Excitation

Muscle Contraction: Excitation Phase

Overview of Muscle Contraction Process

  • The first step in muscle contraction is the excitation of the muscle.
  • Skeletal muscle, which is a voluntary muscle type, requires the brain to send signals for contraction.

Nerve Signal Pathway

  • The brain sends signals down the spinal cord.
  • These signals exit the spinal cord to innervate a group of muscle fibers.
    • Muscle Fiber: A long muscle cell; they typically exist in groups.

Anatomy of Skeletal Muscle

  • Example: Biceps brachii contains multiple muscle cells.
  • Striated Muscle: Skeletal muscle exhibits a banding pattern due to:
    • Sarcomeres: Units composed of alternating dark bands (A bands) and light bands (I bands) observable in microanatomy.

Recruitment of Muscle Fibers

  • When the bicep contracts, a specific number of muscle fibers in the biceps brachii are recruited.
  • Signal initiation likely occurs higher in the spinal cord, due to the proximity of the arms and extremities.

Spinal Cord Structure

  • The spinal cord has bulges in the cervical and lumbar regions, known as:
    • Cervical and Lumbar Enlargements:
    • These enlargements accommodate more sensory information from extremities (arms and legs).
    • Contain a higher number of cell bodies.
    • The spinal cord narrows as it descends due to the increasing number of motor neurons and nerves directing towards muscles.

Spinal Cord Anatomy Details

  • Key structures in the spinal cord include:
    • Central Canal
    • Ventral Lateral Horn
    • Ventral Horn
    • Dorsal Horns
    • The Ventral Root (carries outgoing motor signals) and Dorsal Root (carries incoming sensory signals) which combines to form the spinal nerve.
  • Profile view versus anterior view of the spinal cord is discussed, clarifying spatial orientation.

Motor Signal Preparation and Pathway

  • Before contraction, signal preparation occurs in the:
    • Premotor Cortex: Initial signals are relayed to the Primary Motor Cortex.
    • The Primary Motor Cortex activates neurons associated with arm movements (e.g., biceps brachii).
  • The motor pathways discussed include:
    • Lateral Corticospinal Tract: Involved in controlling voluntary movements.
    • Desiccation occurs in the Medullary Pyramids; signals on the left side of the brain control the right side of the body.

Neuron Pathway Description

  • The pathway includes:
    • First Order Neuron: Transmits signals from the brain to the spinal cord, synapses with an Interneuron.
    • Interneuron: Acts as a relay to the Alpha Motor Neuron.
    • Alpha Motor Neuron (Lower Motor Neuron): This is the key executing neuron that triggers muscle contraction.

Motor Unit Concept

  • A Motor Unit is defined as the combination of an:
    • Alpha Motor Neuron and all the muscle fibers it innervates.
  • An action potential is generated due to signals from the premotor cortex that travels down to cause an excitatory postsynaptic potential on the interneuron, thus exciting the alpha motor neuron.

Action Potential Transmission

  • The action potential generation leads to:
    • Movement down the axon of the alpha motor neuron through the Ventral Root to the periphery.
  • When reaching the axon terminals at the muscle's neuromuscular junction, the process includes:
    • Opening of Calcium Voltage-Gated Channels upon receiving an action potential.
    • Release of Acetylcholine (ACh) neurotransmitter into the synaptic cleft.

Neuromuscular Junction (NMJ) Process

  • ACh binds to receptors at the muscle fiber's motor end plate, initiating contraction processes.
  • Components of NMJ include:
    • Axon Terminal: Releases neurotransmitters.
    • Synaptic Cleft: Space between the neuron and muscle fiber.
    • End Plate: Site of ACh action leading to muscle fiber excitation.

Conclusion

  • This process completes the excitation phase for skeletal muscles, setting the stage for contraction to occur as a result of neuronal signaling.