A neurone consists of a cell body (with a nucleus and cytoplasm), dendrites which carry electrical impulses to the cell, and a long axon which carries the impulses away from the cell.
The axon of one neurone and the dendrites of the next neurone do not actually touch. The gap between neurones is called the synapse.
There are 3 processes involved in nerve transmission:
Generation of a nerve impulse (action potential) of a sensory neurone occurs as a result of a stimulus such as light, a particular chemical or stretching of a cell membrane by sound.
Conduction of an impulse along a neurone occurs from the dendrites to the cell body to the axon.
Transmission of a signal to another neurone across a synapse - A chemical transmitter substance is released across the synapse to allow the electrical impulses to pass from one neurone to the next. This substance causes the next neurone to be electrically stimulated and keeps the signal going along a nerve.
The Central Nervous System
The Central Nervous System comprises the parts that are enclosed and protected by bone - the Brain and the Spinal Cord.
The Brain is composed of millions of interconnected neurones with short axons. It is protected within 3 membranes or meninges as well as the skull or cranium.
The Spinal Cord is a bundle of nerve fibres made of many neurones. It is protected by the 3 meninges also as well as the vertebral column.
Cerebro-spinal Fluid lies inside the meninges and acts as a buffer against hard knocks or jolts.
3 Parts of The Brain
Cerebrum (Forebrain) - the largest section of the brain, which lets us think, interpret sensory messages, carry out voluntary muscle movements, remember and have consciousness
Cerebellum (Midbrain) - helps us to keep our balance, and have repetitive muscle control
Medulla Oblongata (Hindbrain or Brain Stem) - control the vital functions of heartbeat, breathing and blood pressure
The hypothalamus is a small cluster of neurones deep within the brain. It plays a central role by regulating many vital processes (e.g. regulating body temperature, heart rate, water balance and blood pressure, carbohydrate and fat metabolism, appetite, sleep and sex drive).
It also links the nervous system with the endocrine system, because it controls the pituitary gland which is the master gland of the endocrine system.
The Peripheral Nervous System
This is the part of the nervous system that does not include the brain and the spinal cord.
There are 2 types of nerves - sensory and motor nerves.
Sensory Nerves carry information about the surroundings from the sense receptors in the skin, eyes, ears, nose and tongue, along the spinal cord to the brain to be interpreted.
Motor Nerves carry messages from the brain through the spinal cord to the muscles and other organs to produce an action.
Some of the nerves of the peripheral nervous system are under voluntary control (e.g. controlling motor nerves and muscles when writing). Other nerves are involuntary or uncontrolled (e.g. regulating heartbeat).
The Autonomic Nervous System
The autonomic nervous system is not under voluntary control.
It consists only of motor nerves transmitting to major organs such as the heart, lungs, digestive organs and skin.
It is a double system with 2 parts that work together - Sympathetic Nervous System and Parasympathetic Nervous System.
ACTION OF SYMPATHETIC NERVES
ACTION OF PARASYMPATHETIC NERVES
Strengthens and speeds up heartbeat
Weakens and slows heartbeat
Constricts arteries and raises blood pressure
Dilates arteries and lowers blood pressure
Slows peristalsis and decreases activity
Speeds up peristalsis and increase activity
Dilates passages, makes for easier breathing
Causes erection of hair
Causes hair to lie flat
A Reflex Arc
A reflex arc involves transmission of a nervous impulse or message from sensory receptors to the spinal cord and back to muscles. Later, the message also reaches the brain for interpretation.
Example: touching your hand on a hot stove
5 main senses are:
The sense of touch includes touch, pressure, heat, cold and pain.
A stimulus is a factor in the surroundings that causes the sense receptors to function.
Touch, pressure, heat, cold, pain
Skin, joints, internal organs
Internal or external force or temperature
Taste buds on tongue
The Path of Light through the Eye
Functions of the Parts of the Eye
Cornea - thin transparent layer at the front of the eye
Aqueous Humour - watery substance that fills the cavity between cornea and lens
Pupil - a hole to allow to pass to the lens
Iris - a coloured circular muscle that contracts or relaxes to dilate or constrict the pupil
Lens - a transparent elastic ball that focuses light rays onto the retina
Ciliary Ligament - attached to lens, and contracts or relaxes to adjust the lens
Ciliary Body - attaches the ciliary ligament to the eyeball, and produces both aqueous and vitreous humours
Vitreous Humour - a more viscous fluid that fills the cavity behind the lens
Retina - a hemispherical layer of light-sensitive cells (rods and cones) at the back of the eye
Fovea - a small area of the retina which is directly in line with the centre of the cornea and the lens, and is concentrated with the colour-sensitive cones
Optic Nerve - the nerve which connects the retina with the vision area of the brain
'Blind Spot' - the place on the retina where the optic nerve attaches; has no light-sensitive cones and rods
Choroid Coat - sheet of cells next to the retina with a black pigment to absorb extra light, and blood vessels to nourish the retina
Sclera - tough outer coat of the eyeball
Light-Sensitive Sense Receptors - Rods and Cones
Rods are the more numerous cells that detect shades of black, grey and white light. They are more prevalent in the periphery of the eye.
Cones are cells that detect coloured light. They are more prevalent in the centre of the retina, particularly the fovea.
There are no rods or cones in the 'Blind Spot' where the optic nerve meets the retina.
Focus of the Lens - When looking at distant objects, the lens is long and thin in shape. When looking at close objects, the lens is short and wide.
Binocular Vision - Two eyes are important in judging distance and depth.
Pupil Size - In bright light, the iris muscle relaxes and the pupil decreases in size so that less light enters the eye.
In dim light, the iris muscle contracts and the pupil increase in size to allow more light to enter.
Short-sightedness (Myopia) - This is a condition where the person can see close objects well, but not distant objects. Light focuses in front of the retina. It is corrected with concave lenses in spectacles.
Long-sightedness (Hyperopia) - This is a condition where the person can see distant objects well, but not close objects. Light focuses behind the retina. It is corrected with convex lenses in spectacles.
Astigmatism - This is a condition where the cornea is curved unevenly, so that different light rays focus in different places. It is corrected with spectacles.
Did You Know That...?
The average person can see for a distance of 2 million light-years.
When we look at something very colourful, we must look directly at it. This is because the colour-sensitive cones in our retinas are concentrated in the fovea which is directly in line with the front of the eye.
When we are in the dark, we see shades of black white and grey but not colour. This is because our colour-sensitive cones require a higher stimulus of light to function than do rods.
Path of Sound through the Ear
Pinna (OUTER EAR)
Auditory Canal (OUTER EAR)
Eardrum (OUTER EAR)
3 Earbones or Ossicles (MIDDLE EAR)
Oval Window (MIDDLE EAR)
Cochlea (INNER EAR)
Functions of the Parts of the Ear
Pinna - funnel-shaped visible flap that directs sound waves into the auditory canal
Auditory Canal - canal that carries the sound waves to the eardrum
Eardrum - a thin membrane which is vibrated by sound waves
3 Earbones or Ossicles (Hammer, Anvil and Stirrup) - These are the smallest bones in the body. The eardrum vibrates, and this vibrates the hammer, the anvil and the stirrup one after another. The stirrup then vibrates the Oval Window.
Cochlea - This is a spirally coiled tube containing fluid and the actual organ of hearing, the Organ of Corti. Each Organ of Corti contains thousands of hairs that are vibrated by the sound waves. The hairs then initiate nervous impulses in the Auditory Nerve which is connected to the auditory areas on the sides of the brain.
Pitch and Amplitude of Sound
High-pitched sound causes intense stimulation of the hairs in the cochlea, whereas low-pitched sound causes less stimulation.
Loud sounds stimulate a greater number of hairs in the cochlea, than do quiet sounds.
Other Features of the Ear
Semi-Circular Canals - These are 3 fluid-filled canals that detect the position of the head in 3 dimensions. Impulses are sent through via the auditory nerve to the brain.
Eustachian Tube - There is a tube that connects each middle ear to the pharynx to equalise air pressure within the middle ear.