the human nervous system

• homeostasis is how the body maintains sable internal conditions
• thermoregulation is the process of maintaining a constant body temperature in response to internal and external conditions
• osmoregulation is he regulation of water levels
• vasoconstriction is the narrowing of blood vessels to reduce blood flow to the skin
• vasodilation: the dilatation of blood vessels, which decreases blood pressure.
• Homeostasis maintains optimal conditions for enzyme action
• The human nervous system consists of: the central nervous system and peripheral nervous system
• • the central nervous system – the brain and spinal cord
• • the peripheral nervous system – nerve cells that carry information to or from the CNS
• Nerve cells are called neurones. They are adapted to carry electrical impulses from one place to another.
• A bundle of neurones is called a nerve.
• • Cells called receptors, which detect stimuli (changes in the environment).
• • The coordination centre, such as the brain, spinal cord or pancreas, which receives and processes information from receptors around the body.
• Effectors bring about responses, which restore optimum levels, such as core body temperature and blood glucose levels. 
• Effectors include muscles and glands, and so responses can include muscle contractions or hormone release.
• A long fibre (axon) which is insulated by a fatty (myelin) sheath. 
• axons- They are long so they can carry messages up and down the body.
• • Tiny branches (dendrons) which branch further as dendrites at each end. These receive incoming impulses from other neurones.
• Information from receptors passes along neurones, as electrical impulses to co-ordinators such as the central nervous system or CNS. 
•  The CNS is the brain and spinal cord. 
•  Muscles contracting or glands secreting hormones are the response of effectors coordinated by the CNS.
• Stimulus → receptor → coordinator → effector → response
• Receptors are groups of specialised cells. They detect a change in the environment (stimulus) and stimulate electrical impulses in response. Sense organs contain groups of receptors that respond to specific stimuli.
• Effectors include muscles and glands - that produce a specific response to a detected stimulus.
• Skin- Touch, temperature and pain
• Tongue- Chemicals (in food and drink, for example)
• Nose- Chemicals (in the air, for example)
• Eye- Light
• Ear- Sound and position of head
• an example of a effector is a gland releasing a hormone into the blood
• The nerve pathway followed by a reflex action is called a reflex arc.
• Reflexes are rapid responses to stimuli that do not involve conscious thought.
• A reflex arc consists of sensory neurones that carry information from receptors to the CNS, interneurons within the spinal cord or brainstem, and motor neurones that carry impulses away from the CNS to an effector organ.
• 1. Receptor in the skin detects a stimulus (the change in temperature).
  2. Sensory neurone sends electrical impulses to relay neurone, which are located in the spinal cord. They connect sensory neurones to motor neurones.
  3. Motor neurone sends electrical impulses to an effector.
  4. Effector produces a response (muscle contracts to move hand away).
• Where two neurones meet there is a small gap, a synapse.
• 1. An electrical impulse travels along the first axon.
  2. This triggers the nerve-ending of a neurone to release chemical messengers called neurotransmitters.
  3. These chemicals diffuse across the synapse (the gap) and bind with receptor molecules on the membrane of the second neurone.
  4. The receptor molecules on the second neurone bind only to the specific neurotransmitters released from the first neurone. This stimulates the second neurone to transmit the electrical impulse.
• The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions.
• • The cerebrum (the outer layer is called the cerebral cortex), which is split into two hemispheres and is highly folded. It controls intelligence, personality, conscious thought and high-level functions, such as language and verbal memory.
• • The cerebellum, which controls balance, co-ordination of movement and muscular activity.
• • The medulla, which controls unconscious activities such as heart rate and breathing rate,