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Skin The skin is the largest organ in the body. Scattered throughout the skin are millions of tiny sense organs containing sensory nerve endings. Each sensory nerve cell is sensitive to either: 1. Touch e.g. fingertips, tongue, lips (blind person uses fingertips – Braille) 2. Pressure e.g. fingertips, tongue 3. Pain e.g. tendons, muscles, ligaments: distributed evenly in the skin. warning device to say if muscle is over-stretched 4. Temperature e.g. front of fore arm, back of hand: hot and cold receptors distinguish ½ºC difference. The tongue is NOT temperature sensitive – this is why it burns. (cook tells if there are lumps in the food) The distribution or density of each type of sensory receptor varies. Hair movements There are sensory receptor cells at the base of the hair in the skin. These are stimulated when hairs move (by touch or wind). Cats and mice avoid obstacles in the dark using their whiskers. free nerve ending – detects pain/cold capillaries near surface of dermis hair follicle sweat duct / pore epidermis dermis sweat gland layer of fat and connective tissue nerve ending in hair Tongue The tongue is muscle. On its upper surface are taste cells grouped into taste buds. Taste is a property of substances in solution. There are four types of taste bud – each is specialised and sensitive to one particular taste: 1. Sweet property of sugars at tip/front of tongue 2. Sour property of acids at edge/sides of tongue 3. Salt property of strong acids at tip/front of tongue 4. Bitter property of plant alkaloids at back of tongue bitter sweet and salt sour The 4 tastes are grouped into different parts of the tongue. The tongue is temperature insensitive and that is why we burn it. Flavour is a combination of taste and smell. This is shown by not being able to taste properly when you have a cold. Smell: there are small smell sensory receptors high up in the nasal cavity (i.e. the nose!) – generally these are sensitive to harmful substances (e.g. decaying food). Brain The brain is a complex network of neurones and weighs approximately 1.5 kg. It contains thousands of millions of neurones. Each neurone synapses with thousands of other neurones. It needs protection because it is so important and the protection is given by the skull bones. cerebrum medulla oblongata responsible for eyesight hypothalamus pituitary gland cerebellum spinal cord Medulla oblongata positioned at the top of the spinal cord – where the spinal cord joins the brain. It is the reflex centre for unconscious activities (e.g. heart beat, blood pressure, blinking) Cerebellum this is the area concerned with muscular coordination. It receives nerve impulses from all the sense organs concerned with muscular activity Cerebrum this is a dome shaped mass of neurones and is the largest part of the brain. There are two cerebral hemispheres: the right and the left. In the centre of the cerebrum is the grey matter. This is the part, which is the most advanced in humans. It is responsible for all conscious activity. The cerebrum receives nerve impulses from all the sense organs and will be responsible for all the conscious activity. This includes reasoning, decision-making, memory storage and emotions. Hypothalamus this lies on the underneath front part of the cerebrum. It is the part of the brain that monitors and controls the body temperature, the concentration of the salts in the blood and the concentration of CO2 and acid in the blood. It is linked to the pituitary gland. Pituitary gland this controls the activity of all endocrine glands. Therefore, the hypothalamus is the link between the nervous system and the endocrine system in controlling and coordinating the body.