B10: THE HUMAN NERVOUS SYSTEM

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 response

All control system include receptors,coordination centres and effectors

Human Nervous System

The nervous system uses electrical impulses to enable you to react quickly to your surroundings and coordinate your behaviour

Cells called receptors detect stimuli (changes in the environment)

Impulses from receptors pass along sensory neurones to the brain or spinal cord (CNS)

The brain coordinates the response, and impulses are sent along motor neurons from the brain (CNS) to the effector organs

Reflex Actions

Reflex actions are automatic and rapid and do not involve the conscious parts

Reflexes involve sensory, relay and motor neurones

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

The Brain

The brain is made up of billions of interconnected neurons that control complex behaviour

It has different regions eg. cerebral cortex, cerebellum and the medulla, each having different functions

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 techniques

The Eye

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

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

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

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

The optic nerve carries impulses from the retina to the brain

Common Problems of the Eye

Accommodation is the process of changing the shape of the lens of the eye to focus on near and distant object

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