both the type of cellular respiration

Answer 32

Slow oxidative (Type 1) - eg walking Fast oxidative (Type IIa) - eg jogging (aerobic and anaerobic metabolism) Fast glycolytic (Type IIx) - short term, high intensity, eg sprinting or jumping - anaerobic mainly

Question 33

The simplest form of coordinated movement is the monosynaptic stretch reflex (myotatic reflex) In simple words, what happens in the stretch reflex?

Answer 33

The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle

Question 34

What are the sensory receptors for the stretch reflex known as? What are the sensory receptors a collection of?

Answer 34

Sensory receptors for the monosynaptic stretch reflex are known as muscle spindles aka intrafusal muscle fibres The muscle spindles are specialised collections of muscle fibres

Question 35

If the specialised collection of muscle fibres are known as muscle spindles aka intrafusal muscle fibres What are the ordinary muscle fibres known as?

Answer 35

Ordinary muscle fibres are referred to as extrafusal muscle fibres

Question 36

So let’s finish by discussing the monosynaptic pathway A monosynaptic reflex, such as the knee jerk reflex, is a simple reflex involving only one synapse between the sensory and motor neurone. The pathway starts when the muscle spindle is stretched (caused by the tap stimulus in the knee jerk reflex). Which fibres detect the change in length of the muscle fibres? How do they cause the muscle fibres to contract and via which type of neurones?

Answer 36

Intrafusal muscle fibres detect a change in muscle length -> afferent fibres travel to spinal cord where it synapses with the a- motorneuron causing contraction of the agonist extrafusal muscle fibres (quads) and relaxation of the antagonist (hamstrings) The y-motorneurones innervate the intrafusal muscle fibres causing co-contraction of these fibres