BY4: Structure of neurones
Neurones are highly specialised cells that generate and transmit nerve impulses. There are three types of neurones:
• Sensory: which bring impulses from the sense organs or receptors into the CNS.
• Motor: these carry impulses from the CNS to the effector organs (muscles or glands).
• Connector (Intermediate or relay): – these receive impulses from sensory neurones or other intermediate neurones and relay them to motor neurones or other intermediate neurones.
Neurones are highly specialised cells that generate and transmit nerve impulses. There are three types of neurones:
• Sensory: which bring impulses from the sense organs or receptors into the CNS.
• Motor: these carry impulses from the CNS to the effector organs (muscles or glands).
• Connector (Intermediate or relay): – these receive impulses from sensory neurones or other intermediate neurones and relay them to motor neurones or other intermediate neurones.
Action 1: Draw and label a mammalian motor neurone.
Each nerve cell (neurone) consists of a cell body containing a nucleus and granular cytoplasm containing many ribosomes. These ribosomes are grouped together forming Nissl granules, which are concerned with the formation of neurotransmitter substances. Many thin extensions carry impulses towards the cell body. These short extensions are called dendrites. These receive impulses from other nerve cells and carry the information towards the cell body. Some neurones also have a long membrane-covered cytoplasmic extension, the axon, which transmits impulses from the cell body. At its end, an axon divides into branches called axon terminals which form synapses with other neurones (the gap between the two neurones is the synaptic cleft). Peripheral neurones are surrounded by and supported by Schwann cells. In some cases, these grow around the axons of the nerve cells to form a multi-layered fatty myelin sheath, found only in vertebrate nervous systems. This acts as an electrical insulator and speeds up the transmission of impulses. The myelin sheath can to some extent also protect the axon. The myelin sheath has thin areas at intervals, nodes of Ranvier, which are important in impulse transmission as it is here that the axon membrane is not covered by myelin sheath.