Clin Phys Week 3 E Flashcards
What is the microscopic structure of cardiac muscle?
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)
What is the function of intercalated discs?
They anchor the myocytes together using desmosomes and allow passage of ions via gap junctions.
What is the difference between the sarcoplasmic reticulum in cardiac muscles and skeletal muscles?
In cardiac muscles the sarcoplasmic reticulum has much larger t-tubules which admit Ca2+ from extra cellular fluid during excitation
What is the metabolism of cardiac muscle?
Aerobic respiration
Rich in myoglobin and glycogen
Large mitochondria
Organic fuels: fatty acids, glucose, ketones
Fatigue resistant
What is the role of the pacemaker cells?
They have unstable resting potentials.
They initiate action potentials
The depolarise spontaneously and regularly
Explain the depolarisation of the SA node
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)
What is the rate of SA node firing?
Once every 0.8 seconds, approx 75bpm
Explain the depolarisation of myocardium
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.
How does the length tension curve of cardiac muscle differ to that of skeletal muscle?
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.
What is smooth muscle stimulated by
Nerves, hormones, chemical and mechanical forces, physical forces
Describe smooth muscle microscopic structure: shape, filament arrangement, sarcoplasm contents, nuclei, etc
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
What are the 2 types of smooth muscle?
Visceral
Multi unit
What is visceral smooth muscle?
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’
What is multi unit smooth muscle?
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
Describe how smooth muscle is innervated?
Smooth muscle lacks neuromuscular junctions. Innervating nerves have bulbous swellings called varicosities which release neurotransmitters into wide synaptic clefts called diffuse junctions.
How does smooth muscle contract?
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.
Where does Ca2+ come from for smooth muscle contraction?
All comes from extracellular fluid and discusses through calcium channels. Some can come from rudimentary sarcoplasmic reticulum structures.
Neural control of smooth muscle
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.
Acetylcholine and noradrenaline
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.
Smooth muscle reponses to stretch:
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.
What are spike potentials of smooth muscle?
Short duration potentials of 10-50ms.
Elicited by electrical stimulation, hormones, neurotransmitters from nerves, stretch or spontaneous generations in the fibre itself.
How long are the plateaus of smooth muscle AP and why is this important?
Can be up to 1 second.Important for prolonged contraction in smooth muscles such as uterus, ureter and intestines.
Contraction of smooth muscle without neural stimulation:
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
What is muscle memory and what does it involve?
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.
