Nervous System

• Homeostasis

  The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.
• Why is homeostasis needed?

  Maintains optimal conditions for enzyme action and all cell functions.

  In the human body, this includes control of:

  Blood glucose concentration.

  Body temperature.

  Water levels.
• How is homeostasis controlled?

  By automatic control systems involving nervous responses or chemical responses.
• Features of automatic control systems

  1: Receptors - Cells that detect stimuli (change in environment).

  2: Coordination centres - Receive and process information from receptors (includes brain, spinal cord and pancreas).

  3: Effectors - Bring about responses that restore optimum levels (includes muscles and glands).
• The Nervous System: Function

  Enables humans to react to their surroundings and to coordinate their behaviour.
• The Nervous System: Structure

  Information from the receptors passes along nerve cells (neurones) as electrical impulses to the central nervous system (CNS), which consists of the brain and spinal cord.

  The CNS coordinates the response of effectors which may be muscles or glands secreting hormones.

  stimulus → receptor → coordinator → effector → response.
• Reflex actions


  An involuntary response that does not involve the conscious parts of the brain as the coordinator of the reaction.

  Reflex responses are therefore automatic and rapid to minimise damage to the body.
• Reflex arc


  1: Stimulus (e.g. sharp pin/hot pan) is detected by receptor cells in the skin.

  2: Electrical impulse is sent along a sensory neurone to the spinal cord (the coordinator).

  3: Electrical impulse is passed to a relay neurone in the spinal cord.

  4: Relay neurone synapses with a motor neurone.

  5: Motor neurone carries impulse to the muscle (the effector) in the appropriate area (e.g. leg/arm)

  6: Muscle contracts pulling the limb away (the response) from the stimulus when stimulated by the motor neurone.
• Synapses


  The junction (gap) between two neurones.

  When the electrical impulse reaches the end of the first neurone, a chemical is released into the synapse.

  The chemical diffuses across the synapse.

  When the chemical reaches the second neurone, it triggers the impulse to begin again in the next neurone.
• The Brain


  Made of billions of interconnected neurones and controls complex behaviour.

  Contains different regions responsible for different functions.
• The Brain: Components


  1: Cerebral cortex - Outer layer of the brain. Responsible for consciousness, memory, intelligence and language.

  2: Cerebellum - Found under cerebral cortex. Responsible for muscle coordination and movement.

  3: Medulla - Controls unconscious activities like breathing and heart rate.

  
• Why is investigating brain function and treating brain damage so difficult?

  It is incredibly complex; limits our understanding and makes it difficult for neuroscientists to learn how it works.

  It is delicate; easy to cause damage and lead to speech and motor issues.

  It is not fully understood what part of the brain does what.
• How do neuroscientists map out the regions of the brain?

  1: Studying patients with brain damage - If an area of the brain has been damaged, the effect this has on the patient can tell you a lot about what the damaged part of the brain does.

  2: Electrical stimulation - The brain can be stimulated electrically by inserting an electrode into the tissue and giving it a jolt of electricity. Scientists observe the impact of stimulating areas of the brain to understand what they do.

  3: MRI scans - Produce a detailed image of the brain's structures. Used to determine which areas are active.
• The Eye

  A sense organ containing receptors sensitive to light intensity and colour.

  The structures within the eye are adapted to allow the eye to change its shape in order to focus on near or distant objects (accommodation) and to dim light.

  The purpose of the eye is to receive light and focus it onto the retina at the back of the eye.

  
• The Eye: Retina

  Contains the receptor cells sensitive to light intensity and colour.

  When light hits these receptor cells, they send an electrical impulse along the optic nerve to the brain, which interprets the information to create an image.
• The Eye: Optic Nerve

  The sensory neurone that carries impulses from the retina to the brain.
• The Eye: Sclera

  The white outer layer of the eye which is strong to protect the eye from damage and support the other structures.
• The Eye: Cornea

  The transparent lens that refracts/bends light as it enters the eye to focus it onto the retina.
• The Eye: Iris

  Muscles that surround the pupil (the hole that allows light to enter).

  Control the size of the pupil and therefore how much light enters the eye.
• The Eye: Lens

  Transparent disc that can change shape to focus light onto the retina.
• The Eye: Suspensory Ligaments and Ciliary Muscles

  Control the shape of the lens and hold it in place.
• The Pupil Reflex


  A reflex action carried out to protect the retina from damage from bright light and to improve vision in dim light.

  Controlled by two muscles: the radial and circular muscles in the iris.
• Pupil Reflex in Dim Light

  Radial muscles contract.

  Circular muscles relax.

  This widens the size of the pupil.

  More light enters the eye.
• Pupil Reflex in Bright Light

  Radial muscles relax.

  Circular muscles contract.

  This narrows the size of the pupil.

  Less light enters the eye.
• Accommodation


  The process of changing the shape of the lens to focus on near or distant objects.

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

  Changing the lens' shape alters how much light is refracted so that the light is focused onto the retina.

  These changes are brought about by the ciliary muscles which contract or relax.
• Focusing on a Near Object

  Ciliary muscles contract.

  So the suspensory ligaments loosen.

  Lens is thicker and more curved so light rays are refracted more strongly.
• Focusing on a Far Object

  Ciliary muscles relax.

  So the suspensory ligaments tighten.

  Lens is thin and less curved so light rays are only slightly refracted.
• Two Defects of the Eye

  Myopia (short-sightedness) - Unable to focus on distant objects; occurs when lens is too thick/curved or distance between retina and lens is too great so the image is in focus in front of retina and not on retina. Treated with glasses containing concave lens to correct it so light rays focus on retina.

  Hyperopia (long-sightedness) - Unable to focus on near objects; occurs when distance between retina and lens is too small so the image is in focus behind retina and not on retina. Treated with glasses containing convex lens to correct it so light rays focus on retina.

  
• New/Alternative Treatments for Vision Defects: Contact Lenses

  Sit on the surface of the eye and are almost invisible, so are ideal for activities like sports.

  Contact lenses are more comfortable than glasses but carry a higher risk of infection.
• New/Alternative Treatments for Vision Defects: Laser Eye Surgery

  Lasers are used to change the shape of the cornea to change how it refracts light onto the retina.

  For myopia: Cornea is slimmed down, reducing refractive power.

  For hyperopia: Shape is changed so refractive power is increased.

  Risk of unexpected damage during the procedure which may worsen vision or lead to infection.
• New/Alternative Treatments for Vision Defects: Replacement lens surgery

  The lens is completely replaced with an artificial plastic lens.

  More invasive than laser surgery and carries risk of damaging retina leading to complete sight loss.
• Why does body temperature need to be controlled?

  The human body needs to maintain a temperature at which its enzymes work best, which is around 37 degrees.

  Processes such as respiration release energy as heat, and the body loses heat energy to the surroundings constantly and these gains and losses must be regulated to maintain optimum body temperature.
• Monitoring of Body Temperature

  Body temperature is monitored and controlled by the thermoregulatory centre in the brain.

  The TR centre contains receptors sensitive to the temperature of the blood.

  The skin also contains temperature receptors and sends nervous impulses to the thermoregulatory centre.
• When Body Temperature is Too High...

  Temperature receptors in the skin detect that body temperature is too high.

  Impulses are sent to the thermoregulatory centre (coordination centre) which receives the information and triggers responses in the effectors.

  Sweat glands in the skin produce sweat, which evaporates, transferring energy to the environment and cooling the body.

  Blood vessels near the surface of skin dilate (vasodilation), so excess heat is transferred from the skin to environment, cooling the body.

  Skin hairs lie flat.

  
• When Body Temperature is Too Low...


  Temperature receptors in the skin detect that body temperature is too low.

  Impulses are sent to the TR centre which receives the information and triggers responses in the effectors.

  Sweat glands stop producing sweat, reducing heat loss to environment.

  Blood vessels near surface of skin constrict (vasoconstriction), reducing heat loss to environment.

  Skeletal muscles contract rapidly and shivering occurs.

  Skeletal muscle contraction requires energy from respiration, which releases heat so body temperature increases.

  Skin hairs erect to trap air.