Flashcards in Option D.4 The Heart Deck (45)
How skeletal and cardiac muscles similar?
- surrounded and enclosed by SARCOLEMMA, a membrane
- transverse tubules tunnel in and around sarcomeres
- fluid filled system of branching membranous sacs, cardiac has SARCOPLASMIC RETICULUM, skeletal have ENDOPLASMIC RETICULUM
- cardiac are striated in appearance; have similar arrangement of actin and myosin
How are cardiac and skeletal muscles different from one another?
cardiac muscles are:
- shorter /wider than skeletal
- single nucleus, not coenocytic (skeletal)
- branched and joined by intercalated discs in complex (3D) network allows contraction in 3D
- supplied by mitochondria (make up larger volume)
- contract even without stimulation of nerve/not voluntarily controlled
What are intercalated discs?
cardiac muscle cells remain single cells by being interconnected by intercalated discs (are between junctions of cells);
contain gap junctions (openings) for cytoplasm to pass;
What is the benefit of having cardiac cell have gap junctions?
freely sharing cytoplasm allows cardiac muscles to send signals quickly; synchronizing contractions
Is the interconnected system of cardiac muscle fibres separate from other ventricle?
Yes. Network of atria walls is entirely separate from ventricles
What is the effect of having cardiac muscle fibres separate from ventricles?
ensures transmission delay of electrical between atria and ventricle
What stages is a heartbeat divided into?
systole and diastole
What happens during the systole stage (in heart)?
heart muscles contracts
What happens during the diastole stage (in heart)?
heart muscle relaxes
Why do we say the heart is myogenic in origin?
inherit electrical activity triggers continuous beating of heart by network of specialized self-excitable cardiac muscle fibre
What are autorhythmic cells?
specialized, self-excitable cardiac muscle fibres
Where are the autorhythmic cells located?
right atrial wall, close to the points where the vena cavae empty into the heart
What is the network of cardiac muscles fibres?
Sinoatrial (SA) node
How does the atrial systole occur?
electrical charge runs through network of muscle fibres (in wall of atria) via gap junctions (intercalated discs);
atrial walls contract
Do signals fro SAN pass directly to ventricles?
No; muscle fibres from atria and ventricles are separate
What feature of the heart ensures the delays of the SAN being passed along?
coronary muscle fibres from atria and ventricles being completely separat
If signals from SAN cannot directly pass on to ventricles, how are they passed on?
stimulus picked up by AV node (at base of right atrium)
Why is there a delay between arrival and passing on the stimulus at the AV node?
- cells of the AV node taking longer to become excited
- diameter of AV node cells are smaller, slows down conduction
- fewer sodium ion channels in AV node cells (more negative resting potential)
- fewer gap junctions in the intercalated discs (between cells)
What does the delay in conduction allow?
time for atrial systole before the AV vales close; atria contracts and empty blood into ventricular before ventricles contract;
prevents both to contract simultaniously
What do AV bundle do?
conduct signal into ventricles to the point, where signal split into left and right branch (2 ventricles)
What are Purkinje fibres?
conducting fibres that deliver signal to base of each ventricle, and coordinate contractions starting from base
What is ventricular systole?
Conducting fibres coordinate contraction of entire ventricle walls, starting at the base of heart upwards
Why is conduction during ventricular systole fast?
- are less in number
- have larger diameter
- numerous voltage-gated sodium ion channels
- are well supplied with mitochondria and have glycogen store - reserves of respiratory substrate, in effect
What happens after every contraction of the heart?
- period of insensitivity to stimulation, the REFRACTORY PERIOD ( enforced diastole)
- blood passively refills (takes more time)
How do signals from the SA node cause initial contraction in atria?
- SA node initiate action potential without stimulation
- initiation occurs rhythmically
- pacemaker of heart
- gap junctions allow electric charges to flow feely between cells
- SA node spreads across atrium
- atria undergo systole/contraction
What occurs after atria systole?
- signals from SA reaches after delay of passing on stimulus AV node
- delay allows time for atrial systole
- AV bundles receive signal from AV node;
- signal rapidly split between bundle branches;
- at apex of heart, bundle connects to Purkinje fibres
- signals spread more rapidly throughout heart via Purkinje;
- ventricles must undergo systole
- contraction of ventricles starts at apex
What happens if the SA node is damaged/diseased?
normal heart rhythm restored by implantation of artificial pacemaker;
delivers electrical impulse via electrodes to heart wall;
When does a patient get assigned a pacemaker?
- heart with slow heartbeat;
- conducting fibres are faulty;
- can also only be used when normal heart beat missing
What are the causes of the sound of the heartbeat?
two sounds ( lub and dub) caused by closing of AV vales first;
then closing of semi lunar valves soon after