The central nervous system

The nervous system is a complex collection of nerves and specialised cells known as neurons that transmit signals between different parts of the body. The nervous system is responsible for the reflex action which is when for example you touch a hot stove, it relays the message to the brain in order for you to remove your hand off the hot surface.

The nervous system allows for the body to be able to respond to stimuli such as the changes in the temperature etc…It is also responsible for all the normal functions of the body such as walking, thinking as well as everything else that make us human. The central nervous system consists of the brain and spinal cord.

Brain

Cerebrum – Its responsible for higher thought processes, controls the voluntary movements and it receives information from the sense organs and interprets it to initiate a response.

Pituitary gland – Responsible for releasing hormones.

Medulla (also known as the Medulla oblangata) – Involuntary movement such as blinking is controlled by the medulla oblangata. It is also responsible for transmitting impulses between the brain and the spinal cord.

Corpus callosum – It relays messages between the two hemispheres of the brain (left and right) as well as connects the two hemispheres of the brain to each other.

Hypothalamus – This part of the brain controls a lot of functions such as sleep, hunger and the temperature of the body.

Cerebellum – Controls the muscle tension in order to help with balance as well as the coordination of voluntary movement.

In order for the spinal cord and brain to be able to relay messages between them there needs to be cells known as neurons that assist with the transmission of these messages.

Neurons

Neurons are the basic cells of the nervous system, they transmit impulses throughout the body from the brain.

Figure: Neuron

Dendrite – It receives the stimulus and carries the impulses towards the cell body.
Cell Body –It holds the nucleus & most of cytoplasm
Axon – fiber which carries impulses away from cell body
Schwann Cells– cells which produce myelin or fat layer in the Peripheral Nervous System
Myelin sheath – dense lipid layer which insulates the axon – makes the axon look gray
Node of Ranvier – gaps or nodes in the myelin sheath .
Impulses travel from dendrite to cell body to axon

Three types of Neurons

There are 3 main types of neurons that all have different functions in the nervous system. The 3 types of neurons are sensory (afferent) neurons, motor (efferent) neurons and interneurons (or relay).

Neuron	Function
Sensory	Transmits an impulse from the sense organs towards the brain and spinal chord.
Interneuron (Relay)	This neuron is found in the brain and the spinal chord, it is responsible for linking the sensory neuron and the motor neuron.
Motor	This neuron responds to a stimulus. It transmits the impulse from the brain and the spinal chord to the effector which can be a muscle.

Synapse

It is a small gap or space between the axon of one neuron and the dendrite of another. Keep in mind that the neurons do not actually touch at the synapse.
It is a junction between the neurons that uses neurotransmitters to start the impulse in the second neuron or an effector (muscle or gland)
The synapse insures one-way transmission of impulses.

Neurotransmitters

Neurotransmitters – These are chemicals in the synapse which allow for the transmission of impulses to the second neuron.

How is an impulse transmitted ?

When a stimuli is detected such as the change in the environment. The neuron detects it if it is strong enough and converts it into an impulse. Once the nerve impulse starts it continues to the end of the neuron in only one direction- from dendrite then the cell body then to the axon where it causes the release of ions (neurotransmitters) which move across the synapse to the next neuron.

Reflex arc

The reflex arc is the path that an impulse is transmitted on in order to bring about a response to a stimulus during a reflex action.

Reflex action

A reflex action is a quick response that occurs in order for the body to protect itself from harm, it does not involve the brain instead only the spinal chord is involved, the neurons are very important in the reflex action. An example of a reflex action is when you prick your finger on a needle and you quickly remove it without even thinking about it in order to prevent hurting yourself. Refer to the figure below for an example on a reflex arc.

Reflex arc path
Receptor (fire, needle etc…) – Sensory neuron – Interneuron – Motor neuron – Effector (muscle)

The Human eye

The eyes are one of our important organs because they allow us to see and sight is very important. The figure below shows the eye and the different functions of the different components.

Figure : The Human Eye

Part	Function
Cornea	Bends the light which is also known as refraction in order for it to land on the retina to produce a clear image.
Suspensory ligament	They hold the lens in place.
Pupil	Its an opening that allows for light to pass through.
Aqueous humour	Fluid filled cavity that supports the cornea and the front part of the eye.
Iris	The part of the eye that has colour.
Lens	Changes its shape to accomodate near and distant vision.
Vitreous Humour	Jelly like substance that gives the eye its shape.
Blind spot	This area has no vision because it does not contain any rods or cones.
Optic nerve	Responsible for carrying nerve impulses from the retina to the brain.
Fovea (yellow spot)	This area has a lot of cones which cause the ability to see colour and a clear image.
Choroid	Contains the blood vessels that supply the eye with nutrients and oxygen, it is also responsible for reducing reflection.
Sclera	Tough white outer layer that protects the eye from damage.
Retina	Contains the rods and cones that are the receptors for sight.

Accomodation

Accomodation is when the eye adjusts itself in order to be able to see distant objects as well as close objects, the different parts of the eye adjust accordingly.

Distant vision (objects further than 6 m)

The ciliary muscles relax.
The tightening of the suspensory ligaments, they become taut.
There is an increase in the tension in the lens.
Lens becomes less convex.
Light rays are refracted less.
The light rays become focused on the retina.

Near vision (objects closer than 6 m)

The ciliary muscle becomes contracted.
Suspensory ligaments become slacken.
The tension on the lens decreases.
The lens becomes more convex.
Light rays are refracted more.
The light rays become focused on the retina.

Pupillary Mechanism

Bright light

The radial muscles of the iris relaxes.
The circular muscles of the iris contract.
The pupil becomes smaller by constriction.
A small amount of light enters the eye.

Dim light

The radial muscles of the iris contract.
The circular muscle of the iris begin to relax.
The pupil becomes wider.
A lot of light enters the eye.

Visual Defects

Defect	Nearsightedness (Myopia) – Close objects are seen clearly.	Farsightedness (Hyperopia) – Distant objects can be seen clearly	Astigmatism	Cataracts
What happens?	Eyeball is elongated, the lens is unable to become flatter. The light rays are refracted too much due to the lens, this causes the light to fall before it reaches the retina. It results in a blurred image forming It becomes hard to see objects that are far clearly.	The lens can’t become convex properly, which results in the eyeball being smaller then usual. The lens is unable to bend the light rays enough, which causes them to fall behind the retina. This results in a blurred image. It becomes hard to see objects that are near.	It occurs when the cornea (the clear front cover of the eye) is irregularly shaped or sometimes because of the curvature of the lens inside the eye. It causes blurred vision.	A white cloudy cover forms over the lens.
How to correct it.	Wearing glasses with a converging (concave) lens helps this condition.	Wearing glasses with converging (convex) lens helps this defect.	Wearing specialised glasses that are customised to fix the defect.	The lens needs to be replaced with surgery.

Now that we have looked at the eye which is one of the major sense organs, we are now going to now look at another one and this is also a very important sense organ. Lets take a look at the ear.

The human ear

The human ear is split up into three different sections which are the outer, middle and inner ear.

Part of the ear	Function
Pinna	Directs sound waves to the eardrum
Eardrum (Tympanic membrane)	Transmits sound waves towards the middle ear.
Ear canal (Auditory canal)	Transmits the sound waves to the eardrum
Ossicles	They help transmit vibrations
Eustachian tube	Ensures that the pressure is equalised on both sides of the eardrum.
Oval window	Transmits the sound waves towards the inner ear.
Round window	Releases the pressure from the inner ear.
Semi-circular canals	Assists in the bodys balance.
Sacculus and utriculus	Assists in the balance of the body.
Auditory nerve	It transmits the impulses to the brain.
Cochlea	It houses the organ of Corti which converts the sound waves into nerve impulses.

How does hearing work?

The way we hear things is a process that is very important certain vibrations cause certain reactions in the different parts of the ear which are then interpreted by the brain as hearing.

Sound waves enter your outer ear and travel through your ear canal to the middle ear.
The ear canal directs the waves to your eardrum (also known as the tympanic membrane) which is a thin, sensitive membrane stretched tightly over the entrance to your middle ear.
The waves cause your eardrum to vibrate.
The eardrum passes these vibrations on to the hammer, one of the ossicles ; the hammer
vibrating causes the anvil, to vibrate; the anvil passes these vibrations to the stirrup, then from the stirrup the vibrations pass into the inner ear.
The stirrup is in contact with a liquid filled sack and the vibrations travel into the cochlea, which is shaped like a shell.
Inside the cochlea, a vestibular system which is formed by three semicircular canals that are approximately at
right angles to each other and which are responsible for the sense of balance and spatial orientation in the body.
It has chambers filled with a viscous fluid and small particles (otoliths) containing calcium carbonate. The movement of these particles over small hair cells in the inner ear sends signals to the
brain via the auditory nerve and the brain processes the information from the ear and lets us distinguish between different types of sounds.

Balance

The ear is not only responsible for hearing, it is also responsible for the balance of the body.

When there is movement, the change in speed and direction in the body causes the cristae in the semicircular canals to be stimulated. Inside the sacculus and the utriculus the maculae is also stimulated by the changes of the position of the head. Once there is movement the cristae and maculae are stimulated and they then convert the stimuli into nerve impulses. The nerve impulses are then transported via the auditory nerve towards the cerebellum. The cerebellum then sends the impulses to your muscles in order to restore the balance in the body.

Hearing defects

There are different hearing defects that can occur, due to different causes.

Hearing defects	Ear infection (middle ear)	Deafness
Causes	A pathogen infection can cause excess fluid in the middle ear.	Certain damage to the brain part that controls hearing Damage to the ear due to injury The hardening of the ossicles or other tissue in the ear. Hardened wax
Treatment	You can take antibiotics. Inserting grommets is another option.	Inserting a cochlea. The use of hearing aids is also another treatment option.