MUSCLE PHYSIOLOGY

• Muscle cells, like neurons, can be excited to produce an action potential that is transmitted along their cell membrane
• Muscles have a contractile mechanism that is activated by the action potential
• 3 types of muscles:

  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscle:

  • Comprises the great mass of the somatic musculature
  • Has well-developed cross striations
  • Does not normally contract in the absence of nerve supply
  • Lacks anatomic and functional connections between individual muscle fibers
  • Generally under voluntary control
• Cardiac muscle:

  • Has cross-striations but functionally syncytial in character
  • Contracts rhythmically in the absence of external innervation due to pacemaker cells that discharge impulses spontaneously
• Smooth muscle:

  • Lacks cross-striations
  • Those found in the hollow viscera are functionally syncytial in character and contain pacemakers that discharge impulses irregularly
  • Those found in the eye and some other locations are not spontaneously active and resemble skeletal muscle in function
• Skeletal muscle morphology:

  • Made up of individual muscle fibers
  • Begin and end in tendons
  • Muscle fibers are arranged in parallel between the tendinous ends so that the force of contraction of the unit is additive
  • Each muscle fiber is a single cell, multinucleated, long and cylindric in shape with no syncytial bridges between cells
  • Muscle fibers are made up of fibrils divided into individual filaments which are in turn made up of the contractile proteins
• Sarcomere:

  • Functional unit of the muscle
• Contractile proteins:

  • Myosin (thick filament)
  • Actin (thin filament)
  • Tropomyosin (thin)
  • Troponin (thin)
  • Troponin I inhibits the interaction of myosin with actin
  • Troponin T binds the other two to tropomyosin
  • Troponin C contains the binding sites for the Ca++ that initiates contraction
• Sarcotubular system:

  • Muscle fibrils are surrounded by structures made up of a membrane that appear as vesicles and tubules forming the sarcotubular system made up of the T system and the sarcoplasmic reticulum
  • T system: transverse tubules which are continuous with the membrane of the muscle fiber and facilitate rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle
  • Sarcoplasmic reticulum: forms an irregular curtain around each fibril between its contact with the T system and is concerned with Ca++ movement and cell metabolism
• Contractile responses:

  • 2 kinds of muscle response: Electrical and Mechanical
  • A single action potential causes a brief contraction followed by relaxation, known as muscle twitch
• Steps in skeletal muscle contraction:

  • Discharge of motor neuron
  • Release of transmitter substance (acetylcholine) at the motor end plate
  • Generation of end-plate potential
  • Generation of action potential in muscle fibers
  • Inward spread of depolarization along T tubules
  • Release of Ca++ from lateral sacs of sarcoplasmic reticulum and diffusion to thick and thin filaments
  • Binding of Ca++ to troponin C, uncovering myosin binding sites on actin
  • Formation of cross-linkages between actin and myosin and sliding of thin on thick filaments producing shortening
• Steps in relaxation:

  • Ca++ is pumped back into the sarcoplasmic reticulum
  • Release of Ca++ from troponin
  • Cessation of interaction between actin and myosin
• Types of skeletal muscle contraction:

  • Isometric contraction: contraction without appreciable decrease in the length of the whole muscle; no joint movement
  • Isotonic contraction: contraction against a constant load with approximation of the ends of the muscle; there is joint movement
  • Tetanus: response to rapidly repeated stimulation occurring before any relaxation has occurred, where responses fuse into one continuous contraction
• Energy sources of metabolism:

  • ATP → phosphocreatine + ADP → GLUCOSE → GLYCOGEN
  • Fatigue: accumulation of lactic acid
• Rigor:

  • State of extreme rigidity when muscle fibers are completely depleted of ATP and phosphocreatine
• Effects of denervation:

  • Healthy skeletal muscle does not contract except in response to stimulation of its motor nerve supply
  • Destruction of this nerve supply causes muscle atrophy and abnormal excitability of the muscle
• Smooth muscle types:

  • Visceral smooth muscle: occurs in large sheets, has bridges between individual muscle cells and functions in a syncytial fashion, found in the walls of hollow viscera, shows continuous, irregular contractions that are independent of its nerve supply
  • Multi-unit smooth muscle: made up of individual units
• Norepinephrine, when combined with acetylcholine, increases the rhythmic contractions of smooth muscle
• Multi-unit smooth muscle is made up of individual units without interconnecting bridges
• Multi-unit smooth muscle is found in the iris of the eyes and exhibits fine, graded contractions that are discrete and localized
• Multi-unit smooth muscle is involuntary, non-syncytial, and contractions do not spread widely through it
• Multi-unit smooth muscle is sensitive to circulating chemical substances
• Resting membrane potential of cardiac muscle is greater than -80 mV
• Impulses are transmitted from one nerve cell to another at synapses, which are junctions where the axon terminates on another nerve cell
• Types of junctional transmission include chemical, electrical, and both chemical and electrical (conjoint)
• Chemical mediators bind to receptors on the surface of the post-synaptic cell, triggering intracellular events that alter membrane permeability
• Electrical transmission occurs when membranes come close together to form gap junctions, allowing the passage of ions
• The myoneural junction is where the motor nerve terminates on a skeletal muscle fiber
• Terminal buttons at the myoneural junction contain vesicles with acetylcholine, the chemical transmitter at this junction
• Acetylcholine is enclosed in small vesicles found in the terminal buttons of cholinergic neurons
• Acetylcholine acts on receptors of the post-synaptic cell membrane to increase membrane permeability to Na+
• Acetylcholine is rapidly removed from the synapse by being reabsorbed by presynaptic terminals or hydrolyzed by acetylcholinesterase
• Actions of acetylcholine are categorized as muscarinic and nicotinic actions
• Norepinephrine is found at most sympathetic post-ganglionic endings
• Dopamine is secreted by dopaminergic neurons and, along with epinephrine and norepinephrine, forms catecholamines
• Catecholamines are metabolized by monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT)
• Serotonin has the highest concentration in blood platelets, GIT, brain, and retina
• Histamine is formed by decarboxylation of histidine and is metabolized by diamine oxidase
• Nerve fibers in contact with smooth muscles are cholinergic or noradrenergic