Muscle Anatomy & Physiology

Question 1

What is a myocyte?

Answer 1

Muscle cell
Contains lots of nuclei because when they fuse they keep their nuclei

Question 2

Describe the anatomy of a singe muscle fibre

Answer 2

Multinucleates
Packed with mitochondria for energy to activate the muscles
Contains specialised memebrane structures associated with contraction
Transverse tubules & sarcoplasmic reticulum (TRIAD) system
Contains several myofibrils

Question 3

List the components of a muscle fibre that can be seen under a microscope

Answer 3

A band
M line
Z line/ disc
I band
H zone

Question 4

What happens to the I and A bands when a muscle contracts?

Answer 4

I band shortens
A band stays the same length

Question 5

Describe the thin filament of a muscle fibre

Answer 5

Contains 2 principle units
G-actin monomers and F acitn filament- globular in natured
Tropomyosin and Troponin complex- Regulatory protein complex
Both come together to form functional actin filament

Question 6

Describe the thick filament of a muscle fibre

Answer 6

Made up of heavy and light myosin
Light chains make up the head portion (S1) and contain actin binding sites
The S2 portion is the “tall portion’’
Whole myosin filament has a a bipolar arrangement and tails point to to the ideal of the thick filament

Question 7

Where does excitation coupling occur in skeletal muscle?

Answer 7

At the motor end plate of a neuromuscular junction

Question 8

What happens to Ca2+ when excitation is being spread?

Answer 8

Ca2+ is translocated
From sarcoplasm reticulum to main Intracellular space of the muscle fibre

Question 9

What is the difference between the interaction of muscle fibres in a relaxed and contracting muscle?

Answer 9

Relaxed- No physical linkage the interaction of muscle fibres and actin and myosin is inhibited
Contracting- Inhibitory component has moved, the contact between the binding sites is permitted

Question 10

Give the list of events that occurs during excitation contraction coupling crossbridge recycling

Answer 10

1. At rest, ADP and Pi are bound to the myosin head, which is in position to interact with actin - Energised State
2. Inhibition of actin-myosin interaction is removed by binding of Ca2+ to troponin C, myosin head binds to the actin filament
3. Conformational change of the myosin head from 90° to 45°, stretching the myosin neck.
4. Recoil of the neck region creates the power stroke.
5. Still attached crossbridge= rigor state.
   6.Detachment due to new ATP molecule binding to the myosin head.
6. ATP hydrolysis resets the myosin head from a 45 ° to 90° conformation. Return to the rest state.

Question 11

What happens to the sarcomere during contraction?

Answer 11

The sarcomere as a whole shortens due to contracted H zone and I band
A band AWLAYS remains a constant length

Question 12

What is the chain of events that occurs during excitation contraction coupling?

Answer 12

1. Ca2+ going from SR to intracellular space stops with the repolarisation Ca2+ of the membrane – Ca2+ channels close in T-tubule and SR.
2. Ca2+ is actively pumped out from SERCA the muscle fibre into the SR, via Sacroplasmic endoplasmic Calsequestrin reticulum ATPase (SERCA).
3. Within the SR, Ca2+ is sequestered by Calsequestrin to ensure SERCA operates in required mode.
4. Muscle relaxes

Question 13

What is a muscle twitch?

Answer 13

Single activation of muscle fibre

Question 14

List and describe the elastic components and contractile elements of the muscle

Answer 14

Series and elastic components- elastic in nature, tendons and aponeuroses in series with the main muscle belly
Contractile elements- within the main muscle, actin and myosin
Parallel elements- in parallel with CE’s, such a epimysium, sarcolemma etc

Question 15

Describe the behaviour of the elastic components and contractile elements during a passive mule state

Answer 15

Contractile elements= inoperative
Elastic comps= collectively behave like a giant elastic band

Question 16

Define, in physiology terms, contraction

Answer 16

Generation of ensign/ force with/ without shortening of a stimulated muscle

Question 17

What are the 2 types of contraction, in terms of lifting a 5kg weight?

Answer 17

Isometric contraction If the muscle is unable to generate more than a 5kg of force, then the muscle will not shorten.
Isotonic contraction If the muscle is able to generate more than a 5kg of force, then the weight moves as the muscle shortens

Question 18

What happens to muscle length during an isometric contraction and why?

Answer 18

Remains the same
Somerset shorten to generate force, whilst elastic components are stretched to equal the amount of sarcomere shortening

Question 19

Describe the active tension relationship

Answer 19

This is the myosin- actin filament overlap from a constraint much fibre
Active tension is linked to resting sarcomere length
Too much/ little overlap of the thick + thin filaments in resting muscle= dec tension

Question 20

Describe the effect of load on isotonic contraction

Answer 20

Heavier load = Reduced speed and distance that the load can be moved

Question 21

List the 3 motor unit types

Answer 21

Slow
Fast fatigue- resistant
Fast fatiguable

Question 22

Describe spacial summation

Answer 22

Muscle force is dependent upon the number of muscle fibres contracting
Increasing the number of motor units activated at any
one time increases force
Recruitment is ordered, starting with slow, progressing to fast motor units

Question 23

What are the 2 types of cardiac muscle action potentials?

Answer 23

Fast response- Ventricular and atrial muscel and purkinje fibres
Slow response- Sinoatrial and atrioventricular node

Question 24

What is the importance of a long cardiac action potential (AP) duration?

Answer 24

Long AP= fail safe mechanism
Cardiac AP has a long absolute refractory period
Protects heart from o frequent excitation due to no temporal summation, therefore no tenancies contraction
Helps heart work as an effective pump

Question 25

Give the sequence of events that occur during cardiac excitation contraction

Answer 25

1. When Action Potential passes over the sarcolemma and down the T-tubules
2. This trigger the entry of calcium into the cell via the DHP/L-type calcium channels in the T-tubules
3. An increase in intracellular Ca2+concentration induces calcium release via the Ryanodine-sensitive calcium release channels on the junctional SR.
4. When the intracellular free Ca2+ concentration is high (>1.0 μM), cross-bridges form between the thick 3
   and thin filaments found within the muscle.

Question 26

What happens to Ca2+ in cardiac E-C coupling?

Answer 26

Ca2+ unbinds to troponin
Aprox 80% of calcium actively taken back up into SR by SERC pumps
Aprox 20% of calcium extruded rom cell into extracellular fluid via: NA+-Ca2+ exchanger OR Ca2+ ATPas pumps

Question 27

What is iontropism?

Answer 27

Modulation of cardiac myoctye behaviour
Not all Ca2+ released from SR for a standard contraction
Means contraction can be varied by how much Ca2+ released from SR

Question 28

Give 3 things in bed in positive and negative iontropism

Answer 28

Positive:
1.Sympathetic nerve stimulation
2. Exercise
3. Beta receptor agonists
Negative:
1. Sympathetic inhibition
2. Heart failure with reduced ejection fraction
3. Myocardial loss

Question 29

How does smooth muscle compare to striated skeletal muscle?

Answer 29

Much smaller fibres
They have a single nucleus
No myofibrils
Orientation of actin and myosin is in a flat line
No Z-line, instead the fibres are attached to dense bodies (alpha- actin)

Question 30

Where is smooth muscle located?

Answer 30

Hollow/visceral organs
Usually in 2 different layers: circular and longitudinal
GI tract
Cardiac muscle
Renal
Genital
Respiratory tract
Sensory

Question 31

How is the myosin cross bridge cycling activated in smooth muscle?

Answer 31

1. Depolarization of the cell membrane opens voltage gated Ca2+ channels and Ca2+ flows into the cell
2. Ca2+ influx causes release of Ca2+ from the sarcoplasmic reticulum (Ca2+ induced Ca2+ released)
3. Ca2+ binds to calmodulin (CaM). The Ca2+-CaM binds to and activates myosin light chain kinase (MLCK). MLCK phosphorylates myosin and allows it to bind to actin, thus initiating cross-bridge cycling
4. Regulation of myosin phosphorylation is balance between MLCK and myosin light chain phosphatase (MLCP) activity

Question 32

How are nerve muscle arranged in smooth muscle?

Answer 32

Found in both multi and single unit
Multi- discrete, similarity to skeletal muscle as supplied by ANS axon
Found in large blood vessels, large airways, ciliary muscle and iris
Single- represents majority, one ANS axon supplies single muscle fibre within the group, those associated with group are joined by gap junctions
Found in uterus and GIT