Virtual Medical Centre

Central Nervous System (CNS)

• Cells that make up the Nervous System
  • Neurons
  • Glial Cells

Cells that make up the Nervous System

The nervous system can be divided into two sections - the central nervous system (CNS) and the peripheral nervous system (PNS). Our nervous system performs three major functions in the body:

1. It receives information from sites on cells where particular chemicals can bind to and so change the activity of the cell. These sites are called receptors.
2. It processes this information and determines the appropriate response by inergrating all the incoming signals from the receptors.
3. It signals other cells and body organs to perform the appropriate response.

There are two main type of cells that make up the nervous system - neurons and glial cells.

Neurons

An single nerve cell is called a neuron. 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:

• All neurons are very excitable, meaning that they are able to respond to environmental stimuli very well.
• Neurons conduct electricity very well. This allows them to respond to stimuli by producing electrical signals that travel very quickly to cells that may be at a distance.
• Neurons are secretory cells. This means that when an electrical signal is transmitted to the end of the neuron, the cell secretes a particular chemical messenger called a neurotransmitter. The neurotransmitter then stimulates other cells around the neuron.

Neurons are divided into three basic sections:

• Cell body. As the name suggests, this is the main body part of the cell. The key organs needed for cell survival are located in the cell body.
• Dendrites. These are similar to antenna projecting outwards from the cell body. They increase the surface area available to receive signals from other neurons. A neuron can sometimes have up to 400,000 dendrites!
• Axon. The axon is also known as the nerve fibre. It is an enlongated tubular structure that extends from the cell body and ends at other cells. It conducts electrical signals called action potentials away from the neuron. Axons can vary in length, ranging from less than a millimetre to longer than a metre. For example, the axon of the neuron that innervates your big toe must travel the distance from the origin of its cell body which located in the spinal cord in your lower back, all the way down your leg to your toe.
  • The axon hillock is the first portion of the axon, and the region of the cell body from which the axon leaves. The axon hillock is also known as the trigger zone, because this is where action potentials are started.
  • The axon terminal is the end of the axon where action potentials are conducted down to. It is here that neurotransmitters are released.

There are three types of neurons in the nervous system - afferent, efferent and interneurons.

Afferent Neurons

Afferent neurons carry signals towards the CNS - afferent means "towards". 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

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

Interneurons are located entirely within the CNS. They make up about 99% of all neurons and have two main functions:

1. They are located between afferent and efferent neurons, and therefore work to integrate all the information and response from these neurons together. For example, afferent neurons receive information when you touch a hot stove with your hand. Upon receiving this signal, the corresponding interneurons send signals to efferent neurons which then transmit messengers to the hand and arm muscles to tell them to pull away from the hot object.
2. The connections between the interneurons themselves are responsible for various abstract phenomenon of the mind, including emotion and creativity.

Glial Cells

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. Although they are not as well known as neurons, they make up about 90% of cells within the CNS. 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. The glial cells of the CNS therefore support the neurons both physically and chemically via processes needed for cell survival. 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 four major types of glial cells in the CNS - astrocytes, oligodendrocytes, microglia and ependymal cells. There are also two types of glial cells in the PNS - Schwann cells and satellite cells.

Astrocytes

"Astro" means "star" and "cyte" means cell. Astrocytes are so named because they have a star-like shape. They are the most abundant glial cells and have the following crucial functions:

• They act as a "glue" to hold neurons together in their proper positions
• They serve as scaffolding to guide neurons to their proper destination during brain development in the foetus
• They cause the small blood vessels in the brain to change and establish the blood-brain barrier
• They help in repairing brain injuries and in forming neural scar tissue
• They play a role in neurotransmitter activity by bringing the actions of some chemical messengers to a halt by taking up the chemicals. They also break down these taken-up chemicals and transform them into raw materials that are used to make more of these neurotransmitters
• They take up excess potassium ions from brain fluid to help stabilise the ratio between sodium and potassium ions
• They enhance the formation and functioning of synapses by keeping in communication with each other and with neurons.

Oligodendrocytes

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.

Microglia

Microglia act as the immune defence cells of the CNS. 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

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

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

Satellite cells surround the cell bodies of neurons in the ganglia of the PNS. Their function has not been properly defined yet.