Human nervous system

• The function of the nervous system is to allow the body to react to its surroundings and coordinate an appropriate response
• A stimulus leads to a response being carried out by the body through the following sequence of events:

  • Stimulus is converted into an electrical impulse by the receptors
  • The electrical impulse passes along sensory neurones to the central nervous system (CNS)
  • The CNS coordinates an appropriate response and an electrical impulse is sent along motor neurones to the effector, which carries out the response
• A reflex action is an automatic and rapid response that does not involve any conscious input from the brain
• Reflex actions are important as they aid survival by preventing harm to the body
• A reflex action occurs via a reflex arc through the following steps:

  • The stimulus is detected by a receptor
  • An electrical impulse passes along a sensory neurone to the spinal cord (part of the CNS)
  • 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
  • The same process occurs at a synapse between a relay neurone and a motor neurone
  • At the effector, an appropriate response is carried out
• The difference between a reflex pathway and a conscious pathway is that within a reflex pathway, the coordination centre is a relay neurone found in the spinal cord or unconscious parts of the brain, while in a conscious pathway, the coordination centre is in the conscious part of the brain
• The function of the cerebral cortex is to control consciousness, intelligence, memory, and language
• The function of the cerebellum is to control muscular coordination
• The function of the medulla is to control unconscious activities such as breathing and heart rate
• The investigation and treatment of the brain are difficult because:

  • 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 include:

  • Studying patients with brain damage
  • Electrical stimulation of the brain
  • MRI scans
• The receptors of the eye are sensitive to light intensity and colour
• The two main functions of structures found within the eye are:

  • Focusing on near or distant objects (accommodation)
  • Adaptation to dim light
• The structure and function of the retina:

  • 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 structure and function of the optic nerve:

  • The optic nerve connects the eye and the brain
  • It carries impulses to the brain so that an image can be visualized
• The structure and function of the sclera:

  • The sclera is the tough outer layer of the eye which protects its internal structures
• The structure and function of the cornea:

  • 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 onto the retina
• The structure and function of the iris:

  • The iris is a muscle which controls the size of the pupil by contracting
• The iris is a muscle that controls the size of the pupil by contracting or relaxing, allowing the eye to adjust to bright and dim lighting
• The ciliary muscles and suspensory ligaments hold the lens in place and control its shape
• The iris alters the size of the pupil in bright light by having circular muscles contract and radial muscles relax, making the pupil smaller to avoid retinal damage
• In dim light, the iris makes the pupil larger by having circular muscles relax and radial muscles contract, allowing more light to enter the eye
• Accommodation is the alteration of the lens' shape to focus on near or distant objects
• To focus on a nearby object, ciliary muscles contract, suspensory ligaments loosen, and the lens becomes thicker and more curved for strong refraction of light rays
• To focus on a far away object, ciliary muscles relax, suspensory ligaments tighten, and the lens becomes thinner for weak refraction of light rays
• Myopia (short-sightedness) occurs when the lens is too curved, causing light to focus in front of the retina, resulting in blurry images
• Myopia can be treated with glasses with a concave lens to spread out light rays for proper focus on the retina
• Hyperopia (long-sightedness) occurs when the lens is too flat, causing light to focus behind the retina, leading to out-of-focus images
• Hyperopia can be treated with glasses with a convex lens to bring light rays together for focus on the retina
• Contact lenses come in two types: hard lenses made of rigid material that last long and must be kept sterile, and soft lenses made of flexible material that are more comfortable but last for a shorter time
• Laser eye surgery uses lasers to correct visual defects by reducing the cornea's thickness for myopia or altering the cornea's curvature for hyperopia
• Replacement lenses can be used to treat visual defects by either implanting them into the eye along with the natural lens or replacing the natural lens altogether, with risks including retinal damage, cataracts, and infections
• Body temperature is controlled by the thermoregulatory centre in the hypothalamus of the brain
• Temperature is monitored by the body through receptors in the thermoregulatory centre sensitive to blood temperature and skin receptors sensitive to skin temperature, sending impulses to the thermoregulatory centre
• Physiological changes when body temperature is too high include vasodilation for heat radiation and sweating for heat energy dissipation
• Physiological changes when body temperature is too low include vasoconstriction to retain heat, shivering for heat production, and cessation of sweating