Physiology of Smooth Muscle Contraction

Smooth Muscle Contraction

• Unlike action potentials in nerves and muscle, in smooth muscle, calcium ions are responsible for the inward current that brings about depolarization.
• Contraction in smooth muscle contains actin that slides over myosin during contraction, but slower than skeletal muscle.
• Also contains tropomyosin and calmodulin (but no troponin).
• Contraction is activated by calcium ions, which combine to calmodulin.
• Calmodulin-calcium combination joins and activates myosin kinase (MK).
• MK phosphorylates myosin, leading to activation of myosin ATPase.
• Myosin binds with actin, producing contraction.

Cardiac Muscle

Autorythmicity

• The heart has the ability to work independently of any extrinsic stimuli.
• It beats regularly independent of any extrinsic stimuli.
• It has automatic action potential generation capability.
• Spontaneous rhythmic electrical impulses are generated by the conducting system, especially the pacemaker.

Properties of Cardiac Muscle

• Conductivity (Dromotropic Action)
  • Impulses can spread easily between cardiac muscle fibres.
  • Conduction in the heart is normally carried out by the specialized conducting system.
  • Conducting system starts with the pacemaker at the sinoatrial node in the right atrium to the atrioventricular nodes, then to the bundle of His and then purkinje fibres.
• Contractility (Inotropic Action)
  • The heart has the ability to contract in response to intrinsic stimuli in a rhythmic way.
  • Contraction of a cardiac chamber is called systole, and relaxation is called diastole.
  • Like skeletal muscle, action potential gets to the cardiac membrane, excitation-contraction coupling starts with the depolarization of sarcolemma by sodium influx.

Neuromuscular Junction (Myoneural Junction)

• The point at which the axon ending of a motor neuron meets a muscle cell.
• The plasma membrane of the axon ending remains separated from the muscle fibre sarcolemma by a small space (20-30nm) called the synaptic cleft.

Neurotransmission

• For skeletal muscle to contract, the motor nerve controlling it must signal it to do so.
• The signals sent to muscle fibres by motor nerves are through chemical messengers called neurotransmitters.
• Neurotransmitters released onto the skeletal muscle fibre at the NMJ is acetylcholine.
• Acetylcholine is released by the process of exocytosis.

Properties of Neuromuscular Transmission

• Unidirectional - it occurs in one direction only, from nerve to muscle.
• Delay of about 0.5ms, which represents the time needed for release of ACh, change in permeability of muscle fibre, influx of sodium ions and building of depolarization to firing level.
• Easily fatigued due to repeated stimulation, caused by depletion of Ach and ATP needed for synthesis of Ach.
• Can be blocked by drugs, such as curare, a toxic drug that competes with ACh for binding site.

Synaptic Transmission

• Synapses are junctions where the axons or some other part of one nerve cell (presynaptic cell) terminates on the soma, dendrites or some other part of another neuron (the post-synaptic cell).
• Types of synapses: axodendritic, axosomatic, axoaxonic, dendrodenritic, somatosomatic, dendroaxonic.

Clinical Conditions Associated with the Neuromuscular Junction

• Myasthenia Gravis: an autoimmune disease in which antibodies are formed against the acetylcholine receptors, resulting in muscle weakness.
• Lambert-Eaton Syndrome: an autoimmune disease of the neuromuscular junction, affecting the presynaptic neurons.