The Peripheral Nervous System

Sensory Organs

The eye is the sensory organ related to vision.

It detects light variations, colours, and can adapt to light intensity.

Normal eyes can differentiate 2,000 colours.

The Eye

Sclera Rigid, opaque membrane of the eye. Protects the eye from shock and gives it shape. The white part.

Choroid Middle layer of the eye. Blood vessels connect here to nourish the eye.

Retina Innermost layer at the back of the eye. Covered in millions of light-sensitive nerve cells that transform incoming data into nerve impulses.

Cornea Clear and rigid membrane that is an extension of the sclera

Iris The extension of the choroid. This membrane is pigmented, giving eyes colour. There is an opening that regulates the amount of light entering.

Lens Flattened sphere that focuses light information on the retina. The part we can see is the pupil.

Aqueous humour

Transparent liquid that fills the space between the cornea and the lens.

Vitreous humour

Transparent jelly-like substance that fills the space between the lens and the retina.

Structures of the Eye

How the eye works

The lens of the eye works similar to a camera lens. It can adjust its size so as to see things at different distances.

When objects are at a distance, the lens flattens. When they are up close, the lens is rounded.

Once colours and light enter the eye, the retina converts the information into nerve impulses, and sends it from neuron to neuron until it reaches the brain.

Information is sent from the optic nerve, ultimately to the brain.

The brain analyzes the information and allows us to make judgements on the colours that we see, the distances of things, their shapes and their relative sizes.

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Occipital Lobe

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Nerve cells on the retina convert light and colours into nerve impulses.

Some of the nerve cells are called cones. Only able to distinguish colours.

Other nerve cells are called rods. Only distinguish variations in light intensity.

Nerve impulses from the retina are transmitted through the optic nerve and sent to the brain. The brain superimposes the images from each eye into one image that we see.

The ears pick up sounds and convert them into nerve impulses.

The ear is divided into three sections: The Outer Ear The Middle Ear The Inner Ear

The Ear

Structure Description

Outer Ear

PinnaThe visible part of your ear. It is funnel shaped so as to pick up sound vibrations from the air.

Auditory Canal

Slightly curved, 2.5 cm long Canal that carries sound vibrations to the eardum. It is lined with hairs and sebaceous glands that produce wax, preventing debris from entering the ear.

Middle Ear

Tympanic Membrane

The eardrum. It is a thin, flexible membrane that moves to the rhythm of sound vibrations.

Ossicles, (bones)

Miniature bones located in the temporal bone. The three, the hammer, the anvil and the stirrup, move in relation to one another.

Eustachian Tube

Canal that links the middle ear to the pharynx. It equalizes pressure on either side of the eardrum during swallowing.

Structure Description

Inner Ear

Semi-Circular Tubes

Canals that are filled with liquid. They regulate balance when the body is in motion and are linked to the vestibular nerve.

Vestibule

Liquid-filled structure that links the semi-circular canals to the cochlea. Plays a role in balancing the body when not in motion and is also linked to the vestibular nerve.

Cochlea

Liquid-filled structure whose walls are covered with auditory neurons and are linked to the auditory nerve. This is where sounds get transmitted from neurons to send to the brain.

Sounds are created by vibrations in the air. Sound waves cause changes in the air pressure surrounding us, creating sound waves that our ears can pick up.

Sound waves are channeled by the pinna and sent down the auditory canal until they reach the cochlea.

In the cochlea, the sound vibrations are transformed into nerve impulses and sent to the brain via the auditory nerve.

In the brain, these nerve impulses are analyzed, causing us to hear.

How the Ear Works

Temporal Lobe

1. Sound vibrations reach the eardrum, causing it to vibrate.

2. This sets the ossicles in motion. The stirrup transmits the vibrations to the vestibular canal.

3. After passing this canal, vibrations are sent to the cochlea, which transmits the vibrations into nerve impulses.

4. Nerve impulses travel via the auditory nerve to the brain.

Path of Sound

The ear also plays an important role in our balance. The ossicles, (hammer, anvil and stirrup), are each

responsible for knowing when our heads are tilting, moving from side to side, or up and down.

The ossicles are activated by the small hairs that they have. When the fluid within them moves, it activates the hair cells.

They can then situate our bodies by sending nerve impulses from the vestibular nerve to the brain.

This is why we know where we are in space, and why we don’t fall down when we walk, dance, play sports, etc…

Another Important Role

The nose is a sensory organ that has multiple functions, as it is crucial in the respiratory system.

Nerve cells that are sensitive to smells are located in the upper wall of the nasal cavity. They are found on a small surface of only 5 cm², called the olfactory epithelium.

Around 15 million nerve cells are found on the olfactory bulb. This bulb transmits the odours into nerve impulses that are sent to the brain for analysis.

The Nose

Olfactory Epithelium

Small, (5 cm²), location where the nerve cells related to smell are found.

Olfactory Bulb Located above the nasal cavity, this bulb transmits odour information into nerve impulses.

Olfactory Nerve Nerve responsible for the transmission of odour nerve impulses to the brain.

Frontal Lobe

The skin is the sensory organ associated with touch.

This is the bodies largest organ, as it covers the entire surface of the body.

Skin has three distinct layers: the epidermis, the dermis and the hypodermis.

The Skin

Layer Structures Description

Epidermis

Dead LayerOuter skin layer. Atmospheric pressure causes these skin cells to die.

Living Layer

Layer of constantly dividing cells, (mitosis). New cells push older cells to the surface.

Dermis

Sensory Receptors

Structures that pick up stimuli, (pressure, heat/cold).

Blood Vessels

Vessels that nourish the skin cells with oxygen and nutrients.

Sebaceous Glands

Glands that secrete sebum, and oily substance that keeps skin waterproof.

Sweat Glands

Glands that produce sweat, carried to the skin surface through pores.

HairArise from the dermis and protrude out of the skin.

Hypodermis Fat Cells

Layer of fat-containing cells that act as an energy reserve and insulators.

How Skin Works

The sensory receptors, (neurons), in the skin allow us to experience: Tactile sensations, (touch, pressure). Thermal sensations, (heat and cold). Painful sensations

The nerve endings in the skin are either free or protected and each one is responsible for different stimuli.

The different sensory receptors are not equally spread out in the body. This is why parts of our bodies experience more pain than others, and why some are more sensitive to cold.

The tongue is a sensory receptor related to taste.

The tongue is covered in papillae, (one papilla), and within each are the taste buds.

There are three nerves that carry nerve impulses created by the taste buds to the brain.

There are 5 tastes that humans can detect: Sweet, sour, salty, bitter and umami.

The Tongue

All taste buds experience all five tastes, regardless of their position on the tongue.

Taste and smell work together. The taste itself only accounts for about 10% of the taste experience. This is because the smell of food often helps us to identify what we are eating.

Other tongue receptors distinguish temperature, discomfort, (prickly feeling), and the textures of foods.

How the Tongue Works

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