Fun Facts About the Nervous System: A Brief Overview
The nervous system consists of several parts which are responsible for various functions such as movement, sensation, memory, thought and emotion. The central nervous system (CNS) comprises of the brain and spinal cord; it includes all the nerve cells that transmit messages from one part of the body to another. The peripheral nervous system (PNS) comprises of nerves that carry signals between different organs and tissues.
There are two main types of neurons: excitatory and inhibitory. Excitatory neurons stimulate other neurons, while inhibiting them causes them to stop firing. There are also glial cells that support the function of neurons by removing waste products or regulating cell growth. Glia cells are found throughout the body and they include astrocytes, microglia, oligodendroglia and microglial cells.
Neurons communicate with each other through chemical messengers called neurotransmitters. These chemicals pass along axon fibers that connect to dendrites at other neurons. Dendritic spines are specialized structures where these molecules travel along the neuron’s length before reaching their target.
There are a variety of neurotransmitters that play a part in the nervous system and these chemicals can be excitatory or inhibitory. Glutamate is an excitatory neurotransmitter and is involved in the formation of memories and learning. Gamma-Aminobutyric acid (GABA) is an inhibitory neurotransmitter that suppresses unnecessary neural activity. Other important neurotransmitters include acetylcholine, dopamine, serotonin and norepinephrin.
The peripheral nervous system comprises of sensory neurons which bring information from the external environment to the central nervous system; and motor neurons, which carry signals from the brain and spinal cord to muscles and glands. The peripheral nervous system also includes autonomic nerves which innervate blood vessels, glands and smooth muscles.
The brain and spinal cord make up the central nervous system. There are 12 pairs of cranial nerves that arise from the brain and travel to the head, they consist of the olfactory nerve (I), optic nerve (II), oculomotor nerve (III), trochlear nerve (IV), trigeminal nerve (V), abducens nerve (VI), facial nerve (VII), acoustic nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII). There are 31 pairs of spinal nerves that arise from the spinal cord and these include 8 cervical nerves, 12 thoracic nerves, 13 lumbar nerves and 1 coccygeal nerve.
The sympathetic part of the autonomic nervous system innervates the thoracic and abdominal organs. The parasympathetic part of the autonomic system innervates the glands of the head, neck and thorax and the abdomen below the stomach. Control of these two systems is mediated by the hypothalamus. The somatic nervous system consists of nerves that innervate skeletal muscles.
Blood is transported throughout the body via arteries, arterioles, capillaries, venules and veins. Arteries have strong internal walls because they need to withstand higher blood pressures. Arteries gradually merge into smaller blood vessels called arterioles which have a high inner circular and an outer longitudinal muscle to control the diameter of the vessel. Capillaries are very thin blood vessels that act as a interchange between arteries and veins, here oxygen and nutrients are exchanged and waste products are transported away. Venules merge into veins, which have an innermost layer of connective tissue called thin muscled walls.
Blood is transported in blood vessels and the heart pumps the blood around. The four-chambered heart consists of two upper chambers (atria) and two lower chambers (ventricles). Blood enters the heart through the two superior vena cava and the two inferior vena cava, which carry blood from the body and the head respectively. The blood then enters the atria and is pumped through the ventricles and out through the bulbus arteriosus to the lungs. The lungs are separated into bronchi (primary division of the airways) and alveoli (small spherical end buds where gaseous exchange takes place), before returning to the heart.
This completes one circuit of the cardiovascular system.
There are two main types of cells in the blood, these are red blood cells and plasma. Red blood cells or erythrocytes account for over 99% of the blood’s volume and are essentially responsible for transporting oxygen from the lungs to the rest of the body. It is also responsible for transporting carbon dioxide from the body to the lungs where it can be exhaled. The other 1% of the blood’s volume consists of plasma which contains dissolved nutrients and wastes, hormones, clotting factors and cells.
Red blood cells are spherical in shape and have a diameter of 6-8μm, they are able to carry around 10mmols of gaseous oxygen while producing 0.04mmols of carbon dioxide. They are able to carry these gases due to the nature of their internal membrane which contain about 35% protein and other molecules. These red blood cells have a short life span of around 120 days and are constantly being replaced.
There are three types of white blood cells, these include the largest type of lymphocyte (plasma cell), the smallest type of granulocyte (neutrophil) and the largest type of agranulocyte (macrophage). These cells are responsible for combating disease, maintaining the body’s integrity and cleaning up dead or damaged tissue.
There are two types of platelets, these are the large platelets and the small platelets. Large platelets contain cytoplasmic granules that store factors that help promote blood clotting. Small platelets do not contain cytoplasmic granules.
Blood cells are produced in bone marrow which is located throughout bones of the body. Red blood cells mature in the red bone marrow, platelets mature in the yellow bone marrow and the lymphocytes mature in the bone marrow whilst granulocytes mature in the red bone marrow. Blood cells are released into a narrow space between the outer and inner walls of bones called the medulla. The medulla is highly vascularised, meaning that there are many blood vessels within it. Due to the narrow space between the inner and outer bone walls, this pushes the developing blood cells out of the bone marrow into the blood stream when they come into contact with blood vessels.
Blood comes into the heart through the superior and inferior vena cava where it then passes through the right atrium, passes through the tricuspid valve and enters the right ventricle. The blood is then pumped through the pulmonary artery and into the lungs, this process is known as pulmonary ventilation or pulmonic circulation. The oxygenated blood then returns to the heart by passing through the pulmonary veins and enters the left atrium, passes through the bicuspid valve and enters the left ventricle. From here, it is pumped out through the aorta and into the various organs and tissues of the body. As the blood passes through the organs and tissues, it is able to provide nutrients and remove waste materials from these areas.
After passing through the tissues, the blood returns to the heart through the veins. The de-oxygenated blood enters the heart through the inferior vena cava and passes through the right atrium, passes through the tricuspid valve into the right ventricle. It is then pumped through the pulmonary artery and into the lungs, this process is known as pulmonary circulation. The de-oxygenated blood returns to the heart by passing through the pulmonary veins and enters the left atrium, passes through the bicuspid valve and enters the left ventricle. From here, it is pumped out through the aorta and back to the various organs and tissues of the body.
Blood consists of cells (both alive and dead), platelets, plasma and fibrin. The different parts of blood perform different functions within the body. The main parts of blood are-
The blood cells are mainly responsible for transporting oxygen around the body and removing carbon dioxide. There are different types of blood cells and each has its own function:
The normal blood cell count in humans is as follows-
Red blood cells – 4.2-6.2 million/microlitre
White blood cells – 6,780-43,300/microlitre
Platelets – 241-371 thousand/microlitre
The main function of platelets is to help the blood to clot. If a person suffers bleeding injuries, platelets associate together to form clumps that block the bleeding site. Platelets do not move around the body like the other blood cells.
The normal volume of blood in the human body is between 8-10 pints. This does not mean that all of it is in one place, like a container, within the body, there is an equal amount of fluids in the body as there are cells, this is because most of the space inside the body is taken up by the cells. Blood is constantly being produced and used up by the body. The blood in the body has to be a certain level of consistency or it would not be able to perform its functions correctly.
Most of the time blood remains a liquid, but sometimes it is more of a jelly-like substance. The consistency of blood is determined by how much plasma to how many cells it contains.
When an injury occurs in the body and blood leaves the circulatory system, it becomes known as an external loss. In most cases external bleeding can be easily seen and is quite obvious, however sometimes bleeding can occur internally without the person even knowing about it. In these cases only a medical professional can detect the bleeding.
Blood is able to clot quicker than any other bodily fluid because of the platelets, they congregate at the bleeding site and clump together to seal the wound.
In acute bleeding the high pressure within the arteries prevent blood from being lost in large volumes. When a blood vessel is punctured the high pressure is lost and low pressure is quickly introduced, this then causes the blood to pour out of the wound.
There are various important blood vessels in the human body. The ones most commonly thought about are the arteries and veins. Arteries are the blood vessels that carry blood away from the heart and veins bring the blood back to the heart.
Blood vessels can also be classified into three groups-
Arteries are thick, rigid and have a lot of elastic fibre surrounding them. They have a thick lining of smooth muscle to help them maintain a high pressure of blood within them as it passes through. Arteries also contain red blood cells, oxygen and nutrients to the rest of the body. There are three types:
Arteries have the highest pressure of all the blood vessels in the body, this is important as it forces blood to all parts of the body. Arteries can suffer blockages known as arteriosclerosis if fat and other materials build up on the inside wall. This can lead to insufficient blood supply to parts of the body.
Capillaries are very thin, so thin that red blood cells tend to pass through them separately. They are designed to have a very thin wall so that exchange of gases and nutrients can occur rapidly between the blood and the body’s tissues.
Veins have a low pressure of blood within them. They have valves on the inside of their walls which help prevent the backward flow of blood. There is little elastance in veins as there is very little smooth muscle. As the blood is going in the opposite direction to the arteries the pressure within them is lower, towards the heart.
Vessels are divided into three types: Arteries, which carry blood away from the heart; Capillaries, which are very small vessels that connect arteries to veins and take on a “bubbly” appearance due to the close proximity of red blood cells; Veins which carry blood back to the heart.
There are various different factors that can affect the consistency of blood. If the person has been working out and their muscles are fatigued, if they have been sitting down for a long period of time and they are experiencing low blood pressure as a result. If the person has been drinking alcohol and consuming drugs that can also affect the amount of oxygen that reaches the blood. If the person has an infection somewhere within their body this will cause the blood to be thicker due to the body producing more platelets to help it clot. There are also other abnormal chemicals and elements within the blood, some of these can be due to certain foods we eat or diseases that we might have.
Most medical professionals use the term blood typing to classify people into different groups according to their blood types. The ABO system is the most basic method of classifying blood. The ABO system has four different classifications: O, A, B and AB. The first letter refers to the antibodies that occurs naturally in the person’s body. The second letter refers to the antigens that occur naturally in the person’s body.
The table below shows these different types
O Positive: this type is the most common, making up about 43% of the population. It is known as the “default” blood type because this is what a person develops should they receive no or little prenatal care.
A Positive: This is a slightly rarer blood group. It makes up about 13% of the population.
O Negative: This is an even rarer blood type, making up about 5% of the population. It is given to people who need blood transfusions in emergency circumstances if they cannot accept any other blood types.
A Negative: a less common blood type, only making up about 2% of the population.
B Positive: the second most common blood type, making up about 10% of the population. B Positive is known as a “universal receiver”, it can be given to anyone but can only receive B Positive or O Negative blood.
B Negative: this is the rarest of the B blood types, making up about 1% of the population. It is known as a “universal donor”, anybody can receive it but it cannot receive another B negative blood transfusion.
AB Positive: This is one of the two “Universal Receiver” blood types, making up about 3% of the population. It can receive any blood type except another AB Positive blood transfusion.
AB Negative: This is one of the two “Universal Donor” blood types, making up about 1% of the population. It can give any blood type except another AB Negative transfusion.
There are other more advanced methods of typing blood, but this is beyond the scope of this entry.
Sources & references used in this article:
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Brain facts: A primer on the brain and nervous system. by T Orr – 2003 – Scarecrow Press
Therapeutic cannabis: what clinicians need to know… and other fun facts by JH Jackson – British medical journal, 1869 – ncbi.nlm.nih.gov
Lectures on the diseases of the nervous system: delivered at la Salpêtrière by SL Leach – 2017 – utswmed-ir.tdl.org