9FA Nervous Control

• Nervous control of muscular contraction

  • Identify and describe the function of the nerves, spinal cord and motor neurons
  • Describe the relationship between muscle contraction and nerve function
• Muscles and movement: Nervous control of muscular contraction

  • For a muscle contraction to occur, muscle fibres must be stimulated by neural (nerve) impulses sent via motor neurons or nerves
  • A neural impulse is a short-lived electrical signal that travels along neurons
  • The brain will send a neural impulse to the motor (muscle) neuron that will
• Muscular contraction: Nervous control

  Message is transmitted from the CNS (brain and spinal cord) to the PNS (nerves) and then on to the muscle
• Nerves


  • Made up of bundles of neurons, or nerve cells
  • The neurons that transmit impulses to muscles are called motor neurons
• Central nervous system


  • The brain and spinal cord are key parts of the central nervous system (CNS)
  • Your brain sends a message to your muscles to contract and movement occurs
• Motor neuron


  • Refers to neurons located in the CNS that project their axons outside the CNS and control muscles
  • Motor neurons are responsible for carrying impulses away from the spinal cord and brain to the muscles or glands
• Parts of a motor neuron

  • Dendrites - Short branched fibres that protrude from the neuron, receive neural impulses from the CNS and direct them towards the cell body
  • Cell body - Received impulses from the dendrites and directs them towards the axon
  • Axon - Transmits the message to the muscle and away from the cell body
  • Motor end plate - Connects to the muscle fibre to deliver the impulse
• Direction of impulse


  • Motor neuron - From dendrites towards muscle fibre
  • Sensory neuron - Dendrites send information towards brain
• Motor unit


  • A single motor neuron innervates many muscle fibres
  • All muscle fibres respond once the motor neuron is activated and the impulse reaches a certain intensity and frequency
  • A motor unit is said to be large when it contains many muscle fibres
• Muscular contraction: Sliding filament theory

  1. Message from the CNS reaches the motor end plate

  2. Where it is transmitted to the muscle fibre

  3. Stimulating the myosin crossbridges to attach and pull on the actin filaments

  4. The actin are the sliding filaments

  5. The myosin crossbridges detach and then swing back to reattach further along the actin

  6. The actin slide across the myosin into the centre of the sarcomere, shortening the myofibril (in a concentric contraction)

  7. The attaching and detaching of the crossbridges occurs at different times so that tension is maintained in the muscle fibre
• All-or-none principle


  • Once a motor unit receives an impulse that reaches the required threshold, all of the muscle fibres associated with that motor unit will contract at the same time with maximum force
  • If the signal transmitted from the brain is below the threshold required for the muscular contraction, none of the muscle fibres in the motor unit will contract
  • To increase the strength of contraction, the brain sends more signals resulting in the recruitment of more motor units, as well as recruiting larger motor units
• Smaller motor units vs larger motor units

  • Smaller motor units innervate a small number of muscle fibres and generally produce more precise movements
  • Larger motor units innervate a large number of muscle fibres and generally result in the creation of gross motor skills
• Neuromuscular junction summary


  1. Nerve impulse is sent from the spinal cord

  2. Motor neuron

  3. Motor end plates

  4. Brain

  5. Calcium is released into the sarcomere, revealing the binding sites on the actin

  6. This stimulates the crossbridges to attach to the actin

  7. Cross bridges then detach, reattach, and pull, causing the actin filament to slide across the myosin

  8. Resulting in the shortening of the sarcomere and subsequent contraction (concentric) of the muscle
• ATP is broken down and energy is released, causing cross bridges to contract (pull)
• Sliding filament theory