motor neuron

Overview of Motor Neuron Function and Muscle Contraction

Key Topics

  • Due Dates
    • Topic nine and lab six are due today.

Structure of the Motor Neuron

  • Axon
    • The segment of the motor neuron that ends in an expanded region is called the axon.

Action Potential

  • Definition
    • An action potential is the electrical impulse that travels down the axon of a neuron.
  • Function
    • It transmits signals to the target cell that the neuron contacts.

Calcium Ion Channels

  • Opening Channels
    • When the action potential reaches the axon bulb, it causes calcium ion channels to open.
  • Calcium Influx
    • Calcium ions move from outside the cell into the axon bulb.
  • Purpose of Calcium
    • The influx of calcium stimulates synaptic vesicles containing neurotransmitters to move to the edge of the axon bulb and release the neurotransmitter into the synaptic cleft.

Synaptic Transmission

  • Neurotransmitter
    • The neurotransmitter involved in the motor neuron and skeletal muscle cell communication is acetylcholine (ACh).
    • The specific area of the skeletal muscle cell that contains receptors for ACh is called the motor end plate.
  • Binding Process
    • The released neurotransmitter binds to the ACh receptors embedded in the motor end plate of the sarcolemma (the muscle cell membrane).

Resting Potential and Depolarization

  • Resting State
    • The resting charge of the skeletal muscle cell is approximately -90 mV (millivolts).
  • Depolarization
    • Sodium ions (Na+) enter the cell when receptors change into ion channels, making the cell's charge more positive.
    • This process is known as depolarization.
  • Threshold Potential
    • A threshold potential of -55 mV is necessary for muscle contraction to begin.
  • Overshoot
    • Following depolarization, the membrane potential may overshoot and reach up to +30 mV.

Muscle Contraction Mechanism

  • Initial Action Potentials
    • The depolarization leads to muscle contraction when the action potential reaches the motor end plate, generating a new action potential that spreads throughout the muscle cell.
  • T Tubules
    • Action potentials travel down T tubules (transverse tubules) that wrap around myofibrils within the muscle cell.

Sarcoplasmic Reticulum and Calcium Release

  • Sarcoplasmic Reticulum (SR)
    • When the action potential reaches the SR, it prompts the release of calcium ions into the myofibrils.
  • Calcium's Role
    • Calcium binds to troponin, causing a conformational change that allows binding sites on actin filaments to become accessible to myosin heads.

Muscle Fiber Structure

  • Actin and Myosin
    • Actin: Thin filament,
    • Myosin: Thick filament that has globular heads. Each myosin head acts as an ATPase, splitting ATP to release energy necessary for muscle contraction.

Sliding Filament Theory

  • Cross-Bridge Formation
    • When binding sites on actin are exposed, myosin heads attach, forming cross bridges.
  • Power Stroke
    • Myosin heads pull actin filaments inward during the power stroke. This ratcheting motion continues as long as calcium is present.
    • The cycle: Cross-bridge formation → Power stroke → Release → Reset.
  • Sarcomere Changes
    • During contraction, the sarcomere shortens, bringing Z lines closer together.
    • A band remains the same, while the I band shortens, and the H zone diminishes in size.

Relaxation of the Muscle

  • Termination of Action Potential
    • Muscle contraction ceases when action potentials stop firing, leading to a stop in neurotransmitter release.
  • Removal of Acetylcholine
    • Acetylcholine must be removed from the synaptic cleft by:
    • Diffusion out of the cleft,
    • Reuptake into the axon bulb,
    • Breakdown by the enzyme acetylcholinesterase (AChE).
  • Calcium Ion Removal
    • Without calcium, troponin releases tropomyosin, which covers the actin binding sites, preventing muscle contraction.
  • Elastic Recoil
    • The muscle relaxes back to its resting state due to elasticity and recoil.

Study Resources

  • Additional materials like videos and presentations on muscle anatomy and the sliding filament theory are recommended for review before the exam.

Conclusion

  • The mechanisms involved in muscle contraction are crucial for understanding skeletal muscle physiology and pave the way for the study of cardiac and smooth muscle mechanics in subsequent courses.