the human nervous system

• the conditions inside your body are known as its internal environment. your organs cannot work properly if this is constantly changing. the maintenance of your internal environment is called homeostasis.
• enzymes work best in specific pH and temperature, they control all of the functioning of cells so its important to respond to changes in the internal or external environment to maintain optimum conditions for the cellular enzymes. internal condition that are controlled include body temperature, water content of the body and blood glucose concentration.
• homeostasis involves coordination and control. organisms need to be aware of changes in world around them. detecting changes and responding involves an automatic control system. these automatic control systems include nervous responses in your nervous system and chemical responses in your hormone system.
• receptors are cells that detect changes in internal and external environment. these changes are known as stimuli. coordination centres are areas that receive and process the information from the receptors, they send out signals and coordinate the responses of the body including the brain, spinal cord and also the pancreas. effectors are muscles or glands that bring about the responses to the stimuli, these responses restore conditions to optimum conditions
• receptors cells e.g. light receptor cells in the eye are similar to most animal cells. they have a nucleus, cytoplasm and cell membrane. these receptors are usually found clustered together in a special sense organ.
• once a sensory receptors receives a stimulus, the information is sent as an electrical impulse that passes along special cells called neurones. bundles of hundreds of neurones are called nerves. the impulse travels along the neurone until it reaches the central nervous system (the brain and spinal cord) the cells that carry electrical impulse from your sensory organs to your CNS are called sensory neurones
• the brain gets loads of information and coordinates these responses and sends impulses out along special cells called motor neurones. motor neurons send impulses from the CNS to the rest of the body. they carry impulses to the effectors so they can respond. effectors can be muscles or glands. your muscles respond to impulses by contracting and glands respond by secreting chemical substances
• stimulus then receptor then coordinator (CNS) then the effector is the order of the nervous system. sensory neurones go to the CNS and motor neurones go away from the CNS
• in a sensory neurone, the elctrical impulse will start at the sensory receptor, then travel along the neurone fibres which make the neurone a long strand, the cell body will be connected as a little ball to the fibres and the neurone ends in the CNS.
• in a motor neurones, the cells body is connected to the CNS as a ball at the end of the neurone, the neurone fibres form the line that the electrical impulse travels down and the neurone ending is in a muscle or gland
• automatic responses to a stimulus are called reflexes, they help avoid danger for humans and animals because they happen so fast. reflexes are automatic and fast, they do not involve the conscious part of your brain.
• simple reflex actions involve 3 types of neurones; sensory neurone, motor neurones and relay neurones. relay neurones connect the motor and sensory neurones and are found in the CNS.
• as an electrical impulse passes from receptors along the sensory neurones to the CNS. it then passes along the relay neurones to the motor neurones. from there the impulse will arrive at the effector. this pathway is called the reflex arc. the key difference between the reflex arc and the nervous system is that in a reflex arc, the impulse bypasses the conscious areas of the brain meaning the time taken for the reflex action is as short as possible
• your neurones are not joined up directly to each other, there are junctions between them called synapses which form physical gaps between the neurones. the electrical impulses travelling along your neurones have to cross these synapses, they cannot leap the gap.
• the reflex pathway goes

  1. stimulus
  2. receptor
  3. sensory neurones
  4. relay neurones
  5. motor neurones
  6. effector
  7. response
• when an impulse reaches the synapse between neurones, a chemical is released. the chemical diffuses across the synapse to the next neurone where it will set off a new electrical impulse
• the brain controls everything in the body. there are different parts; the cerebral cortex, the cerebellum, the medulla, the hypothalamus and the pituitary gland
• the cerebral cortex is concerned with consciousness, intelligence, memory and language. the cerebellum is concerned with coordinating muscular activity and balance. the medulla is concerned with unconscious activities e.g. heartbeat, movements of gut and breathing. the hypothalamus is involved with controlling body temperature. the pituitary gland produced different hormones that coordinate and control body systems
• the eye is a sense organ that contains may receptors that are sensitive to light intensity and colour. all of the light sensitive cells are arranged together inside the back of the eye in a special layer called the retina
• the sclera is the white outer layer of the eye. it is relatively tough and strong so the eyeball is not easy to damage. the sclera has a transparent area at the front of the eye called the cornea, this lets light into the eye. the curved surface of the cornea is also very important for changing the direction of light rays coming into the eye. making sure that as they enter the eye, they are focused on the retina
• the muscular iris controls the size of the pupil - the hole in which light enters the eye. it is made of muscles that contract or relax to change the size of the pupil and so control the amount of light going onto the retina. in dim light, the pupil is enlarged so as much light as possible can enter. in bright light, the iris makes the pupil very small to reduce the amount of light going into the eye so too much bright light doesn't damage the delicate light-sensitive cells
• once through the pupil, light passes through a clear disc - the lens. the lens is held in place by suspensory ligaments and ciliary muscle. the lens changes the direction of the light rays too, onto the retina. the eyeball is filled with a thick transparent jelly so it maintains its spherical shape
• when light hits the retina, the light sensitive cells are stimulated so they send an impulse to the brain along the sensory neurones in the optic nerve. when the brain receives the impulse, it interprets this message as an image.
• in bright light, the circular muscles contract, the radial muscles relax and the pupil constricts. in dim light, the circular muscles relax, the radial muscles contract and the pupil dilates.
• the point where the optic nerve leaves the eye has no retina so there is a blind spot. you are unaware of the the blind spot because the brain 'fills the gap'
• the light that enters the eye must focus on the light-sensitive cells of the retina to be seen clearly, if the light is focused in front or behind the retina the image will be blurred.
• the cornea, lens and jelly in the eyeball change the direction of the light rays onto the retina but always refract it by the same amount. the changing of the shape of the lens can be controlled by the contraction and relaxation of the ciliary muscles. this changes the amount the lens refracts the light and this is important for seeing both far and near objects clearly.
• accommodation is the process of changing the shape of the lens to see near and far objects. the cornea is what refracts the light rays but it is the lens that helps you see near and far objects well
• to focus on distant object, the ciliary muscles relax so the suspensory ligaments are pulled tight, meaning the lens is pulled flat and thin. meaning the light is only slightly refracted and will focus on the retina. this is because the light rays will be parallel to each other. the focus on a near object, the ciliary muscles contract so the suspensory ligaments loosen. the lens will therefore be thicker and rounder, refracting light more strongly so they focus on the retina. because the light rays are coming from a near point so are diverging more, meaning they need to be refracted more
• common defects of the eye include myopia (short sight) and hyperopia (long sight). in both of these defects, the light rays do not focus on the retina therefore produce a blurred image.
• if you have myopia, you can see near images but not long images. this is because the light rays are refracted too much and don't focus on the retina, instead focus in front of the retina. this could be because your lens is too rounded therefore refracts the light rays inwards too much. to treat this, you could wear glasses with a concave lens, meaning that before the light rays meet the lens, they are slightly diverged, so when refracted by the lens, they meet on the retina
• if you have hyperopia, you can see distant objects but not near objects. this may be because your lens is too flat and thin, meaning the lens cannot refract the light rays enough so they meet behind the retina. to fix this, you could wear glasses with a convex lens so before the light rays meet the eye, they are refracted inwards a bit, so when the lens refracts them, they meet perfectly on the retina.
• contact lenses are placed on the surface of the eye and do the same job as glasses but cannot be seen and make life easier for sports and in general. hard contact lenses are made from a rigid material and last a long time. they have to be removed overnight and kept sterile to prevent eye infections. most contact lenses are soft and they are made of a flexible material that is more comfortable to wear but don't last as long.
• laser eye surgery is an option for adults who have stopped growing and eyesight is stable. lasers are used to treat myopia by reducing the thickness of the lens. to treat hyperopia the lasers change the curve in the lens.
• the nervous system and the endocrine system are both organ systems
• a neurone is a nerve cell, they are long, thin and have lots of branch connections at each end. for 2 to connect, they have a synapse. this is a gap between the neurones. when the electrical impulses reach the synapse, they release a chemical which diffuses across the gap, another electrical impulse is made and travels
• the central nervous system is made from our brain and our spinal cord. this is where all of the 'thinking' takes place. it takes in lots of sensory information, decdes what needs to be done about it, then will send a response
• sensory neurones take information from receptors all over our body to the CNS. e.g. they could carry information about changes in temperature or carbon dioxide concentration in blood. the CNS will decide what to do, then motor neurones will carry response to our effectors which are either muscles or glands which will contract or secrete hormones
• the reflex arc is the nerve pathway that underlies our unconscious reflexes. these reflexes are rapid and automatic. rather than going up to the brain, a relay neurone will pass impulse to motor neurones
• as light hits the eye, the first layer it will pass through is the cornea. the cornea is transparent and has no blood cells so all the oxygen it needs, diffuses into it rather than getting it from the blood. the cornea is needed as it refracts the light a certain amount