NERVOUS COORDINATION

• • A nerve is a bundle of nerve cells.
• • Nerve cells are called neurones.
• • Neurones pass electrical impulses (also known as nerve impulses, or simply impulses) along their length.
• Nerve impulses are electrical.
• Nervous system controls are:

  • communication is by nerve impulses
  • transmission is by nerone
  • transmission is very rapid
  • nerve impulses travel to specific parts of the body
  • response is localised
  • response is rapid
  • response is short-lived
  • effect is usually temporary and reversible
• The structure of a myelinated motor neurone

  A) Dendrite

  B) Cell body

  C) Node of Ranvier

  D) Axon terminal

  E) Synaptic knob

  F) Schwann cell

  G) Myelin sheath

  H) Axon

  I) Nucleus

  
• Synapse - junction between two neurons where an action potential can be transmitted from one neuron to another
• Action Potential - sudden change in membrane potential that travels down axon
• Myelination increases speed of transmission of nerve impulse
• Na+ = sodium ion

  K+ = potassium ion

  Positively charged ions are collectively known as cations.
• • Cell-surface membranes are also known as “plasma membranes”.
• A neurone at rest has an overall positive charge on the outside and an overall negative charge on the inside, despite positive and negative ions on both sides.
• At rest, a neurone has:

  • A higher concentration of sodium ions outside compared to inside.

  A higher concentration of potassium ions inside compared to outside.
• • The term “potential difference” is used to describe charge differences between one point and another.
• • The unit for potential difference is the volt, V.
• • There is a potential difference across the cell-surface membrane of a neurone (positive outside cell, negative inside cell).
• • At rest, a typical neurone cell-surface membrane has a potential difference of -65 mV. We call this the “resting potential”.
• mV stands for millivolt (one thousandth of a volt).
• • Membrane potentials are always measured “inside of cell relative to outside of cell”.
• • The inside of a neurone at rest is overall negative, whilst the outside is overall positive, hence the minus sign on the resting potential.
• • The more negative a membrane potential value is, the more negatively charged the inside of the cell is relative to the outside.
  • The more positive a membrane potential value is, the more positively charged the inside of the cell is relative to the outside
• • A nerve impulse is simply a change in the membrane potential of a neurone that is propagated (sent) along the cell-surface membrane of that neurone.
  • This change in membrane potential that occurs as an impulse passes through is called the action potential.
• Action potentials depend on additional channel proteins in the membrane
• • Voltage-gated sodium ion channel proteins
  • Voltage-gated potassium ion channel proteins

  These channels open in response to specific voltages. They are both closed at resting potential.
• • Depolarisation is the term used whenever a membrane potential becomes less negative or even becomes positive.
• the term electrochemical gradient is used to describe concentration gradients of ions.
• Changes in membrane permeability lead to depolarisation and the generation of an action potential.
• • A nerve impulse is created by an action potential passing along a neurone.
• • The passage of an action potential can vary depending on whether the neurone has a non-myelinated or myelinated axon.
• • Nerve impulses are influenced by axon diameter and temperature.
• • Once sodium ions start diffusion in the axon, they repel other positive ions, accelerating current flow. The positive ions are also attracted to the negatively charged parts of the neurone ahead.
• Axon diameter also affects the speed of impulse transmission
• • Wider diameter axons lead to faster impulse transmission. This is for two reasons:
  • Organelles can get in the way of local current flow in the axon. Wider axons provide more alternate pathways for the ions to take. Therefore, wider axons have less resistance to the flow of local currents.
  • Wider axons have lower surface area to volume ratios. This reduces the leakage of local current ions per unit volume of axons and therefore strengthens these currents.
• Temperature also affects speed of impulse transmission 
• • Higher temperatures increases the speed of impulse transmission;
  • This is because higher temperatures increase kinetic energy of the ions and therefore the speed of ion diffusion;

  Excessively high temperatures will denature the channel and pump proteins and impulse transmission will stop.