Abstract
The nervous system is a complex network of specialised cells called neurons, which coordinate and control the other organ systems.
What are the functions of the nervous system?
The nervous system is a complex network of specialised cells called neurons, which coordinate and control the other organ systems.
The nervous system has three basic functions:
Sensory input. Sensory receptors detect changes in the external and internal environments. In response to a stimulus, the sensory receptor generates an electrical signal – an action potential – which is then relayed to the brain and spinal cord (collectively known as the central nervous system, CNS).
Integration. Sensory input is received and processed by the CNS. Decisions are made on the basis of this sensory integration.
Motor output. Neurons relay action potentials from the CNS to the muscles and glands. Motor output is the only way we can interact with our external environment; even salivation requires smooth muscle contraction. Much of the nervous system is therefore dedicated to producing movement.
Neurons communicate between themselves and other organs through two forms of signalling:
Neural. An action potential is conducted from the cell body of a presynaptic neuron to its terminus. The signal is then transmitted from the presynaptic neuron to a postsynaptic cell (which may be another neuron, a muscle cell or a gland) by means of an electrical or chemical synapse. Whilst nerve conduction is relatively rapid (up to 120 m/s in large myelinated nerves), conduction across a chemical synapse is slower.
Endocrine. The brain synthesises and releases hormones into the circulation, which conveys them to the target organ. Endocrine signalling is therefore much slower than neural signalling.
Describe the structure of a neuron
The neuron is the functional unit of the nervous system. Although neurons vary in their detailed cellular structure, all include the following components (Figure 44.1):
The cell body, which in common with other cell types contains cytoplasm, a nucleus and organelles. The cell body is involved in protein synthesis and the generation of ATP. However, as mature neurons lack a centriole, which is necessary for cell division, neurons cannot undergo mitosis. The cell bodies form the grey matter in the brain and spinal cord. Groupings of cell bodies are termed nuclei in the CNS and ganglia in the peripheral nervous system (PNS).
Dendrites are branched projections that receive signals from other neurons through synapses and propagate them towards the cell body.
The axon is a long projection originating at the cell body. Action potentials are generated at the axon hillock1 and conducted along the axon, away from the cell body. The axon may be either unmyelinated or myelinated. Myelin is made up of multiple layers of electrically insulating lipid and protein, produced by the Schwann cells in the PNS or oligodendrocytes in the CNS. Lipid gives axons a white colour and therefore forms the ‘white matter’ of the brain and spinal cord. Myelin increases the speed of action potential propagation (see Chapter 52).
An axon terminal, the distal end of the axon. At the axon terminal, the neuron communicates with another cell through a synapse. In a motor neuron, this synapse is the neuromuscular junction.
These components can be arranged to give differing nerve morphologies. The major neuron classes are (Figure 44.2):
Unipolar. These neurons have an axon projecting from a cell body. They are not common in humans, but are found in the cochlear.
Bipolar. These neurons have a cell body between the dendrites and the axon. Bipolar cells are found in the retina and the olfactory neurons.
Pseudounipolar. Some bipolar neurons may look unipolar – the axon is interrupted by the cell body approximately midway down. Most sensory neurons are pseudounipolar.
Multipolar. The dendrites insert directly into the cell body without coalescing. The classical example of a multipolar neuron is a motor neuron.
Anaxonic. Dendrites and axons are indistinguishable, looking like a large tree of insertions into a cell body. These are exemplified by amacrine cells in the retina.
Pyramidal cells. These have a triangular cell body (hence the name), a single axon and a large number of dendrites – through these dendrites, pyramidal cells can integrate many afferent signals. They are commonly found in the cerebral cortex and hippocampus.
