Human Nervous Systems

• The nervous system allows the body to react to its surroundings and coordinate an appropriate response.
• • Stimulus is converted into an electrical impulse by the receptors
  • The electrical impulse passes along sensory neurones to the central nervous system
  • The CNS coordinates an appropriate response and an electrical impulse is sent along motor neurones to the effector, which carries out the response
• • Stimulus
  • Receptor
  • Coordinator
  • Effector
  • Response
• A reflex action is an automatic and rapid response which does not involve any concious input from the brain
• Reflex actions aid survival by preventing harm to the body
• Reflex Arc:

  • the stimulus is detected by a receptor
  • An electrical impulse passes along a sensory neurone to the spinal cord (CNS)
  • At a synapse between a sensory neurone and relay neurone, a chemical diffuses across the gap and stimulates a new impulse which passes along the relay neurone
  • The same occurs at a synapse between a relay neurone and a motor neurone
  • appropriate response carried out at effector
• In a reflex pathway the coordination centre is a relay neurone found in the spinal cord.
• In a conscious pathway the coordination centre is the conscious part of the brain.
• The cerebral cortex controls consciousness, memory, intelligence, and memory
• The cerebellum controls muscular coordination
• The medulla controls unconscious activities
• Treatment of the brain is difficult:

  • The brain is a complex and delicate organ
  • The brain is easily damaged and destroyed
  • Certain membranes prevent drugs from entering the brain
  • The exact function of each part of the brain is not known
• Methods scientist use to determine brain function:

  • Studying patients with brain damage
  • Electrical stimulation of the brain
  • MRI scans
• Eye receptors are sensitive to light intensity and colour
• Structures in the eye work together to focus on objects and adapt to changing light conditions
• The retina is a light sensitive layer found at the back of the eye. Light stimulates the retinal cells, resulting in impulses being sent to the brain.
• The optic nerve connects the eye and the brain. It carries impulses to the brain so that an image can be visualised.
• The sclera is the tough outer layer of the eye which protects its internal structures
• The cornea is the curved transparent layer at the front of the eye. It lets light into the eye and allows light to be focused on the retina.
• The iris is a muscle which controls the size of the pupil by contracting or relaxing. This allows the eye to adjust to bright and dim lighting.
• The ciliary muscles and suspensory ligaments hold the lens in place and control its shape
• Iris:

  • Bright light: circular muscles contract and radial muscles relax - makes pupil smaller to avoid retinal damage
  • Dim light: circular muscles relax and radial muscles contract - makes pupil larger so more light can enter the eye
• Accommodation is the alteration of the lens' shape in order to focus on near or distant objects.
• Eye focusing on nearby object:

  • ciliary muscles contract
  • Suspensory ligaments loosen
  • Lens becomes thicker and more curved - light rays are refracted strongly
• Eye focusing on far object:

  • Ciliary muscles relax
  • Suspensory ligaments tighten
  • Lens becomes thinner - light rays are refracted weakly
• Myopia = Short - sightedness
• Myopia occurs when the lens of the eye is too curved. Light is focused in front of the retina so images appear blurry
• Myopia can be treated with concave lens glasses, which spreads out the light rays so they can be focused on the retina
• Hyperopia = long - sightedness
• Hyperopia occurs when the lens of the eye is too flat, light is focused behind the retina so things appear out of focus
• Hyperopia can be treated with convex lens glasses, which brings the light rays together so they can be focused on the retina
• Type 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 is the use of lasers to fix visual defects in adults
• To treat myopia, lasers reduce the thickness of the cornea so light is refracted less strongly
• To treat hyperopia, lasers alter the curvature of the cornea so that light is refracted correctly
• A replacement lens can either be implanted in the eye (along with the natural lens), or it may replace the natural lens altogether. Risks of lens replacement include retinal damage, cataracts and infection.
• Body temperature is controlled by the thermoregulatory centre in the hypothalamus of the brain.
• How is temperature monitored:

  • Thermoregulatory centre has receptors sensitive to blood temperature
  • Skin has receptors sensitive to skin temperature - sends impulses to thermoregulatory centre
• Vasodilation = blood vessels ear the surface of the skin dilate - more heat is radiated away
• Sweating = evaporation of water takes away heat energy from the surface of the skin