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NERVOUS SYSTEM

The nervous system is the control center for your body, and it helps all the parts of the body to communicate with each other. It also reacts to changes both outside and inside the body. It interprets the things your body senses, and it sends information to the muscles and

glands, telling them what to do. The nervous system uses both electrical and chemical means to send and receive messages.

It also runs the systems you don’t have to think about, like the digestive and cardiovascular systems. The nervous system is also responsible for your moods and your thoughts. The structures of the nervous system include your brain, spinal cord, and nerves that reach to every part of your body. Some common problems of the nervous system include:

Epilepsy: Storms of abnormal electrical activity in the brain causing seizures Meningitis: Inflammation of the membrane covering the brain Multiple sclerosis – the myelin sheaths protecting the electrical cables of the central nervous system are attacked. Parkinson’s disease – death of neurons in a part of the brain called the midbrain. Symptoms include shaking and problems with movement. Stroke – a lack of blood to part of the brain.

The nervous system can be divided up into two common parts: 1. Central and peripheral: The central nervous system (CNS) includes the brain and spinal cord, and the

peripheral nervous system (PNS) includes all the nerves that branch out from the spinal cord and the brain and travel throughout the body.

2. Somatic and autonomic: The somatic nervous system regulates the movement of skeletal muscles, and the

autonomic nervous system controls involuntary actions such as heart rate, digestion, and perspiration.

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CENTRAL NERVOUS SYSTEM (CNS)

The CNS receives signals from sensory nerves, coordinates responses and directs the action. The focus of the central nervous system is on coordinating incoming and outgoing neural impulses. It’s also responsible for your thought processes. Brain: Coordinates actions and response, both voluntary and involuntary. Spinal cord: Carries messages from the brain to the peripheral nervous system and carries information from the peripheral nervous system to the brain.

PERIPHERAL NERVOUS SYSTEM (PNS)

All the nerves outside the central nervous system (CNS) belong to the peripheral nervous system (PNS). The peripheral nervous system connects the central nervous system with the peripheral parts of the body. Peripheral nerves are made up of bundles of nerve fibers, and they’re categorized as either cranial nerves that arise from the brain or spinal nerves that arise from the spinal cord.

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The PNS is subdivided into the: Somatic nervous system (SNS): • Transmits messages to the skeletal muscles. • Specialises in the control of voluntary movements and the communication of

information to and from the sense organs. Consists of sensory neurons (carry information from receptors to the CNS) and motor neurons (carry information from CNS to muscles, except for the organs, smooth muscles, and glands).

• It deals with the parts of the body you can move voluntarily. Autonomic nervous system (ANS): • The autonomic nervous system works with and regulates the involuntary parts of the

body, including the muscles of the heart, the digestive system, pupil dilation and the glands.

• The autonomic nervous system is responsible for initiating the flight and fight response

(discussed further on). • Does not require conscious control (automatic). • Functions continuously.

The autonomic nervous system includes both visceral afferent fibers and the visceral efferent fibers. • Afferent fibers: These fibers carry pain and other impulses from the internal organs

and help regulate visceral functions. • Efferent fibers: These nerve fibers stimulate smooth muscles, glands, and the heart. The autonomic nervous system has two divisions: the sympathetic and parasympathetic nervous systems.

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Sympathetic system: Generally, (but not always) heightens activity of the internal systems, preparing the body for action or emergency situations, also known as fight-or-flight reactions. For example, through the dilation of pupils, increase in heart and breathing rate, raises blood pressure and inhibits digestion. The point is to supply more blood to the brain, heart, and muscles by reducing blood flow to the skin and to the digestive system. When the perceived threat has passed the parasympathetic nervous system reverses the above activity, and returns the body to a state of homeostasis. Parasympathetic system: Is generally active during times of rest and normal conditions by decreasing the heart rate and stimulating the digestive system. This part of the autonomic system helps you rest and digest. It is known to lessen activity of internal organs and return the body to a resting state. For example, through the constriction of pupils, decreasing heart and breathing rates, and stimulation of digestion.

OVERVIEW OF SYMPATHETIC AND PARASYMPATHETIC NERVOUS SYSTEM

Body Organs This organ is

responsible for… During activation of the sympathetic nervous system…

During activation of the parasympathetic nervous system…

Adrenal glands

Secreting adrenalin and noradrenalin.

Increased secretion stimulating heart rate, blood pressure, respiration, relaxation of intestinal muscles.

Inhibits (decreases) hormone secretion, calming autonomic functions.

Heart Pumping blood. Heart rate is increased, pumping more blood through the body.

Heart rate is decreased.

Bronchioles in lungs

Breathing. It dilates (expands) the bronchioles, increasing lung capacity.

Contracts bronchioles, decreasing lung capacity.

Liver Produces bile to aid digestion, maintains glucose level in blood.

Inhibits (decreases) the production of bile. Increases conversion of glycogen to glucose.

Stimulates (increases) the production of bile. Decreases conversion of glycogen to glucose.

Sweat Glands Regulate body temperature.

Increase production of perspiration.

Decrease production of perspiration.

Pupils Regulating the amount of light entering the eyes.

Pupils dilate allowing more light to enter eyes so vision is enhanced.

Pupils constrict.

Bladder Urine storage. Relaxes. Increases contractions.

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Salivary Glands

Aids digestion. Salivation decreases and digestion is suspended. Participant gets a dry mouth.

Salivation increases and digestion resumes.

Stomach and digestive system

Digestion. Decreases contractions. Blood flow can be diverted to other critical areas.

Increases contractions.

Gall Bladder Stores bile. Inhibits the release of bile thus slowing digestion.

Increases the release of bile thus increasing digestion.

Key points

• The nervous system uses electrical and chemical means to help all parts of the body to communicate with each other.

• The brain and spinal cord make up the central nervous system (CNS).

• Nerves everywhere else in the body are part of the peripheral nervous system (PNS).

NEURONS

ROLE OF THE NEURON Neurons (also known as nerve cells) are the basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information. How many neurons are in the brain? The human body is made up of trillions of cells. Cells of the nervous system, called nerve cells or neurons, are specialised to carry "messages" through an electrochemical process. It has been estimated that the adult brain has around one hundred billion neurons and an even larger number of glial cells (discussed further on).

FACTS • The basic building block of the nervous system is a nerve cell, or neuron. Neurons

are shaped differently depending on where they are in the body and the role they play.

• Some of the smallest neurons have cell bodies that are only 4 microns wide. Some of the biggest neurons have cell bodies that are 100 microns wide.

(Remember that 1 micron is equal to one thousandth of a millimeter!)

• Unlike other cells in the body, neurons aren’t easily replaced if they die or are damaged by infection or injury.

• Neurons are similar to other cells in the body because:

• Neurons are surrounded by a cell membrane. • Neurons have a nucleus that contains genes. • Neurons contain cytoplasm, mitochondria and other organelles. • Neurons carry out basic cellular processes such as protein synthesis and energy

production.

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A NEURON

STRUCTURE OF A NEURON All neurons have finger-like branches called dendrites and a long fiber called an axon. The axon, extending from the cell body, is coated by a specialized membrane called a myelin sheath.

The axon feathers out into small branches at the end of an axon called axon collaterals. At the end of the axon collaterals are axon terminals. Each axon terminal has a small knob-like swelling at its tip called a terminal button (also known as synaptic knob, synaptic button or synaptic vesicle).

The terminal button is a small structure like a sac that stores and secretes a chemical called neurotransmitter that is manufactured by the neuron and carries its message to other neurons or cells. Each terminal button sits near to a dendrite from another neuron. The space between the terminal button and the dendrite is called a synapse. Messages jump the synapse from one neuron to the next, using neurotransmitters (special chemicals).

NEURON PARTS AND FUNCTIONS Cell body (soma): The part of the neuron that contains the nucleus and is involved in regulation and coordination. Dendrites: Extensions of the cell body which contain receptors that receive information from other neurons and bring electrical signals to the cell body. Axons: Take information away from the cell body, and transmits neural electrical impulses away from the cell body.

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Axon terminals: Transmit chemical messages (neurotransmitters) to other neurons.

Neurotransmitters: chemicals released by the axon terminal of one neuron and bind to the receptor site on the dendrite of the next neuron. Myelin sheath: Many neuron axons are myelinated, and without the coating, called the myelin sheath, interference may occur, like that which can be experienced when a movie is being played and a lawn mower outside is in operation. Myelin is a white, fatty substance that coats and helps insulate the axon from the activity of other nearby axons. This fatty layer is composed of Schwann cells, and is wrapped around the neuron’s axon which is used to increase the speed of neural impulses. Neurons communicate with each other through an electrochemical process. Synapse: The site of chemical communication between two neurons, where the neurotransmitter of one neuron binds to the receptor site of the next.

SYNAPSE BETWEEN 2 NEURONS

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TRANSMISSION ALONG AND BETWEEN NEURONS When a neuron is stimulated it generates an action potential. Action potentials are considered to be “all or nothing” responses; meaning an action potential is either generated or not once generated an action potential travels along the axon until it reaches the axon terminal, where it causes the release of neurotransmitter into the synapse.

TYPES OF NEURONS There are three types of neurons: 1. Sensory neurons (or afferent neurons): Transmit information from sensory receptors

(e.g. in skin, eyes, nose, tongue, ears) about the change in both internal and external environment TOWARD the central nervous system.

For example, the sensory neurons in the eyes respond to light, but not to sound waves. You are able to take your hard away from a hot stove because sensory neurons (skin receptors) specialised for ‘touch’ are transmitting information from your fingertips to your brain.

2. Motor neurons (or efferent neurons): Transmit information AWAY from the central

nervous system to the muscles or glands. 3. Interneurons (or connecting neurons): Mostly located in the central nervous system,

send information between sensory neurons and motor neurons. REMEMBER: Types of neurons – SAME IC! * * Sensory (Afferent); Motor (Efferent), Interneurons (Connecting).

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Did you know? Neurons are the oldest and longest cells in the body! You have many of the same neurons for your whole life. Although other cells die and are replaced, many neurons are never replaced when they die. In fact, you have fewer neurons when you are old compared to when you are young. On the other hand, data published in November 1998 show that in one area of the brain (the hippocampus); new neurons CAN grow in adult humans. Neurons can be quite large – in some neurons, such as corticospinal neurons (from motor cortex to spinal cord) or primary afferent neurons (neurons that extend from the skin into the spinal cord and up to the brain stem), can be several feet long!

GLIAL CELLS Glial cells (also known as glia and neuroglia) are specialised cells that: • Provide insulation, nutrients and support for neuronal function.

• Aid repair of neurons and eliminating waste materials.

GLIAL CELLS VS. NEURONS The brain is made up of more than just nerve cells (neurons). Although there are about 100 billion neurons in the brain, there may be about 10 to 50 times that many glial cells in the brain. Glial cells outnumber neurons ten times over with 860 billion cells. It was only in 2010 that glial cells were found to assist neurons in forming synaptic connections between each other. Once thought to be simply support cells, cleaning up and helping with myelination, they are now known to also promote dendrite growth, and to be as important as neurons in forming the neural network.

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Fun fact: Interestingly, when they examined Einstein's brain after he died to see if he had more neurons than the average person, what they found instead was that he had way more glia than most people.

DIFFERENT TYPES OF GLIAL CELLS • Astrocyte (Astroglia): Are the largest and most numerous of glial cells; are star-

shaped cells that provide physical and nutritional support for neurons: 1. Protects brain by blocking certain substance such as toxins, that may inure the

brain; 2. Clean up brain "debris"; 3. Transport nutrients to neurons; 4. Hold neurons in place; 5. Digest parts of dead neurons; 6. Forms new connections between neighbouring neurons.

• Microglia: Are small cells and like astrocytes, microglia digest parts of dead neurons

and protect neurons from intruders (acts like immune system cells). • Oligodendroglia: Provide the insulation (myelin) surrounding the axon, to neurons in

the central nervous system (CNS), which help to speed up the process of communication. They also absorb chemicals that the neuron secretes and secrete chemicals that the neuron absorbs, which are supposed to contribute to a neuron’s nutrition and function.

• Ependymal: type of neuronal support cell that forms the epithelial lining of the

ventricles (cavities) in the brain and the central canal of the spinal cord. • Schwann Cells: predominantly have functions

similar to those of oligodendroglia, but provide the insulation (myelin) to neurons in the peripheral nervous system (PNS).

• Satellite Cells: Physical support to neurons in

the peripheral nervous system (PNS). © 1996-2015 Eric

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REMEMBER: Glial cells – SAME SO!* * Satelite; Astrocytes; Microglia; Ependymal; Schwann; Oligodendrocytes

QUESTION 18

A single neuron is simply a single____________________________________.

QUESTION 19 Identify the three main types of neuron in the human nervous system.

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

FURTHER RESEARCH: Neurons and Glia: https://canvas.brown.edu/courses/851434/pages/neurons-and-glia

© canvas.brown.edu

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ACTIVITY 1 WORKSHEET ON THE HUMAN NERVOUS SYSTEM 1. Make a simple sketch of the human body and use arrows and labelling to indicate the

direction information is carried around the nervous system (and body) by each type of neuron.

2. _____________________________ are chemicals manufactured by the body which

enable communication between neurons because they cross tiny gaps between

neurons called ________________________________.

3. Identify the main function of the somatic nervous system.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

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4. Identify the main function of the autonomic nervous system.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

5. Identify the main function of the parasympathetic branch of the autonomic nervous system.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

6. Identify the main function of the sympathetic branch of the autonomic nervous system.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

7. Using an example, explain the fight-flight response.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

Key point: The brain is like a computer controlling all functions of the body, whereas the nervous system’s role is to relay messages to the parts of the body.