National 5 Biology Course Notes Unit 2 : Multicellular Organisms Part 3 : Control and communication

Nervous control The nervous system The nervous system consists of: The central nervous system (CNS) – the brain and spinal cord The peripheral nervous system – all the other nerves Central nervous system (brain and spinal cord) Peripheral nervous system Structure and function of the brain cerebrum cerebellum medulla Part of the brainFunction cerebrum Thoughts, memories, reasoning, receives messages from sense organs, conscious muscle control cerebellum Controls balance medulla Control of involuntary actions, e.g. heartbeat, breathing rate

Neurons Neurons are nerve cells. There are three kinds of neurons: Sensory neurons These carry nerve impulses from sense organs to the central nervous system Motor neurons These carry nerve impulses from the central nervous system to muscles and glands Relay neurons Found in the central nervous system. Carry nerve impulses from sensory neurons to motor neurons.

Reflex actions Reflex actions are fast inborn – don’t have to be learned carried out in the same way be all members of the species for protection Examples of reflex action Pupil size decreasing in bright light Sneezing when dust enters the nose Pulling hand away from a very hot object

Reflex arc A reflex arc is the path followed by a nerve impulse when carrying out a reflex action. A stimulus is detected by a receptor, e.g. receptors in the skin detect a hot object A nerve impulse passes along a sensory neurone to the central nervous system The effector responds (e.g. the muscle contracts and pulls the hand away from the hot object) A nerve impulse passes along a relay neurone in the central nervous system A nerve impulse passes along a motor neurone to a muscle Relay neurone Motor neurone muscle Sensory cells in skin Sensory neurone synapse Spinal cord Effectors Effectors can either be muscles bringing about a fast response, for example when the hand is pulled away from a hot object or glands bringing about a slow response, e.g. sweat glands producing sweat when the temperature increases.

Nerve impulse reaches the end of the first neurone Synapse Where two nerve cells meet, there is a microscopic space between them called a synapse. When a nerve impulse reaches the end of the first neurone, it causes release of a chemical that triggers a nerve impulse in the second neurone. End of first neurone Start of second neurone Synapse This causes release of a chemical that diffuses across the synapse And triggers a nerve impulse in the second neurone

Hormones As well as the nervous system, parts of the body can communicate through hormones. Hormones are chemical messengers produced by endocrine glands. The endocrine glands release their hormones directly into the blood as it flows through the gland. Hormones travel around the body in the blood but they have an effect only in certain parts called the target tissues of the hormone. This is because the target tissues have receptors on their cells that the hormone can bind to. Control of blood glucose Blood glucose is controlled by two hormones produced in the pancreas called insulin and glucagon. Blood glucose increases after eating Pancreas releases insulin into the blood Insulin reaches the liver Liver changes glucose in the blood to glycogen and stores it Blood glucose level decreases back to normal Blood glucose decreases after fasting Pancreas releases glucagon into the blood Glucagon reaches the liver Liver changes stored glycogen into glucose Blood glucose level increases back to normal

Diabetes Diabetes results either from the body producing no insulin or too little insulin (type 1 diabetes) or from body cells not responding to insulin (type 2 diabetes). Type 1 diabetes is treated by insulin injections while type 2 may be treated by lifestyle changes, e.g. healthy diet and exercise. Untreated diabetes can result in damage to small blood vessels especially of the eyes and kidneys resulting in blindness and/or kidney failure.