Homeostasis

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


  Enables humans to react to their surroundings and to coordinate their behaviour
• Information in the nervous system

  Sent as electrical impulses - electrical signals that pass along nerve cells known as neurones
• Neurone


  A bundle of neurones is known as a nerve
• Neurones


  • Have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites
  • Some human neurones have axons over a metre in length (but only 1 - 4 micrometres wide)
  • The axon is insulated by a fatty myelin sheath with small uninsulated sections along it (called nodes) which the impulse jumps along
• Pathway through the nervous system

  1. Stimulus

  2. Receptor

  3. Coordinator

  4. Effector

  5. Response
• Reflex arc


  An involuntary (or reflex) response that does not involve the conscious part of the brain as the coordinator of the reaction
• Reflex arc


  1. Stimulus detected by receptor

  2. Sensory neurone sends electrical impulses to spinal cord

  3. Relay neurone in spinal cord

  4. Motor neurone carries impulse to muscle

  5. Muscle contracts and pulls foot away (response)
• Types of neurones in a reflex arc

  • Sensory
  • Relay
  • Motor
• Sensory neurone


  Carries impulses from sense organs to the CNS (brain or spinal cord)
• Relay neurone


  Found inside the CNS and connects sensory and motor neurones
• Motor neurone


  Carries impulses from the CNS to effectors (muscles or glands)
• Synapse


  Junctions (gaps) between neurones where neurotransmitters are released and diffuse across to stimulate the next neurone
• Neurotransmitters


  • Chemical messengers released into the synaptic cleft that diffuse across and bind to receptors on the next neurone
  • Drugs can act on neurotransmitter receptors to affect the nervous system
• Reaction time is the time taken to respond to a stimulus and is usually very quick, less than a second
• Reaction time experiment


  1. Choose a factor to investigate

  2. Carry out experiments to measure reaction time

  3. Repeat several times to calculate a mean

  4. Change the factor and repeat
• Reaction time experiment


  • Control variables include: using dominant hand, dropping ruler from same height, no caffeine/noise before first run
• The brain and spinal cord are part of the central nervous system and are responsible for controlling complex behaviours
• Regions of the brain

  • Cerebral cortex
  • Cerebellum
  • Medulla
• Cerebral cortex


  Outer layer of the brain divided into two hemispheres, responsible for higher-order processes like intelligence, memory, consciousness and personality
• Cerebellum


  Underneath the cerebral cortex, responsible for balance, muscle coordination and movement
• Medulla


  Responsible for unconscious activities like heart rate and breathing
• Mapping regions of the brain

  1. Studying patients with brain damage

  2. Electrically stimulating different parts of the brain

  3. Using MRI scanning techniques
• Functional MRI


  Produces images of different regions of the brain that are active during different activities
• Receptors


  Groups of specialised cells that can generate an electrical impulse in a sensory neurone
• Types of receptor cell in the eye

  • Rod cells
  • Cone cells
• Pupil reflex


  A reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light
• Pupil reflex in dim light

  1. Pupil dilates (widens)

  2. Allows more light into the eye
• Pupil reflex in bright light

  1. Pupil constricts (narrows)

  2. Prevents too much light entering the eye
• Effects of pupil reflex

  • Protects the retina from damage in bright light
  • Protects us from not seeing objects in dim light
• Accommodation


  The process of changing the shape of the lens to focus on near or distant objects
• Focusing on a near object

  1. Ciliary muscles contract

  2. Suspensory ligaments loosen

  3. Lens becomes thicker and refracts light more strongly
• Focusing on a distant object

  1. Ciliary muscles relax

  2. Suspensory ligaments are pulled tight

  3. Lens becomes thinner and refracts light less
• Myopia (short-sightedness)


  Rays of light do not focus on the retina
• Hyperopia (long-sightedness)


  Rays of light do not focus on the retina
• Treatments for eye defects

  • Hard and soft contact lenses
  • Laser surgery
  • Lens replacement surgery
• Thermoregulatory centre


  Monitors and controls body temperature
• Controlling body temperature when too high

  1. Blood vessels dilate (vasodilation)

  2. Sweat is produced from sweat glands
• Controlling body temperature when too low

  1. Blood vessels constrict (vasoconstriction)

  2. Sweating stops

  3. Skeletal muscles contract (shiver)
• Homeostasis involves the maintenance of constant internal environment; temperature control is an example of negative feedback