Human Nervous System

• Nervous system


  Allows the body to react to its surroundings and coordinate an appropriate response
• How a stimulus leads to a response being carried out by the body

  1. Stimulus is converted into an electrical impulse by the receptors

  2. The electrical impulse passes along sensory neurones to the central nervous system (CNS)

  3. The CNS coordinates an appropriate response and an electrical impulse is sent along motor neurones to the effector, which carries out the response
• Sequence of events describing how the nervous system works

  Stimulus → receptor → coordinator → effector → response
• Reflex action


  An automatic and rapid response which does not involve any conscious input from the brain
• Reflex actions


  • Aid survival by preventing harm to the body
• How a reflex action occurs via a reflex arc

  1. The stimulus is detected by a receptor

  2. An electrical impulse passes along a sensory neurone to the spinal cord (part of the CNS)

  3. At a synapse between a sensory neurone and a relay neurone, a chemical diffuses across the gap and stimulates a new impulse which passes along the relay neurone

  4. The same process occurs at a synapse between a relay neurone and a motor neurone

  5. At the effector, an appropriate response is carried out
• Reflex pathway


  The coordination centre is a relay neurone found in the spinal cord/unconscious parts of the brain
• Conscious pathway


  The coordination centre is in the conscious part of the brain
• Cerebral cortex


  • Controls consciousness, intelligence, memory and language
• Cerebellum


  • Controls muscular coordination
• Medulla


  • Controls unconscious activities eg. breathing, heart rate
• Investigating and treating the brain

  • The brain is a complex and delicate organ
  • The brain is easily damaged and destroyed
  • Certain membranes prevent drugs from reaching the brain
  • The exact function of each part of the brain is not known
• Methods used by scientists to determine brain function

  • Studying patients with brain damage
  • Electrical stimulation of the brain
  • MRI scans

  
• Receptors of the eye

  • Sensitive to light intensity and colour
• Structures within the eye

  • Focusing on near or distant objects - accommodation
  • Adaptation to dim light
• Retina


  A light-sensitive layer found at the back of the eye. Light stimulates the retinal cells, resulting in impulses being sent to the brain.
• Optic nerve


  Connects the eye and the brain. It carries impulses to the brain so that an image can be visualised.
• Sclera


  The tough outer layer of the eye which protects its internal structures.
• Cornea


  The curved transparent layer at the front of the eye. It lets light into the eye and allows light to be focused onto the retina.
• Iris


  A muscle which controls the size of the pupil by contracting or relaxing. This allows the eye to adjust to bright and dim lighting.
• Ciliary muscles and suspensory ligaments

  Hold the lens in place and control its shape.
• Iris


  A muscle which controls the size of the pupil by contracting or relaxing, allowing the eye to adjust to bright and dim lighting
• Ciliary muscles


  Hold the lens in place and control its shape
• Suspensory ligaments


  Hold the lens in place and control its shape
• How the iris alters the size of the pupil

  1. Bright light: Circular muscles contract, radial muscles relax - makes pupil smaller

  2. Dim light: Circular muscles relax, radial muscles contract - makes pupil larger
• Accommodation


  Alteration of the lens' shape in order to focus on near or distant objects
• How the eye focuses on a nearby object

  1. Ciliary muscles contract

  2. Suspensory ligaments loosen

  3. Lens becomes thicker and more curved - light rays are refracted strongly

  
• How the eye focuses on a far away object

  1. Ciliary muscles relax

  2. Suspensory ligaments tighten

  3. Lens becomes thinner - light rays are refracted weakly
• Myopia


  Short-sightedness, usually occurs when the lens of the eye is too curved so light is focused in front of the retina
• Hyperopia


  Long-sightedness, usually occurs when the lens of the eye is too flat so light is focused behind the retina
• How myopia is treated

  Using glasses with a concave lens, which spreads out light rays so they can be focused on the retina
• How hyperopia is treated

  Using glasses with a convex lens, which brings the light rays together so they can be focused on the retina
• Types of contact lenses

  • Hard - rigid material, last a long time, must be kept sterile
  • Soft - flexible material, last for a shorter time, more comfortable
• Laser eye surgery


  Use of lasers to fix visual defects in adults. To treat myopia, lasers reduce the thickness of the cornea. To treat hyperopia, lasers alter the curvature of the cornea.
• Replacement lenses


  Can either be implanted into the eye (along with the natural lens) or may replace the natural lens altogether
• Risks of lens replacement include retinal damage, cataracts and infections
• Thermoregulatory centre


  In the hypothalamus of the brain, controls body temperature
• How temperature is monitored by the body

  1. Thermoregulatory centre has receptors sensitive to blood temperature

  2. Skin has receptors sensitive to skin temperature - sends impulses to thermoregulatory centre
• Physiological changes when body temperature is too high

  1. Vasodilation - blood vessels near the surface of the skin dilate, more heat is radiated away

  2. Sweating - evaporation of water takes away heat energy from the surface of the skin
• Physiological changes when body temperature is too low

  1. Vasoconstriction - blood vessels near the surface of the skin constrict, less heat radiated away

  2. Shivering - respiration allows muscles to contract, exothermic process releases heat energy

  3. Sweating stops