musculoskeletal system: smooth and cardiac muscle

smooth muscle: general features

• lack visible cross-striations - actin and myosin filaments not as regularly arranged

• long, spindle-shaped cells with single nucleus

• cells usually arranged in sheets within muscle

• no z lines - dense bodies instead

• less mitochondria and more poorly developed sarcoplasmic reticulum

• no troponin, tropomyosin doesn’t block cross-bridge binding sites

smooth muscle: organisation of thin and thick filaments

• actin and myosin filaments at slight diagonal from side to side - diamond shaped lattice

• myosin molecules arranged in thick filaments so cross-bridges present across entire length - allows thin filaments to be pulled along the thick filaments

• intermediate filaments part of cytoskeletal framework supporting cells shape

• dense bodies contain same proteins as z lines

smooth muscle: contraction

• Ca²⁺ ions bind to calmodulin to form a complex

• Ca²⁺-calmodulin complex activates myosin kinase

• myosin kinase phosphorylates myosin heads using ATP hydrolysis

• phosphorylated myosin can interact with actin filaments to form actin-myosin crossbridges so contraction can take place

• when Ca²⁺ ions removed from sarcoplasm they dissociate from calmodulin so myosin kinase activity falls

• myosin is dephosphorylated by myosin phosphatase

• but Ca²⁺ ion removal is slower so contraction is longer and gives a more graded response

smooth muscle: different types

• multi-unit smooth muscle:

neurogenic - stimulated by nerves

made from multiple discrete units which function independently - no gap junctions

units must be separately stimulated by nerves to contract

• single-unit smooth muscle:

myogenic - no nervous stimulation required for contraction

fibres excited and contract as a unit - gap junctions present so AP generated in one smooth muscle cell spreads to all cells in that 

slow and energy efficient contraction

functional syncytium - contract as one

smooth muscle: single-unit contraction

• can be phasic or tonic

• phasic - contracts in bursts, triggered by action potential which leads to increase in cystolic Ca²⁺

• tonic - often partially contracted at all times, doesn’t show burst of activity like phasic but varies in increments above or below usual tonic state

smooth muscle: factors affecting contractile activity

• spontaneous depolarisation of cells

• signaling molecules:

neurotransmitters from autonomic neurons - neurotransmitters of (para)sympathetic postganglionic cells alter membrane potential of smooth muscle cells to make them more/less likely to fire APs and contract

hormones

• local changes in extracellular fluid

• stretch:

vascular smooth muscle cells respond to stretch by contracting - membranes of stretched cells depolarise and fire APs, causing cells to contract

smooth muscle can continue to develop tension even when very stretched - stress relaxation response

cardiac output

stroke volume x heart rate

cardiac muscle: general features

• found in walls of heart

• regular arrangements of actin and myosin filaments - striations

• Z lines present

• cells joined at intercalated discs - provide strong mechanical adhesions between adjacent cells so muscle can withstand high pressures when pumping blood

• Gap junctions between cells - syncytium, AP initiated at one point in cardiac muscle spreads so that a large number of cardiac cells contract simultaneously

• T-tubule system present

• innervated by autonomic nervous system

cardiac muscle: generation of AP

• resting membrane potential - approx - 90mv

• heartbeat autorhythmic - generated by heart muscle

• AP generated by pacemaker cells

• pacemaker cells have low density of actin and myosin filaments, initiate and coordinate rhythmic contractions of heart

• Each AP triggers full contraction followed by relaxation

cardiac muscle: T tubule system

• T tubules larger

• have DHP voltage gated ion channels which open to allow Ca²⁺ ions into sarcoplasm - Ca²⁺ ions bind to ryanodine receptors so more Ca²⁺ ion channels opened which leads to fibre contraction

• this is known as Ca²⁺- induced Ca²⁺ release

cardiac muscle: action potentials

• spontaneous, rapid depolarisation of cells occurs

• plateau phase - slow as Ca²⁺ ions move in slowly

• slow repolarisation

• rhythmic firing

• SA and AV nodes pacemaker cells - result in certain firing rate with vagus nerves

• Firing rate controlled by sympathetic and parasympathetic nervous systems - modifies rate at which pacemaker cells fire

• latent pacemakers located in conduction system - keeps heart beating if other pacemakers fail

cardiac muscle: firing of pacemaker cells

• movement of Na⁺, K⁺ and Ca²⁺ via ion channels

• at -60mv, Na⁺ ion channels open - slow inward current

• slow depolarisation as Ca²⁺ ion channels open

• K⁺ ion channels open to cause repolarization, when closed followed by slow depolarisation again

• long refractory period prevents summation (tetanus) - second AP can’t be generated until excitable membrane has recovered from first