Human Nervous System

Principle of Homeostasis

- homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes

- homeostasis is important for maintaining optimal conditions for enzyme action and all cell functions

- in the human body homeostasis includes control of blood glucose concentration, body temperature, and water levels

- the automatic control systems may involve nervous or chemical responses

- all control systems include receptors, coordination centres and effectors

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Structure and Function

- the nervous system uses electrical impluses to enable you to react quickly to your surroundings and coordinate your behaviour

- cells called receptors detect stimuli (changes in the environment)

- impluses from receptors pass along sensory neurones

    to the brain or spiral cord (CNS) the brain coordinates the response,

    impluses are sent along motor neurones from the brain (CNS)

    to the effector organs

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Reflex Actions

- reflex actions are automatic and rapid and do not involve the concious parts of the brain

- reflexes involve sensory, relay and motor neurones

- these actions control everyday bodily functions, such as breathing and digestion and to help you avoid danger

- the main stages of a reflex arc are:

    stimulus ---> receptor ---> sensory neurone ---> relay neurone ---> motor neurone ---> effector ---> response

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The Brain

- the brain is made up of billions of interconnected neurones that control complex behaviour

- it has different regions, each having different functions

    cerebral cortex: concerned with consiousness, intelligence, memory and language

    cerebellum: concerned mainly with coordinating muscular activity and balance

    medulla: unconsiousness activities, controlling heartbeat, movements of gut, breathing

- scientists map regions of the brain to their functions by studying patients with brain damage, by electrically stimulating different areas of the brain, and by using MRI scanning technqiues

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The Eye

- the eye is a sense organ containing receptors in the retina that are sensitive to light intensity and colour

- tough outer sclera has a transparent region at the front called the cornea that lets light in and refracts (changes direction of) light towards the retina

- muscular iris controls the size of the pupil and the amount of light entering the eye

- ciliary muscles and suspensory ligaments change the shape of the lens to fine focus light on to the retina

- optic nerve carries impluses from the retina to the brain

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Problems with the Eye

- accommodation is the process of changing the shape of the lens of the eye to focus on near or distant objects

- to focus on close objects the ciliary muscles contract, the suspensory ligaments loosen, and the lens becomes thicker so it can refract light rays strongly

- to focus on distant objects, the ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is pulled thin so it only refracts the light rays slightly

- sight defects can be treated using spectacle lenses (that refract light rays so they focus on the retina) hard and soft contact lenses, laser surgery and replacement lenses in the eye

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