Skeletal Muscle: Structure and Function

Question 1

What is the epimysium?

Answer 1

The outermost layer that surrounds the entire muscle cell

Question 2

What is the perimysium?

Answer 2

Surrounds the individual bundles of muscle cell fibres- these individual muscle cells are called fascicle

Question 3

What is the endomysium?

Answer 3

Each fascicles is surrounded by connective tissue called the endomysium

Question 4

What is the sarcolemma?

Answer 4

The cell membrane surrounding the muscle fibre cell, located above this are satellite cells that are responsible for muscle growth and repair through dividing and contributing nuclei

Question 5

What is the sarcoplasm?

Answer 5

Beneath the sarcolemma are the sacrcoplasms, which contain the myofibrils.

The very end/ smallest part of the muscle is the sarcomeres within the myofibrils.

Question 6

Define sarcomeres?

Answer 6

A functional unit of skeletal muscle- where the movement occurs.

Question 7

What are the three structural proteins to be aware of within the muscle cell?

Answer 7

Titin= acts as a molecular spring to allow passive elasticity
Nebulin= structural function
Desmin= structural function

Question 8

Describe/ explain the sliding filament Model?

Answer 8

1. The actin and myosin myofilaments slide across each other during muscular contraction due to the action of numerous cross bridges extending out as ‘arms’
   3.During contraction, actin myofilaments at each end of the sarcomere slide past the myofilaments towards each other, the Z disks are brought closer together and the sarcomere shortens
   4.In a fully contracted muscle, the ends of the actin myofilaments overlap. The ‘pulling’ of actin over the myosin molecule results in muscle shortening and the generation of force

Question 9

Explain the concept of the cross-bridge cycle?

Answer 9

1.Troponin and tropomyosin are the two regulating proteins that are in the actin molecule- control the interactions between myosin and actin
2.Action potential arrives
3.Ca++ released from the sarcoplasmic reticulum
4.Ca attaches to troponin
5.Tropomyosin moves uncovering the binding sites
6.Hydrolysis of ATP changes angle of the myosin head= CROSS BRIDGE FORMED
7.Pi released from myosin head – changes the angle of the myosin head= POWERSTROKE
8.Myosin head picks up another ATP and bond with actin is released
9.Continues until Ca++ or ATP levels drop

Question 10

Describe the length- tension relationship?

Answer 10

If there is a greater increase of muscle length then there’s a lack of actin and myosin overlap within the sarcomere
This leads to a lack of cross bridges forming and therefore a lack of tension :(
Considers the AMOUNT of cross bridge connection

Question 11

Describe the force- velocity relationship?

Answer 11

The faster the muscle contracts, the less cross bridge connections will take place, decreasing the force that can be generated
Considers the RATE of cross bridge connections

Question 12

What does calcium bind with to move the tropomyosin binding sites and form a cross bridge?

Answer 12

Troponin- (CALCIUM + TROPONIN= movement of tropomyosin binding sites)

Question 13

Which are the two functioning units of the sarcomere that create a cross bridge during contraction?

Answer 13

Actin and Myosin

Question 14

What has an excitatory response to rapid change in muscle length?

Answer 14

MUSCLE SPINDLES

Question 15

What are the two main branches of the nervous system?

Answer 15

CNS- includes the brain and the spinal cord
Peripheral Nervous System- nerves outside the CNS

PNS= Afferent (sensory) and Efferent (motor)
Efferent= Somatic (skeletal muscle) and Autonomic (involuntary effector organs- glands and smooth muscle in gut)
Autonomic= sympathetic and parasympathetic

Question 16

Briefly outline the main structure of a motor neurone?

Answer 16

1. Cell body
2. Dendrites
3. Axon- covered by Schwann Cells
4. Myelin Sheath
5. Node of Ranvier
6. Terminal Branches of the Axon

Question 17

Define Action Potential?

Answer 17

A postive electrical charge
When depolarisation reaches a critical value/ threshold the sodium gates open wide and a nerve impulse is formed - starts a series of ionic exchanges between Sodium (Na+) and Potassium (K+)

Question 18

What does an excitatory transmitter do ?
(Neurotransmitter)

Answer 18

Increases neuronal permeability to sodium and and increases the amount of sodium within the axon.

Question 19

What does an inhibitory transmitter do?
(Neurotransmitter)

Answer 19

Decreases neuronal permeability and causes the neuron to become more negative (hyper polarised) = INHIBITS

Question 20

Explain how feedback from sensory receptors can regulate muscle contraction?

Answer 20

Sensory receptors= Golgi Tendon and Muscle Spindles

Golgi Tendon= monitor tension developed
Serve as safety devices that help prevent excessive force during muscle contractions.

Muscle Spindles= monitor relative muscle length
Serve to make a muscle shorten and contract and resist being stretched.

Question 21

Explain the key stages that allow muscle contraction to take place?

Answer 21

1. Action potential arrives
2. Axon terminal depolarised
3. Calcium influx into axon terminal
4. Encourages release of Acetylcholine into synaptic cleft
5. Acetylcholine interacts with sodium to depolarise the motor end plate
6. As the charge of muscle cell membrane increases, voltage gated calcium channels open up
7. This increased charge also leads to a release of calcium from the sarcoplasmic reticulum
8. When the action potential ceases the acetylcholine re-enters the neurone as choline and acetic acid

Question 22

Explain the key stages that cause depolarisation within a motor neuron?

Answer 22

At REST: More sodium (Na+ is outside, than inside, the axon)

When action potential arrives: (Na+ channels open up and Na+ enters the axon)

Increased Na+ within the axon causes the exposed part of the axon between the myelin sheaths to DEPOLARISE and Na+ gates close

REPOLARISATION takes place as Potassium (K+) gates open at +30mV
K+ leaves the axon and the inside becomes less positive (hyperpolairied ) = continuous positive and negative wave