Nerves and muscles

• Describe the sequence of a reflex arc

  1. Receptors in the skin detect a stimuli/change in the environment
  2. Sensory neuron passes this nerve impulse to the spinal cord
  3. Coordinating system passes on the nerve impulse to relay neurons
  4. Motor neuron carries the nerve impulse to an effector (muscle/gland)
  5. Effector brings about a response
• Why are reflex arcs important?

  They are rapid, involuntary responses that protect the body from harm
• What do sensory neurones do?

  Transmit impulses from receptors towards the central nervous system (brain and spinal cord)
• What do motor neurones do?

  Motor neurons transmit signals away from the brain and spinal cord to muscles and glands (effectors), allowing for voluntary and involuntary movement and control. They form part of the autonomic nervous system
• What do relay neurons do?

  Relay neurones transmit impulses between sensory and motor neurones, in the central nervous system
• Label the motor neuron:

  .

  A) dendrites

  B) nodes of ranvier

  C) axon terminals

  D) myelin sheath

  E) schwann cells

  F) axon

  G) nucleus

  
• What is the function of the cell body?

  Control centre of the neuron. It directs impulses from the dendrites to the axon
• What is the function of dendrites?

  Fine hair-like extensions on the end of the neuron that receive incoming impulses
• What is the function of the axon?

  Conducts impulses from the cell body to the opposite end of the neuron
• What is the function of Schwann cells?

  They wrap around the axon, and their cytoplasm is rich in myelin, an insulating lipid
• What is the function of the myelin sheath?

  Made up of Schwann cells, and it insulates the impulse as it passes down the axon
• What is the function of the Nodes of Ranvier?

  Gaps between Schwann cells where membrane is unmyelinated, causing the speed of an impulse to increase as it jumps from node to node - saltatory conduction
• What is the resting potential?

  The potential difference across the membrane of a neuron when it isn't transmitting an impulse. The inside of the membrane is more positively charged than the outside, therefore is polarised (-70mV)
• How is resting potential maintained?

  1. Active transport of Na+ and K+ ions across the membrane by transport proteins

  2. Na+ ions are pumped out faster than K+ ions are pumped in (3:2 ratio)

  3. Passive diffusion of Na+ and K+ ions across the membrane through ion channels

  4. Na+ ions diffuse in more slowly than K+ ions diffuse out, therefore membrane loses positive charge quicker than it gains it, making it more negative

  5. Large anions (negatively charged proteins inside the axon e.g glucose) cannot diffuse out of the membrane, as they're large, charged and polar

  6. Rate of diffusion of K+ ions out is higher than the rate of K+ ions diffusing in, due to electrochemical gradient
• What is an action potential?

  The change in membrane potential when an electrical stimulus depolarises the membrane - the inside is now positive and the outside is negative

  -70mV becomes +40mV
• Resting potential

  The inside of the neuron is negative compared to the outside, and most of the ion channels are closed
• Action potential propagation

  1. Electrical stimulus applied to axon

  2. Action potential initiated - Na+ channels open, and Na+ ions diffuse into the axon, making it less negative

  3. When threshold is reached (-55mV) more Na+ ion channels open

  4. Repolarisation - Action potential moves to next patch of membrane along the axon - Na+ ion channels close, and K+ ion channels open, allowing K+ ions to diffuse out of axon down concentration gradient, making the inside of the membrane negative again

  5. Hyperpolarisation - K+ ions flood out too fast, and reduce membrane potential to -80mV

  6. Restoring resting potential - resting potential restored to -65mV. K+ ion gates now close and Na/K pump uses ATP(energy) to restore Na+/K+ gradients back to -70mV
• Label the graph on transmission of nervous impulses:

  .