Central nervous system (CNS) anatomy

• Cells that make up the Nervous System
  • Neurons
  • Glial Cells
• Synapses
  • What does a synapse look like?
  • What happens at a synapse?
  • Excitatory synapses
  • Inhibitory synapses
• What is the CNS?
• Interactions between the central and peripheral nervous systems
• Parts of the CNS
  • Grey matter and white matter
  • Spinal cord
  • Brain stem
  • Midbrain
• Diencephalon
• Cerebral hemispheres
• Meningeal layers
• Cerebrospinal fluid (CSF)
• How does the CNS develop?
• What happens to the CNS as we age?
• How do maternal factors affect brain development during pregnancy?
  • Alcohol
  • Drugs
  • Maternal diabetes

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.

Synapses

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 100 000 synaptic inputs!

What does a synapse look like?

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.

What happens at a synapse?

1. A electrical signal (an action potential) is initiated and transmitted to the axon terminal of the presynaptic neuron. This stimulates voltage-regulated calcium ion channels in the synaptic knob to open.
2. The concentration of calcium ions becomes much higher outside the neuron compared to inside, so calcium ions flow into the synaptic knob through the open calcium channels.
3. The increased calcium ion concentration inside the neuron causes the release of neurotransmitter from the synaptic cleft.
4. The neurotransmitter moves across the synaptic cleft, and binds to receptors on the postsynaptic neuron.
5. Binding of the neurotransmitter to its receptor causes the opening of chemically-regulated ion channels on the postsynaptic neuron, allowing different ions to enter or leave the postsynaptic neuron.

Excitatory synapses

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).

Inhibitory synapses

An inhibitory synapse is one where the postsynaptic neuron becomes less excitable as a result of synaptic events. At such a synapse, a neurotransmitter binds to its receptor on the postsynaptic neuron. This leads to potassium ions leaving the cell, and chloride ions entering the cell. 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).

What is the Central Nervous System (CNS)?

The central nervous system is one part of the body’s overall nervous system. 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.

Interactions between the central and peripheral nervous systems

The peripheral nervous system (PNS) is made up of nerves and ganglia (clusters of nerve cells). The PNS and CNS work together to send information between the brain and the rest of the body. 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. The sensory division carries signals from all over the body back to the CNS to be decoded, while the motor division carries signals from the CNS to cells all over the body to carry out the body’s responses to this information.

Parts of the CNS

There are six main parts of the CNS. These are:

1. Spinal cord
2. Medulla
3. Pons and cerebellum (which along with the medulla, form the brain stem)
4. Midbrain
5. Diencephalon
6. Cerebral hemisphere

The last 5 components of the CNS mentioned above are all part of the brain.

Grey matter and white matter

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. 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.

Spinal cord

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.

Brain stem

The brain stem is made up of the medulla, pons and cerebellum. It has the following functions:

1. Receive incoming information from structures in the skull.
2. Transmit information between the spinal cord and higher brain regions.
3. Put together the actions of the different parts of the brain stem to regulate levels of stimulation.

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.

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:

1. Controlling heart rate and how hard the heart pumps
2. Controlling blood pressure
3. Controlling how fast and how hard breathing is

The medulla also plays important roles in speaking, swallowing, coughing/sneezing, vomiting, sweating, salivation, and tongue and head movements.

Pons and cerebellum: The pons is a bulge at the front of the brainstem, while the cerebellum is located underneath the cerebrum. The pons transfers information from the cerebrum to the cerebellum, and is also involved in sleeping, hearing, balance, facial sensation/expression, breathing, and swallowing. The cerebellum has roles in muscle coordination, emotion, and cognitive processes such as judgement.

Midbrain

The midbrain connects the hindbrain and the forebrain to each other. It is divided into different regions:

• Cerebral peduncles
• Tegmentum
• Substantia nigra
• Central grey matter
• Tectum
• Medial lemniscus

Diencephalon

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. 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.

Hypothalamus: The hypothalamus is the major control centre of the autonomic nervous system, therefore playing important roles in ensuring all the systems in the body function smoothly. It is also involved in the release of hormones from the pituitary. The hypothalamus is involved in many body functions including the following:

1. Hormone secretion
2. Autonomic effects (acting as a control system for the body)
3. Regulating body temperature
4. Detecting food and water intake (making you feel hungry or thirsty)
5. Sleep and waking
6. Memory
7. Emotion and behaviour

Cerebral hemispheres

The cerebral hemispheres are made up of four major parts:

1. Cerebral cortex
2. Basal ganglia
3. Hippocampus
4. Amydala

Cerebral cortex: The cerebral cortex is located on the surface of the cerebral hemispheres. It is highly convoluted and folded. This allows a large surface area to fit inside the confined space of the skull. The cerebral cortex is divided into four lobes called the frontal lobe (front lobe), the parietal lobe (between front and back lobes), the occipital lobe (back lobe) and the temporal lobe (side lobes).

Basal ganglia: Basal ganglia are collections of cells that are located deep inside the brain and have important roles in many higher brain functions. One function in which they play an important part is the control of movement.

In Parkinson’s disease, the basal ganglia are damaged. Patients with Parkinson’s disease experience tremors and a slowing of movement as a result. Basal ganglia also influence other aspects of behaviours such as cognition and emotion.

Hippocampus: The hippocampus has an important role in the formation of memories. It is also part of the limbic system, which influences thought and mood.

Amydala: The amydala coordinates the release of hormones and the actions of the autonomic nervous system. It is also part of the limbic system, and has a role in emotion.

Meningeal layers

The meningeal layers are sometimes referred to as meninges. They are three separate layers that enclose the brain and spinal cord. Their roles are mainly to protect the brain and to circulate blood to and from the brain. The three layers are:

1. Dura mater
2. Arachnoid mater
3. Pia mater

Dura mater: Dura mater is the outermost of the meningeal layers. It is the thickest membrane. The dura around the cerebral hemispheres and the brainstem is actually made up of two layers. The outer of these layers is attached to the inside of the skull.

Arachnoid mater: The arachnoid mater is the middle meningeal layer. It lies next to the dura mater, but is not tightly bound to it. The space existing between the two layers is known as the subdural space. Breaking of a blood vessel in the dura mater can cause bleeding and a formation of a blood clot in this subdural space, resulting in a subdural haematoma. This is dangerous because the blood clot can push the arachnoid and dura layers apart, compressing the brain tissues.

Pia mater: The pia mater is the innermost meningeal layer, adhering to the brain and the spinal cord. It is a delicate layer and is separated from the arachnoid mater by a space known as the subarachnoid space. The space is filled with cerebrospinal fluid (CSF) and contains the veins and arteries overlaying the surface of the CNS.

Cerebrospinal fluid (CSF)

Cerebrospinal fluid (CSF) bathes the inside of the brain through a network of cavities within the CNS known as the ventricular system. CSF has the following functions:

1. Buoyancy. The brain neither sinks nor floats in CSF, but instead remains suspended in it because the two components have very similar densities. This allows the brain to grow to an attainable size without being impaired by its own weight. If the brain were allowed to rest on the floor of the skull, the pressure from its own weight would kill the nervous tissue.
2. Protection. CSF protects the brain from striking the inside of the skull when the head is jolted. However, there is a limit to this protection as a severe jolt can still result in the brain damaging itself by striking or shearing against the floor of the skull.
3. Chemical stability. CSF ends up being absorbed into the bloodstream. This provides a way of clearing wastes from the CNS, and also allows it to maintain its optimal chemical environment. Slight changes in its composition can cause malfunctions of the nervous system. For example, if the CSF is too basic (not acidic enough), it can lead to dizziness and fainting.

How does the CNS develop?

A human embryo consists of three major cell layers known as the ectoderm, mesoderm and endoderm. The CNS develops from a specialised region of the ectoderm called the neural plate. The process through which the neural plate starts to form the nervous system is called neural induction.

The neural plate lies along the midline of the embryo. A midline indentation forms and deepens along the neural plate to form a groove known as the neural groove. This groove then closes to form a hollow tube known as the neural tube. All major components of the CNS are then present including the spinal cord and brain stem.

What happens to the CNS as we age?

The functioning of the nervous system changes from childhood to old age, reaching its peak development at around the age of 30. Different aspects of brain function tend to be affected at different ages. For example, vocabulary and the use of words start to decline at around age 70, while the ability to process information can be maintained until age 80 if no neurological disorders are present.

As aging occurs, the overall number of nerve cells starts to decline. A brain generally weighs 56% less at the age of 75 than at the age of 30 due to this decrease in brain cells. Overall brain function is also slowed due to several factors. These include less efficient synapses and the slowing down of the transmission of electrical signals between neurons.

Engaging in mental and physical activity (ie. exercise) can help to slow the decline in brain functioning, especially in the area of memory. Conversely, consuming 2 or more standard alcoholic drinks per day can speed up the decline in brain activity.

However, not all functions of the CNS are affected in the same way by old age. Although skills such as motor co-ordination, intellectual function and short-term memory decline, language skills and long-term memory can be retained, in the absence of any neurological pathology. Elderly people often remember things in the distant past better than recent events.

How do maternal factors affect brain development during pregnancy?

Alcohol

Foetal alcohol syndrome (FAS) and other congenital abnormalities are freqently linked to alcohol exposure. FAS is one of the most frequent causes of non-genetic mental retardation. Features of FAS include:

• Facial abnormalities, including small eye openings, flattened cheekbones, depressed nasal bridge, and an underdeveloped groove between the nose and upper lip
• Growth retardation, resulting in low birth weight
• Brain dysfunctions ranging from moderate learning difficulties to severe mental retardation
• Defects in vision and hearing

There is no “safe” amount of alcohol that a pregnant woman can consume without any risk for her foetus. It is strongly recommended that pregnant women do not consume any alcohol at all.

Drugs

Heroin and methadone: Heroin and its substitute, methadone, are often taken together with other toxins such as cocaine, alcohol or tobacco. The exact nature of these drugs on the developing brain is not well studied. However, laboratory studies suggest that they can greatly influence brain development, causing changes in brain cells under laboratory conditions.

Cocaine: Like most other toxins, cocaine is associated with an increased risk of prematurity and intra-uterine growth retardation. Cocaine exposure during development has been linked to microcephaly, malformations of the brain, and several other brain defects. After birth, effects of cocaine can include sleep disturbances, difficulties in feeding, and epileptic fits. These symptoms generally disappear within the first year of life.

However, some chidren who were exposed to cocaine as a foetus develop long-term neurological difficulties. Their IQ is generally within the normal range, but they may often exhibit difficulties concentrating, becoming distracted easily, and behave aggressively or impulsively. They are also at increased risk of developing anxiety or depressive disorders.

Caffeine: Caffeine is broken down more quickly during pregnancy, and some animal studies suggest that caffeine is concentrated in the developing brain. Caffeine by itself, when taken in low to moderate amounts, does not appear to greatly increase the risk of foetal malformations.

Smoking: Maternal smoking is major risk factor for sudden infant death symdrome (SIDS). It is also linked to increased risk of growth retardation and conduct disorder (a psychiatric disorder). Two susbstances found in cigarette smoke, carbon monoxide and nicotine, affect the foetal brain by acting directly on it, or by causing a lack of oxygen supply.

Maternal diabetes

Maternal diabetes can be type I, type II, or gestational diabetes. All three increase the risk of foetal brain malformation. However, these can be prevented by following a special program designed for pregnant diabetic women to keep their condition under control. Patients’ doctors will normally advise diabetic pregnant women on these programs.

References

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