Neuromuscular Physiology

Motor unit

• nerve body

• axon

• muscle fiber innervated

Nerve conduction

• action potential passed down nerve segments

• signal remains same voltage throughout nerve

Frequency modulation

• increased frequency of action potentials → increased intensity of muscle contraction

Electrical potential

• ability of electrons to do work/generate energy

• dependent on gradient of electrons (comparison of two sides)

Membrane potential

• potential for electrons to flow across cell membrane and do work

• excitable cells = higher potential

Nernst/Goldman equations

• concentration of ions on both sides & permeability of membrane determines membrane potential

Resting membrane potential

• K+ is main contributor, ATP-dependent Na+/K+ pump maintains

• K+ is very permeable when membrane is in resting/inactive state

• -90mV charge compared to outside of cell

• high K+ concentration in the cell, high Na+ & Cl- concentration outside of the cell

Excitable cells

• higher resting membrane potential

• ability to rapidly depolarize (change potential)

• ex: muscle & nerve cells

Action potential

• stimulus causes membrane to completely depolarize

• occurs when stimulus reaches threshold (~ -60mV)

• cell becomes hyperpolarized before returning to resting potential

• unidirectional down the axon

Excitability

• difference between resting potential & threshold

• lower threshold/higher resting potential = more excitable

• low extracellular K+/high extracellular Ca++ = decreased excitability

• high extracellular K+/low extracellular Ca++ = increased excitability

  • K+ influences resting potential, Ca++ influences threshold

Depolarization

• rapid entry of Na+ via voltage-gated Na+ channels when membrane potential hits threshold value

• positive feedback loop (Na+ channels opening causes more channels to open)

Repolarization

• K+ channels always open to allow K+ to leak out of the cell

• slower process

Refractory period

• period of time after an action potential in which the cell membrane is unable to mount another action potential

• voltage-gated Na+ channels can’t open again for a period of time

Absolute refractory period

• no impulse can cause action potential

• during action potential

Relative refractory period

• maximal/suprathreshold impulse can cause action potential but normal threshold stimulus will not cause action potential

Total refractory period

• absolute + relative refractory periods

Neuromuscular junction

• junction between the terminal nerve fiber & the muscle fiber

• voltage-gated Ca++ channels open at end of nerve & Ca++ enters cell

• vesicles from cell release acetylcholine into cleft to bind with cholinergic nicotinic receptors on motor end plate of muscle

  • can re-bind or get degraded

• Na+ channels on motor end plate open & cause end plate potential

A band

• dark/myosin w/overlapping actin/thick filaments

• remains same length

• finger-like projections w/resting potential energy

I band

• light/actin/thin filaments

• varies in length depending on contraction

• binding sites w/high affinity for myosin, covered by tropomyosin w/troponin in resting state

Sarcomere

• between Z lines (where actin filaments of two sarcomeres join), center M line (myosin)

Contraction

• Ca++ contained in sarcoplasmic reticulum released to actin molecule, binds to troponin & displaces tropomyosin

• myosin heads bind to actin

• ATP releases myosin head from actin

• ATP breakdown to ADP resets myosin head to potential energy

Relaxation

• Ca++ pumped back into sarcoplasmic reticulum + released from troponin

• actin binding sites re-covered, cross-bridges detached

Passive tension

• tension generated by inherent elasticity of muscle

• increases as length increases

Active tension

• created by myosin-actin cross-bridges

• tension wanes at very high length - maximum tension when all myosin heads engaged

• total tenson follows curve

Summation

• multiple firings in short period of time that increases force of contraction