1.6 nervous system and hormones

• a stimulus is a change in the environment
• To respond to a stimulus, it requires a coordination system
• There are two different types of coordination systems : the nervous system and the endocrine system (hormones)
• The hormonal (endocrine) system is composed of glands, which secrete messages in the form of chemicals called hormones. Hormones are transported in the blood, which travel slowly. These responses tend to be longer lasting.
• The nervous system is made of neurons and communicates using nerve impulses. Messages are transmitted along nerves and they travel very fast. The effect of the message lasts a very short time.
• Hormones are chemicals produced by special glands.
• Stimulus > Receptor > (TRAVELLED BY SENSORY NEURONES) > synapse > Coordinator > synapse > (TRAVELLED BY MOTOR NEURONES) > effector > response
• A neurone is adapted to its function by having a cell body with the nucleus, an axon which carries impulses quickly along neurones, an insulating sheath which speeds up the transmission of nerve impulses
• Effectors are either muscles (which respond by contracting) and glands (which respond by secreting chemicals)
• Some examples of receptors are : eye (detecting light), ears (detecting sounds/gravity/movements), nose/tongue (detecting chemicals), skin (detecting pain, pressure, temperature)
• There are three different types of neurones : sensory neurones (carry nerve impulses from receptors to the CNS - central nervous system), relay neurones (connect sensory neurones to motor neurones) and motor neurones (carry nerve impulses from the CNS to the effectors)
• Synapses are tiny gaps which are between neurones. They function as junctions between neurones.
• Vesicles are tiny sacs containing chemicals (neurotransmitters)
• The impulse arrives at the end of the first neurone. This causes a release of chemicals from tiny sacs stored at the end of the neurone. Then, the chemicals are released and travelled by diffusion across the synapse. Finally, the chemicals attach to the receptors on the surface of the next neurone.
• IF A HIGH ENOUGH CONCENTRATION BUILDS UP, A NEW NERVE IMPULSE IS TRIGGERED IN THE SECOND NEURONE.
• Nerve impulses cannot travel the 'wrong way' across the synapse. There are two reasons for this. The sacs of the chemical are only stored on one side of the synapse (the pre-synaptic neurone) and the receptors for the chemical are only found on the other side (the post-synaptic neurone)
• A reflex is an automatic response to a specific stimulus
• Reflexes are very fast that they happen before you have time to think. Since reflexes are protective responses, any damage done to the part of your body should be minimal.
• A withdrawal reflex refers to pulling a part of your body away from a painful stimulus
• Withdrawal reflex : Stimulus > Sensory neurone (Dorsal root) > Synapse > Spinal cord > Synapse > Motor neurone (Ventral root) > Effector > Response
• Reflexes involve just three neurones in a pathway called the reflex arc. Having only three neurones in a reflex arc means that only two synapses are in the pathway, which is why reflex reactions is so fast.
• The eye is a slightly asymmetrical glove, about 2-3cm in diameter.
• Light passes through the transparent front of the eye (the cornea and the aqueous humour). Light projects through the pupil, at the centre of the iris and then through the lens to the back of the eye.
• The inside lining of the eye is covered by the retina, which contains special light sensitive receptor cells. The retina converts light energy into electrical impulses. The optic nerve carries these impulses to the brain. The fovea is the most sensitive point of the retina to give the clearest imagery.
• The conjunctiva is a thin transparent skin which protects the cornea at the front of the eyeball from infection
• The sclera is a tough white outer coat which protects the eyeball against mechanical damage.
• The cornea is a clear front of the eye which allows passage of light. It begins refraction of the light rays.
• The choroid is a dark layer which prevent reflection of light within the eye. It also contains blood vessels to supply the retina.
• The ciliary muscle is a ring of muscle which supports the lens. The muscles contract to change the shape of the lens to focus light from different distances. The ciliary body also secretes aqueous humour.
• The iris is a sheet of coloured muscle which controls the size of the pupil at its centre and adjusts how much light enters the eye
• The lens completes refraction to focus the image.
• The retina is a layer of light receptor cells called rods and cones.
• The fovea is the most sensitive part of the retina which contains only cone cells
• The blind spot is a region where the optic nerve leaves the inside of the eyeball and it contains no light receptor cells.
• The vitreous humour is a clear jelly which fills the back of the eyeball. It helps to support the lens and maintains the shape of the eyeball.
• The aqueous humour is a watery liquid which maintains the shape of the front chamber of the eyeball
• The optic nerve relays nerve impulses from the retina to the brain.
• The iris is a sheet of muscle with a hole in the middle called the pupil. By the iris contracting or relaxing, the iris can change the size of the pupil and let less or more light into the eye
• The iris has two layers of muscle. Circular muscle (which runs in circles around the pupil) and radial muscle (which radiates out from the pupil, like a bicycle wheel)
• In dim light, the radial muscle contracts, dilating the pupil and letting more light into the eye