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Overview of the Nervous System, Brain Structures & Neuron Types

With all the billions of neurons that are present in the body, things would be a complete mess if these neurons were not organized in some way. So to help tidy things up a bit, the body arranges all these neurons together as part of the nervous system.

The nervous system therefore acts very much like the body’s communication network, as it contains all the highways of information which our brain uses to send and receive information to and from different parts of the body. There are two main divisions of the nervous system, these are: the central nervous system and the peripheral nervous system with each serving a specific role.

In this article, we are going to start by looking at what the basic functions of these divisions are, and then later, look at the endocrine system, the different structures of the brain, and finally, provide a brief overview of the different types of neurons.

The Central Nervous System

The central nervous system (CNS) consists of the brain and the spinal cord.

Central and peripheral nervous system

The central nervous system includes the brain and spinal cord.

The spine, which runs up the centre of your back, carries sensory messages from various parts of the body (e.g. hands, feet, legs etc…) up to the brain. In turn, the brain also sends messages down the spine in the form of motor messages, which then result in the movement of your body.

The importance of these messages running up and down the spine can be seen in people who have suffered from spinal injuries. In severe cases, where the spinal cord has become irreversibly damaged, paralysis can occur resulting in a person who becomes unable to move their legs or other parts of their body.

The Peripheral Nervous System

Central and peripheral nervous system

The peripheral nervous system is "peripheral" because it lies outside the central nervous system.

The peripheral nervous system (PNS) consists of all the neural tissue that lies outside the central nervous system. It can be further subdivided into the autonomic nervous system and the somatic nervous system.

The Autonomic Nervous System

The autonomic nervous system (ANS) mainly deals with involuntary (or automatic) responses. This means that it can sometimes act independently from the brain without receiving any information from it.

As a result, many of the processes that occur in the body which you don’t have to consciously think about (e.g. digestion, breathing, regulation of body temperature), are under the control of the autonomic nervous system. These are known as involuntary responses.

In addition to this, the autonomic nervous system can itself be divided into two types (or divisions); the sympathetic division and the parasympathetic division.

The sympathetic division (fight or flight)

The sympathetic nervous system tends to be activated when there is an increase in autonomic activity. For example, when you exercise, the sympathetic division of the autonomic nervous system will increase your pulse and rate of respiration.

The parasympathetic division (time to rest)

The parasympathetic division of the autonomic nervous system works in reverse to the sympathetic division. It is activated when there is a decrease in autonomic activity. For example, when you are very relaxed (such as during meditation) the parasympathetic division causes your pulse and rate of respiration to decrease.

We can summarize both divisions of the autonomic nervous system by saying;

The sympathetic division speeds things up and prepares your body for activity.

The parasympathetic division progressively slows things down so that you can relax.

sympathetic and parasympathetic division nervous system divisions

The parasympathetic division deals with activities relating to relaxation and digestion. The sympathetic division deals with fight-or-flight responses.

The somatic nervous system

The somatic nervous system is the other major division of the peripheral nervous system, and its role is to control the actions of the body (soma). For example, walking, talking or any other form of voluntary bodily movement.

diagram of nervous system showing peripheral and central nervous system components

Now that you know a little bit about the nervous system and its different components, let’s turn our attention to another very important organizational system within the body, the endocrine system.

The Endocrine System

The body has a series of glands, known as endocrine glands, which play a big role in shaping your moods throughout the day. They do this by secreting hormones into the blood, which then dock with specific receptors causing an effect at their target. Collectively, all these glands working together are known as the endocrine system.

Below we examine some of the major glands of the endocrine system and the types of hormones that they release.

The Pineal Gland (sleep & puberty)

Pineal Gland

The pineal gland secretes melatonin, a hormone that's important for sleep.

The pineal gland is a small gland located roughly in the centre of the brain. It is called the “pineal” gland because it is oval in shape, looking almost like a pine cone. The pineal gland secretes a hormone called melatonin, which is important for sexual maturation and controlling the sleep cycle.

The Pituitary Gland (the master gland)

Pituitary Gland

The pituitary gland secretes hormones that affect other glands.

Below the pineal gland and roughly in line with the centre of your forehead, is the pituitary gland. This gland is often called the “master gland” because it secretes hormones which affect the action of other glands.

For example, the adrenocorticotrophic (ACTH) hormone affects the adrenal glands, the thyrotrophin hormone affects the thyroid gland and follicle stimulating hormone (FSH) affects the gonads.

The pituitary gland also secretes growth hormone, a hormone that affects the growth of the body. In some people who have a tumor in their brain, their pituitary gland secretes abnormal amounts of growth hormone which causes them to grow very large. This condition is known as gigantism.

The Thyroid Gland (fat or thin)

Thyroid Gland

The thyroid gland is important for regulating your metabolism.

Located towards the base and front of the neck is a butterfly shaped gland called the thyroid gland. The thyroid gland secretes a hormone called thyroxine which helps to regulate your metabolism (i.e. the rate at which you burn food).

If low levels of thyroxine are secreted, your metabolism will slow and you will put on weight easily. You may also feel lazy and lack energy. On the other hand, if too much thyroxine is secreted, then your metabolism will become abnormally high making it difficult for you to put on weight. You may also feel hyperactive and full of energy.

Iodine is very important for the proper functioning of the thyroid gland. In countries where people don’t get enough iodine, they develop a disease called goitre where the thyroid gland swells to roughly the size of an orange. Due to modern-day dietary habits, many people have a underactive thyroid gland (hypothyroidism) which may partly explain the rapid rise in obesity over the past few decades.

Things which interfere with the body’s ability to absorb iodine are known as goitrogens, and include things such as fluoride (in water, toothpaste), bromine (in bread, foods) and chlorine (shower, water).

Unfortunately, many of these are competitive inhibitors, which means that they bind to the same receptor sites as iodine. In order to reverse this, it is recommended to take iodine/iodide supplements under your doctor’s supervision at a high dosage for around 3 months.

Adrenal Glands (fight or flight)

Adrenal Glands

The adrenal glands secrete adrenalin which is involved in the fight-or-flight response.

Your adrenal glands (2) are located on top of your kidneys, or just above either side of your belly button.

They produce two main types of hormones. The first are the corticosteroid hormones which regulate the way that the body uses substances such as glucose, salt and water during times of stress.

The second type of hormone is epinephrine (adrenalin) which puts you in a heightened state of awareness so that you can rapidly respond to danger. This is also known as the fight or flight response, part of the sympathetic division of the autonomic nervous system which we discussed earlier.

If you are constantly under stress then your body may release epinephrine throughout the day. This may then stress the body so much that it leads to the development of disease or even an early death.

The Pancreatic Gland (insulin)

diagram of liver, large & small intestine, stomach & pancreas

The pancreatic gland secretes insulin, a hormone important for regulating blood sugar levels.

The pancreatic gland is located roughly in the same location as your belly button, and it secretes the hormone insulin. Insulin is a very important hormone because it helps to regulate blood sugar levels. When the body is unable to produce enough insulin to regulate blood sugar levels, a person is said to have developed the disease of diabetes.

Some people are born genetically predisposed to diabetes (Type I diabetes). However, it has become increasingly more common for regular people to develop diabetes later in life (Type II diabetes).

This is largely due to the over consumption of sugary foods and refined carbohydrates, as such foods lead to insulin resistance in the body. This means that over time insulin become less effective at being able to control your blood sugar levels after you eat.

Insulin also causes the body to store fat, which is why eating lots of sugar can cause you to put on weight quickly and easily.

Gonads (sexual characteristics)

male gonads (testes) & female gonads (ovaries)

The male gonads are the testes, the female gonads are the ovaries.

The gonads are the sexual glands. In males, these are the testes, and in females, they are the ovaries.

The ovaries produce the hormone estrogen, which results in the development of female sexual characteristics such as no facial hair and large breasts. In males, the testes produce the hormone testosterone, which also plays a role in the development of sexual characteristics such as facial hair and muscle growth.

However, it is important to note that both males and females have estrogen and testosterone, and that it is the relative amounts which determine the effects they have on the body.

For example, men who have abnormally high levels of estrogen and low levels of testosterone may develop man breasts, a condition known as gynecomastia. Likewise, women who have abnormally high levels of testosterone but low levels of estrogen may develop facial hair, a condition known as hirsutism.

The reason why hormonal levels may become unbalanced in the body can be due to many things, although one of the major factors is due dietary habits. For example, it is known that the majority of plastics contain estrogen mimicking compounds such as Bisphenol A which are leached into food and water from food packaging and plastic water bottles.

The water supply may also cause an imbalance of these sex hormones as a result of people flushing medications such as birth control pills down the toilet.

The effect of this was first noticed on frogs, which, over time, appeared to change sex due to the chemicals in the water. Overall, this is leading to a society where men are becoming increasingly demasculinized and women increasingly defeminized.

Structures of the Brain

Sitting right at the top of the spine is the brain which acts as the body’s central command centre. Below you will find a brief description of its major components.

Brain Stem

Brain Stem

The brain stem makes up the ancient reptilian part of the brain.

The brain stem connects the brain to the spinal cord, and is considered to be the oldest part of the brain. Some people call this area of the brain the “reptilian brain” because it deals with the regulation of basic autonomic bodily functions.

Reticular Activating System

Reticular Activating System

The reticular activating system can increase or inhibit the amount of information processed by the brain.

Inside the brain stem is a structure known as the reticular activating system (RAS), which projects nerves into various regions of the brain.

The RAS is involved in the regulation of the sleep-wake cycle, and is what causes you to fall asleep quickly or wake up in the morning. Think of the RAS as a light switch. When you switch it on you wake up, when you switch it off you fall asleep.

The RAS is also involved in how much information your brain processes at any one time. For example, when you are feeling drowsy or tired, the RAS reduces the flow of incoming information to the brain.

This is why working when you are tired is generally not very effective, because it will take you much longer to understand something when that information is being processed slowly by the brain.

The reverse of this is when you are very alert and attentive. In this case, the RAS increases the flow of information that your brain receives.

A good example of this occurs during life threatening situations, where the brain becomes hyper-vigilant rapidly absorbing lots of information at once. A peculiar side effect of this, is that time seems to slow down.

Medulla Oblongata & Pons (vital functions)

Diagram showing pons, midbrain and medulla oblongata

The medulla oblongata and pons deal with autonomic bodily functions.

Right at the bottom of the brain stem and connected to the spinal cord, is the medulla oblongata. This is responsible for controlling autonomic bodily functions such as the beating of the heart and the rate of respiration.

Above the medulla is the found a bundle of nerve structures called the pons. The pons is involved in the regulation of breathing, sleep and attention.

Cerebellum (balance & movement)

Cerebellum

The cerebellum is important for making coordinated muscle movements.

Towards the back of the brain stem is the cerebellum. This structure helps to maintain your sense of balance and coordinate muscle movement. The cerebellum is sometimes called the “little brain” because it contains two hemispheres which resemble the larger brain.

Hypothalamus (regulation of biological drives)

Hypothalamus

The hypothalamus helps to regulate the temperature of the body.

The hypothalamus is located just above the brain stem and below the thalamus. This is easy to remember because hypo means “under”. The role of the hypothalamus is to regulate primitive drives such as eating, drinking and sex.

It is also involved in the control of autonomic bodily functions such as temperature regulation and the release of hormones via the pituitary gland.

Pituitary Gland (master gland)

Pituitary Gland

The pituitary gland secretes hormones that affect other glands.

The pituitary gland, which is located towards the front of the hypothalamus, is a small oval-shaped gland which produces hormones that control other endocrine glands.

For this reason it is sometimes called the “master gland”. The pituitary gland also influences the growth of bone structure, sexual maturation and metabolism.

Thalamus (sensory relay station)

Thalamus

The thalamus is responsible for sensory awareness.

Just above the hypothalamus is the thalamus, and its purpose is to pass sensory information from the sense organs to the cerebral cortex.

For example, the optic nerves transmit visual information to the thalamus which then passes it on to the cerebral cortex. The thalamus is responsible for giving us awareness of all our senses, except for smell.

Cerebral Cortex (thinking brain)

cerebral cortex showing frontal, temporal, parietal and occipital lobes.

This is the part of your brain that you think with.

The cerebral cortex is the outermost part of your brain, and what some people call the “thinking brain” because it allows for higher levels processes such as learning, thinking and memory. It consists of the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe.

The Limbic System (emotions)

Limbic System

The limbic system deals with emotions.

This region of the brain is associated with basic needs and emotions such as hunger, pain, pleasure, sex and satisfaction. Damage to the limbic system can lead to an inability to experience pleasure.

The Cerebral Hemispheres

Left and right cerebral hemispheres

The left and right cerebral hemispheres are connected via the corpus callosum.

If we look at the brain from above, we can see that it has two distinct cerebral hemispheres. These two hemispheres are connected by a structure known as the corpus callosum which allows both hemispheres to communicate with each other.

As treatment for some epileptic patients, the corpus callosum is cut to prevent the spread of epileptic seizures from one hemisphere to the other.

Right hemisphere (creative)

The right hemisphere is the creative part of the brain, and is used for things which involve you having to create mental images in your mind. For example, drawing, dancing or meditation. It is also used for facial recognition, music and spatial tasks (3D images).

The right hemisphere is sometimes called the (R)omantic hemisphere.

Left hemisphere (logical)

The left hemisphere deals with verbal and mathematical thinking, and is used for things such as writing, reading, talking, maths and the majority of the work you do at school.

The left hemisphere is sometimes called the (L)ogical hemisphere.

Note: Although one hemisphere may predominantly deal with a certain type of thinking (i.e., lateralization of brain function), it is important to remember that the less dominant hemisphere can also play a role in that same function.

Neuron Structure

Neurons make up the core components of the brain, and specialize in transmitting messages via electrochemical impulses. Structurally, neurons can be simplified by breaking them down into dendrites, the axon and the axon terminal.

Neuron showing dendrites, cell body, axon and axon terminals.

Information flows from the dendrites, via the axon, to the axon terminals as electro-chemical impulses.

Dendrites

Dendrites are the roots of a neuron, and project from the cell body just like roots do from a plant. Their purpose is to pick up information from the surrounding environment and carry it towards the cell body.

Axon

The axon transmits impulses away from the cell body towards the axon terminal. To help speed up this process, the axon is insulated with a myelin sheath that is made up of Schwann cells.

nerve impulse

The presence of a myelin sheath allows nerve impulses to be sent faster by "hopping" along the nerve at the nodes of Ranvier.

Axon terminal

At the very end of the axon is the axon terminal, and this is what transmits information to a neighbouring neuron via the release of neurotransmitters.

Once these neurotransmitters have been released, they will then travel across the synaptic cleft and dock with receptor sites on the dendrites of an adjacent neuron.

Axon terminal showing synaptic vesicles releasing neurotrasmitter

At the axon terminal neurotransmitters cross the synaptic cleft and dock with receptors on the postsynaptic membrane.

And so the whole process begins again, with the dendrites sending information to the cell body and the cell body sending information down the axon and onto the next neuron.

Neurotransmitters & mental disorders

It is believed that neurotransmitters may be responsible for many mental disorders. For example, low levels of the neurotransmitter serotonin are thought to cause depression, whilst excessive amounts of dopamine are thought to cause schizophrenia.

Certain drugs such as ecstasy, work by influencing the activity neurotransmitters. In the case of ecstasy, the massive release of serotonin it causes results in feelings of euphoria.

“Firing” of neurons

When a neuron releases its neurotransmitters, it does so through a process known as depolarization. This involves the neuron alternating from a resting negative electrical potential to a positive electrical potential and then back again to a negative electrical potential.

Neuron Types

The brain has roughly 3 billion neurons, and the nervous system has around the same amount. This is almost the same number as the entire population of Earth, which as of March 2009, is said to be at around 6.76 billion people.

Below you will find a brief description of the three main types of neurons; the sensory neuron, the interneuron and the motor neuron.

Sensory Neuron

Sensory Neuron

The sensory neuron receives information from your senses.

Sensory neurons allow you to receive information from the outside world via your senses. For example, there are sensory neurons that allow you to see light, hear sound, feel things, taste food and smell odors.

Interneuron

Interneuron

"Inter" means between, so an interneuron connects other neurons together.

Interneurons, also called association neurons or connector neurons, communicate with other neurons and represent the majority of the neurons in your brain. They allow you to think, remember and perceive your surrounding environment.

Motor Neuron

Motor Neuron

Motor neurons make muscle movements possible.

Motor neurons are neurons which communicate with muscle fibres, the result of which makes them contract or relax thereby allowing you to move your body.

Nerves

A nerve is a bundle of axons that are twisted together just like the individual fibres of a rope. Nerves can be classified as being afferent or efferent.

Afferent nerve

Afferent nerves carry information inwards towards the brain. For example, the optic nerve is an afferent nerve because it transmits visual information to the brain.

Afferent = (A)nything that comes into the brain.

Efferent nerve

Efferent nerves carry information outwards from the brain. For example, motor nerves carry information to muscle fibres which then causes them to contract.

Efferent = Everything that (e)xits the brain.

Note: It is possible for nerves to carry two-way information (i.e. be both afferent and efferent).

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