12.3 - Neurones and Synapse

The nervous system is made up of millions of nerve cells called neurons. A typical neuron has several distinct parts:

Figure 6 - The basic structure of an efferent neuron.

Characteristics and Functions of a structure efferent neuron

• Cell body

  • Contains a nucleus and many dendrites.
  • Functions by integrating and coordinating metabolic activities.

• Axon

  • Elongated branch of the cell body.
  • Sends impulses to other neurons or effectors.

• Dendrite

  • Short branches of the cell body.
  • Receives impulses from other neurons or the outer environment and sends them to the cell body.

• Myelin sheath

  • Thick coat that insulates the axon.
  • Protects and insulates the axons
  • Speeds up transmission of nerve impulses.
  • Provides nutrients to the axon.

• Node of Ranvier

  • The part of the axon that is not insulated by the myelin sheath.
  • Speeds up the transmission of nerve impulses.

• Synaptic knob

  • Located at the end of the axon.
  • Transmits signals to the effectors or neuron dendrites.

Neurons transmit nerve impulses to other nerve cells, glands or muscles. There are 3 types of neurons – afferent neurons, efferent neurons and relay neurons.

Figure 7 - Motor/efferent neuron

• Situated in the ventral root of the spinal nerve.
• Carry information relay neuron to the effectors such as muscles or glands.
• Cell body is found in the grey matter of the spinal cord.

Figure 8 - Sensory/afferent neuron

• Found in the dorsal root of the spinal nerve.
• Carry sensory information from the receptors to the brain and spinal cord.
• Cell body is present in the dorsal root ganglion.
• Dendrites receive the impulses which are passed to the next neuron.

Figure 9 - Relay neuron

• Connects sensory neuron to motor neuron.
• Cell body is present in the grey matter of the CNS.
• Transmits impulses from the sensory neuron to the motor neuron.

Structure and Functions of Synapses

The transmission of information along the neuron is through electrical signals called nerve impulses. A nerve impulse moves along the axon of a neuron until it reaches the synaptic terminal. A synapse is the site where the two neurons or a neuron and an effector cell communicate.

Transmission of Impulse Across a Synapse

The transmission of information across a synapse involves the conversion of electrical signals to chemical signals in the form of neurotransmitters. Some examples of neurotransmitters are acetylcholine, noradrenaline, serotonin and dopamine.

Figure 10 - 1. When an electrical impulse reaches the synaptic knob, it triggers the synaptic vesicles to release neurotransmitters into the synapse. 2. The neurotransmitters diffuse across the synapse and bind to specific receptors on the receiving neuron. 3. The binding of the neurotransmitters to the receptors leads to generation of the next impulses.