12.3 - Neurones and Synapse

Basic Structure of a Neurone

• Dendrites: Short branches that receive nerve impulses and send them to the cell body.
• Cell body: Contains a nucleus and dendrites; integrates signals and coordinates metabolic activities.
• Axon: Elongated branch that carries impulses out of the cell body to other neurones or effectors.
• Myelin sheath: Insulating membrane that coats the axon, protects the neurone, insulates electrical impulses and supplies nutrients to the axon.
• Node of Ranvier: Regular gap not insulated by myelin; accelerates impulse flow by allowing impulses to jump from one node to the next.
• Synaptic knob: Swelling at the end of an axon branch that sends signals to muscle cells, gland cells or dendrites of other neurones.

Types and Functions of Neurones

Sensory Neurone

• Located in the dorsal root of the spinal nerve.
• Carries impulses from sensory organ receptors to the central nervous system.
• Cell body is in the dorsal root ganglion.

Relay Neurone

• Nerve fibre located in the central nervous system.
• Connects sensory neurones to motor neurones.
• Cell bodies occur in clusters in the grey matter of the central nervous system.
• Sends impulses from sensory neurones to the central nervous system and from the central nervous system to motor neurones.

Motor Neurone

• Located in the ventral root of the spinal nerve.
• Receives impulses from relay neurones and sends them to effectors such as muscles or glands.
• Cell body is in the grey matter of the spinal cord.

Synapse

• A synapse is the narrow gap separating a synaptic knob from the dendrite of the receiving neurone.
• Synapses allow impulses to travel in one direction and control the types of impulses that pass through them.
• Transmission across a synapse is slow because it occurs chemically.

Transmission of Impulses Across a Synapse

1. An electrical impulse reaches the synaptic knob.
2. Synaptic vesicles release neurotransmitters into the synapse.
3. Neurotransmitters diffuse across the synapse and bind to specific receptor proteins on the dendrite of the receiving neurone.
4. The binding initiates the next electrical impulse in the receiving neurone.

• Examples of neurotransmitters: acetylcholine, noradrenaline, serotonin and dopamine.
• Synaptic knobs contain many mitochondria to generate energy for nerve impulse transmission.