Coordination, responses and homeostasis

• The human nervous system consists of the central nervous system (CNS) - the brain and the spinal cord, and the peripheral nervous system (PNS) - all of the nerves in the body
• Nerve impulses


  Electrical signals that pass along nerve cells known as neurones
• Nerve


  A bundle of neurones
• Types of neurones


  • Sensory
  • Relay
  • Motor
• Sensory neurones


  • Carry impulses from sense organs to the CNS (brain or spinal cord)
• Relay neurones


  • Found inside the CNS and connect sensory and motor neurones
• Motor neurones


  • Carry impulses from the CNS to effectors (muscles or glands)
• Neurones


  • Have a long fibre (axon)
  • Axon is insulated by a fatty sheath with small uninsulated sections (nodes)
  • Cell body contains many extensions called dendrites
• Sensory neurones are long and have a cell body branching off the middle of the axon
• Relay neurones are short and have a small cell body at one end with many dendrites branching off it
• Motor neurones are long and have a large cell body at one end with long dendrites branching off it
• Voluntary response


  A response where you make a conscious decision to carry out a particular action
• Involuntary (reflex) response


  A response that does not involve the brain as the coordinator and is automatic and rapid
• Reflex arc


  1. Stimulus detected by receptor

  2. Sensory neurone sends impulse to spinal cord

  3. Relay neurone connects to motor neurone

  4. Motor neurone carries impulse to effector (muscle)
• Synapse


  The junction between two neurones
• Transmission of impulse across a synapse

  1. Electrical impulse triggers release of neurotransmitters from presynaptic neurone

  2. Neurotransmitters bind to receptors on postsynaptic neurone

  3. This stimulates postsynaptic neurone to generate an electrical impulse
• Synapses ensure impulses only travel in one direction
• Receptor


  A group of specialised cells that detect a change in the environment and stimulate electrical impulses in response
• Parts of the eye

  • Cornea
  • Pupil
  • Iris
  • Lens
  • Retina
  • Optic nerve
• Blind spot


  • Part of the retina where the optic nerve joins, with no light-sensitive rod and cone cells
• Pupil reflex


  1. In dim light, pupil dilates to allow more light

  2. In bright light, pupil constricts to allow less light
• Accommodation


  The function of the eye in focusing on near and distant objects by changing the shape of the lens
• Accommodation for near objects

  Ciliary muscles contract, suspensory ligaments loosen, lens becomes fatter, more refraction
• Accommodation


  1. Extended

  2. The function of the eye in focusing on near and distant objects

  3. The way the lens brings about fine focusing

  4. The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose

  5. The changes are brought about by the contraction or relaxation of the ciliary muscles
• Focusing on distant objects

  1. Ciliary muscles relax

  2. Suspensory ligaments tighten

  3. Lens becomes thinner

  4. Light is refracted less
• Focusing on near objects

  1. Ciliary muscles contract

  2. Suspensory ligaments loosen

  3. Lens becomes fatter

  4. Light is refracted more
• The focusing of the eye on distant and near objects is complex and it can be hard to remember what is happening
• Staring at your hand right in front of your eye will make your eyes feel tight and tired after a few seconds because the ciliary muscles are contracted
• Staring at an object far away feels relaxing and comfortable because the ciliary muscles are relaxed
• Rod cells


  • Sensitive to dim light
  • Found all over the retina, other than the area where the optic nerve attaches to the retina
• Cone cells

  • Distinguish between different colours in bright light
  • There are 3 types sensitive to different colours of light (red, blue and green)
  • Found mostly in the fovea
• There are no light-sensitive cells at all in the area where the optic nerve attaches to the retina, so it is known as the blind spot
• Night vision is black and white because the low light intensity isn't enough to stimulate cone cells, so only rod cells are stimulated
• Dim stars disappear when looked at directly but reappear when looked at slightly to the side, because the light falls on the fovea which has more cones so fewer rods, so the low light intensity won't be enough to stimulate the rods
• Hormone


  A chemical substance produced by a gland and carried by the blood that alters the activity of one or more specific target organs
• Major endocrine glands in the body

  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pancreas
  • Gonads
• Endocrine glands


  Have a good blood supply to get hormones into the bloodstream as soon as possible so they can travel around the body to the target organs
• Hormone receptors


  Cells have target receptors that the hormone can bind to, either on the cell membrane or inside cells
• The liver regulates levels of hormones in the blood; transforming or breaking down any that are in excess
• Important hormones in the human body

  • Insulin
  • Glucagon
  • Adrenaline
  • Thyroxine
  • Testosterone
  • Oestrogen