muscle contraction

Action Potential and Muscle Contraction

Definition of Action Potential

  • The action potential is described as an electric impulse that results from the movement of sodium (Na+) and potassium (K+) ions in and out of the cell membrane.

  • This impulse is the message that conveys necessary actions such as:

    • Muscle contraction

    • Secretion of hormones

    • Secretion of enzymes

  • The origin of the signal is always the brain.

Motor Neurons and Sensory Neurons

  • Motor Neuron:

    • Responsible for causing an action.

  • Sensory Neuron:

    • Takes information from stimuli to the brain for processing.

  • The brain processes this sensory information and sends a message via the motor neuron to initiate muscle contraction.

Synapse

  • Refers to the space between the motor neuron and the muscle cell membrane.

  • The neurons do not physically touch the muscle cells; instead, this gap is essential for communication between them.

  • In order for the muscle to contract, a neurotransmitter must cross this synapse.

Neurotransmitter Release

  • The neurotransmitter involved in muscle contraction is acetylcholine (ACh).

  • Neurotransmitter Definition: Carries a nerve message across the synapse.

  • For ACh to be released into the synapse:

    • Calcium (Ca2+) ions must enter the motor neuron through voltage-gated channels.

    • Voltage-gated channels: Open in response to a change in electrical charge.

    • If the electrical charge is insufficient to reach the end of the motor neuron, calcium cannot enter, and the message cannot be transmitted.

ACh Receptors

  • ACh binds specifically to ACh receptors on the muscle cell membrane.

  • ACh Receptor: A specialized site where acetylcholine can bind to trigger a response in the muscle cell.

  • Each receptor is uniquely shaped to ensure specificity; only the correct neurotransmitter can fit.

Mechanism of Muscle Contraction

  1. Phase One: Communication

    • Upon binding of ACh to the receptor, an action potential is generated in the muscle cell membrane.

    • This changes the charge inside and outside of the membrane.

  2. Phase Two: Calcium Release

    • The action potential travels down the muscle cell membrane into the T-tubules (named for their T shape).

    • Adjoining is the sarcoplasmic reticulum (SR), an organelle that stores calcium ions.

    • The action potential causes the release of calcium from the sarcoplasmic reticulum into the cytoplasm of the muscle cell.

  3. Phase Three: Contraction

    • Calcium binds to troponin, moving it away from binding sites on actin.

    • Now myosin heads can attach to actin filaments, initiating a power stroke which is the actual muscle contraction, enabling shortening of the sarcomere.

    • In order for contraction to occur, energy must be supplied, which is obtained from ATP (adenosine triphosphate).

    • During contraction, ATP is broken down to ADP + P (adenosine diphosphate plus a phosphate), fueling the power stroke.

Energy Pathways for ATP Generation

  • ATP must be regenerated to sustain muscle contraction. There are three pathways for ATP regeneration:

    • Creatine Phosphate (CP):

    • Provides an immediate source of energy for muscle contraction but is short-lived.

    • Anaerobic Respiration:

    • Occurs without oxygen and is used for quick bursts of energy.

    • Aerobic Respiration:

    • Requires oxygen and is preferred for longer, more sustained activities.

  • All three energy pathways operate simultaneously, but certain pathways may dominate based on the type of muscle activity (e.g., aerobic during long runs, anaerobic during sprints).

Muscle Relaxation

  • After contraction, muscles must return to a relaxed state, which requires the removal of calcium ions from the cytoplasm back into the SR.

  • This is facilitated by an enzyme called ACh esterase that breaks down ACh, stopping the signal for contraction.

  • Once calcium is removed, the troponin shifts back to block the binding sites on actin, preventing further contraction.

Applications and Implications

  • Botox:

    • Temporarily paralyzes facial muscles by blocking ACh receptors or inhibiting the influx of calcium, thus preventing muscle contraction.

  • Muscle Relaxants:

    • Work by disrupting the chain of communication between motor neurons and muscle fibers.

Summary of Muscle Function

  • The cycle of contraction and relaxation in muscles is rapid and allows for fluidity in motion across various activities. Muscle function integrates biochemistry with physiological responses to maintain homeostasis during tasks, whether it be a quick movement or prolonged physical effort.