Muscles

Question 1

What features does skeletal muscle have?

Answer 1

Large multinucleate cells that are striated
Endomysium between individual myofibril
Perimysium around groups of fibres
Epimysium around entire muscle

Question 2

What features does cardiac muscle have?

Answer 2

Striated but smaller, branched and uninucleate
Cells are joined by junctions - intercalated disks
Order goes endocardium, myocardium, epicardium, parietal pericardium (fibrous layer)

Question 3

What features does smooth muscle have?

Answer 3

Fibres are small, no striations and uninucleate

Electrical excitation spreads from cell to cell via gap junctions due to lack of order of muscle

Question 4

How is a striated muscle sarcomere arranged?

Answer 4

Thick and thin filaments arranged parallel to each other
M line links the middle of thick filament
Z line connects to end of thin filament
A filament composed of titin stretches from the Z line to the M line
A band - Dark area full of thick filament
I band - Light area full of thin filament

Question 5

What are the thick and thin filaments in a myofibril?

Answer 5

Thick filament - myosin, made of 2 heavy and 4 light chains with two heads that can create cross bridges
Thin filament - mostly actin, but also tropomyosin and troponin

Question 6

What is the Sarcoplasmic Reticulum?

Answer 6

Like the endoplasmic reticulum is a repeating series of networks around the myofibril, A-I band in length and acts as a reservoir for Ca2+
Ca2+ is stored in end of the segment in terminal cisternae

Question 7

What is a T-tubule?

Answer 7

Sits between 2 terminal cisternae and a T-tubule has voltage-sensor proteins activated by membrane depolarisation and causes release of Ca2+

Question 8

Does each muscle fibre need to be innervated and if so why?

Answer 8

Yes, because the endomysium seperates all of the muscle fibres from one another

Question 9

How does innervation occur?

Answer 9

ACh is released from the motor end plate which stimulates nicotinic receptors and opens a channel causing a small depolarisation
This opens Ca2+ channels causing more depolarisation that stretches to the T-tubules which are physically linked to the Ca2+ channels in the terminal cisternae
Ca2+ then diffuses into cytosol and binds to troponin, changing the formation of tropomyosin
This allows the myosin head to attach to the actin filament

Question 10

What are the 3 stages of sarcomere contraction?

Answer 10

Resting stage - filaments slightly overlap
Concentric contraction stage - increase in overlap of filaments
Eccentric contraction stage - thin and thick filaments do not interact

Question 11

What is the contraction cycle?

Answer 11

Attachment - myosin head is attached to actin
Release - ATP binds causing myosin to relax
Bending - ATP allows head to bend
Force generation (power stroke) - increased binding affinity
Reattachment

Question 12

How does relaxation of a muscle occur?

Answer 12

ACh is reabsorbed by synaptic knob and active transport pumps in SR pump Ca2+ back up
Tropomyosin can now regain original formation and myosin can no longer bind to actin

Question 13

How do contraction and relaxation occur in cardiac muscle?

Answer 13

Action Potential originates spontaneously from the pacemaker cells, then the AP simulates opening of Ca voltage-gated channels and Ca2+ then binds to troponin
Relaxation is same as skeletal

Question 14

How does contraction occur in smooth muscle cells?

Answer 14

Ca2+ enters cell and induces release of Ca2+ from sarcoplasmic reticulum
This binds to calmodulin, which activates myosin light chain kinase (MLCK) which results in phosphorylation and activates the myosin head

Question 15

What opens ion channels?

Answer 15

Voltage
Ligands: ACh, GABA, ATP, Glutamate
Stretch, light, 2nd Messengers, Temperature

Question 16

What 2 main properties affect voltage ion channels?

Answer 16

Voltage-dependence and time-dependence/time-independence

Question 17

What features does the neuronal action potential have?

Answer 17

Short refractory period (500mcs) and long relative refractory period

Question 18

What features does the skeletal action potential have?

Answer 18

Long refractory period (1ms) and short relative refractory period

Question 19

What features does the cardiac action potential have?

Answer 19

Very long refractory period that curves down gradually (200-400ms) and tiny refractory period

Question 20

Why does cardiac muscle have long refractory periods?

Answer 20

it prevents tetanus (when lots of stimuli cause an elongated tension period which happens in skeletal muscle) in a heart it would lead to re-entrant arrhythmias, the refractory period prevents that

Question 21

What are the two types of smooth muscle contraction?

Answer 21

Time-dependent action potentials - most smooth muscle

Time-independent action potentials - most vascular muscle

Question 22

How is calcium released in all 3 types of muscle?

Answer 22

Skeletal - voltage induced calcium release
Cardiac - calcium induced calcium release
Smooth muscle - membrane potential modulated calcium release