There are two main type of cells that make up the nervous system — neurons and glial cells. An single nerve cell is called a neuron.
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There are about a trillion neurons in the human nervous system! These important cells enable communication within the nervous system. To carry out this function, neurons possess certain crucial properties:. There are three types of neurons in the nervous system — afferent, efferent and interneurons.
They provide information about the external environment and the regulatory functions being carried out by the nervous system. An afferent neuron has a receptor at its ending that generates action potentials in response to a particular stimulus. These action potentials are transmitted along the length of the axon towards the spinal cord which is part of the CNS. Efferent neurons are mainly located in the peripheral nervous system, but their cell bodies orginate in the CNS. Many incoming signals from the CNS converge onto the efferent neurons, which then affect the outgoing signals to various organs in the body.
These organs then carry out the appropriate response. Interneurons are located entirely within the CNS. As previously mentioned, in addition to neurons, glial cells are the other major cell type that make up the nervous system. Glial cells are also called neuroglia. However, they only occupy about half of the space in the brain because they do not have extensive branching like neurons. Unlike neurons, glial cells do not conduct nerve electrical signals.
They instead serve to protect and nourish the neurons. Neurons depend on glial cells to grow, nourish themselves, and establish effective synapses. In addition, they maintain and regulate the composition of the fluid surrounding the neurons in the nervous system.
This is very important because this environment is highly specialised, and very narrow limits are required for optimal neuronal function. Glial cells also actively participate in enhancing synaptic function. There are also two types of glial cells in the PNS — Schwann cells and satellite cells. Astrocytes are so named because they have a star-like shape. They are the most abundant glial cells and have the following crucial functions:.
Oligodendrocytes form sheaths around the axons of the CNS that serve as insulation. These sheaths are made of myelin , which is a white material that enables the conduction of electrical impulses. They are made of the same tissues as monocytes , which are a type of white blood cell that leaves the blood and sets up a front-line defence against invading organisms throughout the body. Ependymal cells line the internal cavities of the CNS. The ependymal cells that line the cavities of the brain also contribute to the formation of cerebrospinal fluid CSF.
These cells have tail-like projections called cilia. The beating of this cilia assists the flow of CSF throughout the brain cavities. Ependymal cells also act as stem cells in the brain, and have the potential to form other glial cells and new neurons which are only produced in specific site of the brain. Neurons in most of the brain are considered to be irreplaceable. Schwann cells are wound repeatedly around nerve fibres in the peripheral nervous system, producing a myelin sheath similar to the membrane produced by oligodendrocytes in the CNS.
They also play a role in the regeneration of damaged fibres. Satellite cells surround the cell bodies of neurons in the ganglia of the PNS. Their function has not been properly defined yet. Book your health appointments online Find and instantly book your next health appointment with HealthEngine.
Find health practitioners. A synapse typically involves a junction between an axon terminal of one neuron, known as the presynaptic neuron, and the dendrites or cell body of a second neuron, known as the postsynaptic neuron. Less frequently, axon-to-axon or dendrite-to-dendrite connections occur. Some neurons within the CNS have been estimated to receive as many as synaptic inputs!
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The axon terminal of the presynaptic neuron conducts electrical signals called action potentials towards the synapse. The end of the axon terminal has a slight swelling known as the synaptic knob. This is where chemical messengers called neurotransmitters are made and strored. The synaptic knob of the presynaptic neuron is located near the postsynaptic neuron. The space between the two neurons is called the synaptic cleft, and is too wide to allow current to pass directly from one cell to another, preventing the transference of action potentials between neurons. Synapses only operate in one direction.
Presynaptic neurons influence the cell membrane voltage known as the cell membrane potential of postsynaptic neurons, but postsynaptic neurons cannot directly affect presynaptic neuron membrane potentials. An excitatory synapse is one where the postsynaptic neuron becomes more excitable as a result of synaptic events. At such a synapse, a neurotransmitter binds to its receptor on the postsynaptic neuron. This leads to a few potassium ions moving out of the cell, and many sodium ions moving into the cell. Both potassium and sodium ions carry one positive charge, so the overall effect is that the inside of the cell membrane becomes slightly more positive, making it easier for action potentials to be elicited compared to when the cell is at rest.
This change in membrane voltage at an excitatory synapse is called an excitatory postsynaptic potential EPSP. An inhibitory synapse is one where the postsynaptic neuron becomes less excitable as a result of synaptic events. This leads to potassium ions leaving the cell, and chloride ions entering the cell.
Motor Neuron Diseases Fact Sheet | National Institute of Neurological Disorders and Stroke
Potassium ions carry a positive charge while chloride ions carry a negative charge, so the overall effect is that the inside of the cell membrane becomes slightly more negative, making it more difficult for action potentials to be elicited compared to when the cell is at rest. This change in membrane voltage at an inhibitory synapse is called an inhibitory postsynaptic potential IPSP.
It is made up of the brain and the spinal cord , which are located within and protected by the skull and the vertebral column respectively. The other part of the nervous system is called the peripheral nervous system PNS.
This is made up of all the parts of the nervous system that are not part of the CNS. The peripheral nervous system PNS is made up of nerves and ganglia clusters of nerve cells. Nerves emerge from the CNS through the skull and vertebral column, using the PNS to carry information to the rest of the body. The PNS is made up of two divisions — sensory and motor. Within these six divisions, there are other sub-regions. These are divided according to what kind of structures they are primarily made up of. One region is called grey matter.
Brain Basics: The Life and Death of a Neuron
Grey matter is mainly made up of cell bodies and dendrites. It is called grey matter because it has a grey appearance in fresh material. The other region is called white matter , and has a white appearance in fresh tissue. White matter is mainly composed of axons, which give it its white colour because of a membrane around the axons known as a myelin sheath.
The spinal cord has in important role in controlling the muscles of the limbs and the trunk, as well as the functions of internal body organs. It also processes information from these structures, and sends information to and from the brain. The spinal cord is divided into many segments. It also contains a pair of roots called the dorsal and ventral roots. These roots become intermingled with the spinal nerves, and contain sensory and motor axons which are part of the PNS.
The axons and spinal nerves work together to transfer information between the muscles and organs of the body, and the spinal cord. Medulla : The medulla is located just above the spinal cord. It contains structures known as pyramids that carry signals from the cerebrum to the spinal cord.
This stimulates the skeletal muscles in the body, which are generally the muscles used to create movement. The medulla also receives information from the spinal cord and other parts of the brain, and transfers it to the cerebellum.
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Parts of the medulla also receive information from the taste buds, the pharynx , as well as the chest and abdominal cavities. The cell structures that receive this information have several functions, including:. Pons and cerebellum : The pons is a bulge at the front of the brainstem, while the cerebellum is located underneath the cerebrum.
The cerebellum has roles in muscle coordination, emotion, and cognitive processes such as judgement. The midbrain connects the hindbrain and the forebrain to each other. It is divided into different regions:. The diencephalon is made up of two components called the thalamus and the hypothalamus. Thalamus : The thalamus has an important role in transferring information to the cerebral hemispheres.
In turn, it receives information from areas in the cerebrum.
Motor Neuron Diseases Fact Sheet | National Institute of Neurological Disorders and Stroke
Signals from all over the body are also sent to the thalamus, which directs this information to the cerebrum to be processed. The thalamus is closely interconnected with the system responsible for emotion and memory — the limbic system. Eye movements, taste, smell, hearing and balance are also linked to the thalamus.
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