6.2.1 Neurones

Structure of Neurones

1. Cell Body:

• Contains the nucleus and cytoplasm with organelles such as mitochondria and ribosomes to produce proteins and energy required for impulse transmission.
• Site of most metabolic activity within the neurone.

2. Dendrites:

• Branched extensions from the cell body.
• Conduct nerve impulses towards the cell body.

3. Axon:

• A long, single extension that transmits impulses away from the cell body to the next neurone or effector.
• Enclosed in a myelin sheath in some neurones to increase the speed of impulse transmission.

4. Axon Terminals:

• End of the axon that forms synapses with other neurones or effectors, such as muscles or glands.

5. Schwann Cells:

• Surround the axon in myelinated neurones, producing the myelin sheath, which acts as an electrical insulator.

6. Nodes of Ranvier:

• Gaps in the myelin sheath.
• Allow for saltatory conduction, enabling faster transmission of impulses.

Types of Neurones

1. Sensory Neurones:

• Carry impulses from receptors (e.g., skin, eyes) to the central nervous system (CNS).

2. Relay Neurones:

• Found in the CNS.
• Connect sensory neurones to motor neurones.

3. Motor Neurones:

• Transmit impulses from the CNS to effectors such as muscles or glands.

Resting Potential in Neurones

• At rest, a neurone's membrane is polarised, meaning there is a difference in charge between the inside and outside of the membrane.
• Resting potential is maintained at approximately -70 mV by:
  • Sodium-potassium pumps: Actively transport 3 $Na⁺ ions$ out and 2 $K⁺ ions$ in, creating an electrochemical gradient.
  • Potassium channels: Allow K⁺ to diffuse out, further contributing to the positive charge outside.
  • The inside of the neurone is negatively charged compared to the outside due to the presence of large, negatively charged proteins and fewer positive ions.

Action Potentials

• When stimulated, the membrane becomes depolarised:
  • Sodium channels open, allowing Na⁺ ions to flood into the cell, making the inside more positive.
  • If the stimulus is strong enough, it reaches the threshold potential and triggers a full action potential.
  • The action potential propagates along the neurone to transmit the signal.