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