Clin Phys Week 3 E

Question 1

What is the microscopic structure of cardiac muscle?

Answer 1

Striated - short, branched, single nucleated cells

Connected by a series of intercalated discs

The connective tissue endomysium acts as both tendon and insertion

Complete latticework of cardiac muscle is a syncytium (helps with conduction of action potentials)

Question 2

What is the function of intercalated discs?

Answer 2

They anchor the myocytes together using desmosomes and allow passage of ions via gap junctions.

Question 3

What is the difference between the sarcoplasmic reticulum in cardiac muscles and skeletal muscles?

Answer 3

In cardiac muscles the sarcoplasmic reticulum has much larger t-tubules which admit Ca2+ from extra cellular fluid during excitation

Question 4

What is the metabolism of cardiac muscle?

Answer 4

Aerobic respiration

Rich in myoglobin and glycogen

Large mitochondria

Organic fuels: fatty acids, glucose, ketones

Fatigue resistant

Question 5

What is the role of the pacemaker cells?

Answer 5

They have unstable resting potentials.

They initiate action potentials

The depolarise spontaneously and regularly

Question 6

Explain the depolarisation of the SA node

Answer 6

Membrane potential starts at -60m in order for the cell to reach its threshold of -40mV, there is a slow influx of Na+ into the cell. at threshold of approx -40mV, fast Ca2+ open and Ca2+ rushes into the cell.as the cell depolarises to about 0mV, K+ channels open and K+ rushes out of the cell into order to make the cell less positive.in order to repolarise, the K+ leaves the cell until the cell reaches approx -60mV again. At this point the K+ channels close.Each depolarisation creates one heart beat (contraction)

Question 7

What is the rate of SA node firing?

Answer 7

Once every 0.8 seconds, approx 75bpm

Question 8

Explain the depolarisation of myocardium

Answer 8

Cardiomyocytes have a resting potential of -90mV

Depolarisation: Stimulus from pacemaker cells opens Na+ channels, allowing Na+ to enter the cell until the cell reaches threshold. Once threshold is reaches, more Na+ channels open in positive feedback cycle. Action potential peaks at +30mV and the Na+ gates close quickly. K+ channels open and K+ starts to leave cell until the cell reaches approx 0mV. At this point there is a plateau, which sustains the contraction. During the plateau EXTRACELLULAR Ca2+ slowly enters the cell which simultaneously, K+ exits the cell. This maintains the absolute refractory period for 200-250 milliseconds. After 200-250ms, the Ca2+ channels close and Ca2+ stops entering the cell. The out flux out K+ rapidly repolarises the cell and returns it to its -90mV resting potential.

Question 9

How does the length tension curve of cardiac muscle differ to that of skeletal muscle?

Answer 9

Resting tension is passively developed by applying load (preload)

Preload elongates the cardiac muscle.Cardiac muscle is less compliant because it has less titans.Active tension is the developed tension during twitch (contraction) and can be calculates by subtracting the resting tension from the total tension. Resting tension increases more rapidly with increasing muscle length.Cardiac muscles operate on the ascending limb of the active length tension curve

We never reach Lmax (length at which maximum tension is generated) because it takes so much force to stretch the muscle to that length that venous return and contraction of the atria do not produce enough force.This ensures that the intact heart can respond to an increase of filling by improving contraction.

Question 10

What is smooth muscle stimulated by

Answer 10

Nerves, hormones, chemical and mechanical forces, physical forces

Question 11

Describe smooth muscle microscopic structure: shape, filament arrangement, sarcoplasm contents, nuclei, etc

Answer 11

Elongated, spindle shaped cells

Single central nucleus per cell

Sarcoplasm is filled with fibrils

Fine endomysium

Filament arrangement: scattered throughout sarcoplasm, connected randomly by dense bodies.

Regulatory protein: calmodulin (no troponin or tropomyosin)

No z-discs

Non striated

Sarcoplasmic reticulum is rudimentary

Sarcolemma has no t-tubules but has Ca2+ channels placed into puts called calveoli.

Cells have gap junctions to allow the propagation of AP

Question 12

What are the 2 types of smooth muscle?

Answer 12

Visceral

Multi unit

Question 13

What is visceral smooth muscle?

Answer 13

Cells act together in groups and are arranged in sheets or bundles of closely packed fibres.Gap junctions between cells slow action potentials to go from one cell to the next.

Has inherent tone without nerve activity

Exhibit stress-relaxation response’

Question 14

What is multi unit smooth muscle?

Answer 14

Act more like skeletal muscle with individual controls over each cell. Respond to stimulus.

Are found in large areas such as the lungs, large arteries, arrector pili muscles, internal eye muscles etc.

Rare gap junctions

Infrequent spontaneous depolarisations

A rich nerve supply which forms motor units.

Graded contracts in response to neural stimuli

Question 15

Describe how smooth muscle is innervated?

Answer 15

Smooth muscle lacks neuromuscular junctions. Innervating nerves have bulbous swellings called varicosities which release neurotransmitters into wide synaptic clefts called diffuse junctions.

Question 16

How does smooth muscle contract?

Answer 16

The AP reached the cell and depolarises the cell membrane, causing an influx of Ca2+ from the sarcoplasmic reticulum and from the extracellular space through the calcium channels in the calveoli. Ca2+ interacts with calmodulin in the cytosol and activates it. Activated calmodulin activates myosin light chain kinase enzyme. Myosin light chain kinase enzyme transfers a phosphate from ATP to myosin cross bridges Phosphorylated cross bridges interact with actin to produce shortening. Smooth muscle relaxes as intercellular Ca2+ drops. Myosin phosphatase splits the phosphate from the myosin chain and cycling stops. This occurs when the Ca2+ ion concentration drops below critical level.

Question 17

Where does Ca2+ come from for smooth muscle contraction?

Answer 17

All comes from extracellular fluid and discusses through calcium channels. Some can come from rudimentary sarcoplasmic reticulum structures.

Question 18

Neural control of smooth muscle

Answer 18

Intracellular Resting membrane potential is about -50 to -60mV. AP is caused by the unflux of Ca2+ rather than Na+ due to more Ca2+ channels. Ca2+ also acts direct in smooth muscle contractions.Ca2+ channels also remains open longer

Transmission is signed can vary, transmitted by ACh or noradrenaline. These can be excitatory or inhibitory.

Question 19

Acetylcholine and noradrenaline

Answer 19

Most important neurotransmitters secreted by autonomic nerves that innervate smooth muscle at the neuromuscular junction. When ACh excites muscle fibres Noradrenaline inhibits it, but when Noradrenaline excites it ACh inhibits it. This occurs because there are different types of receptors for each neurotransmitter on the muscle cell.

Question 20

Smooth muscle reponses to stretch:

Answer 20

Smooth muscle exhibits a phenomenon called stress-relaxation response in which:

Smooth muscle responds to stress only briefly and then adapts to its new length where the new length retains it’s ability to contract.

This enables organs such as the stomach and bladder to temporarily store it’s contents.

Question 21

What are spike potentials of smooth muscle?

Answer 21

Short duration potentials of 10-50ms.

Elicited by electrical stimulation, hormones, neurotransmitters from nerves, stretch or spontaneous generations in the fibre itself.

Question 22

How long are the plateaus of smooth muscle AP and why is this important?

Answer 22

Can be up to 1 second.Important for prolonged contraction in smooth muscles such as uterus, ureter and intestines.

Question 23

Contraction of smooth muscle without neural stimulation:

Answer 23

Local tissue factors: lack of O2, excess CO2, increased H+, adenosine, lactic acid, K+, decreased Ca2+, body temp not all alter function

Hormonal action:

Noradrenaline, adrenaline, angiotensin, vasopressin, serotonin, histamine, ACh. Cause muscle contraction

Question 24

What is muscle memory and what does it involve?

Answer 24

A type of movement that muscles gradually become familiar with

Involves:Cortex, basal ganglia and cerebellum.

Once actions are memorised by the brain, the muscles must be trained to act in a quick, fluid manner.

Coordinate transmission and muscle recruitment

Attitude: ability to train brain to forget bad shot and remember a good one.

Question 25

Muscle regeneration:

Answer 25

• Injured muscle fibre: dormant myoblasts (satellite cells)
• Proliferation and migration of satellite cells.
• Fusion of myoblasts into myotubules (low level excitation and stretching is important for correct orientational growth of new tissue)
• Myofibrillar synthesis and assembly
• Regeneration of muscle fibre

Question 26

Smooth muscle properties after repair

Answer 26

• Variable muscle diameter
• Some fibres do not mature fully and retain internal nuclei
• Groups of fibres are enclosed in fibrous sheaths.
• Incomplete fusion leads to branching and splitting
• Contraction times, maximal shortening velocity, fibre length and cross sectional area all return to normal
• Muscle mass and peak tetanic tension remains lower
• Muscle endurance is lower