CH9-10 Homeostasis + Eye

• Homeostasis


  Maintenance of a constant internal environment that is optimum for physiological processes within the body
• Negative feedback


  A process by which a receptor detects a restoration of the internal environment to the norm and stimulates the effector to reduce the intensity of the homeostatic corrective mechanism to allow self-regulation
• Homeostasis


  1. Receptor detects change in internal environment

  2. Signal sent to effector organs to carry out corrective mechanism

  3. Normal point achieved, negative feedback occurs to prevent over-correction
• To maintain homeostasis, the human body needs to be able to detect changes, both within and outside the body, and set in place a series of actions to regulate and return the internal environment of the body to its ideal state
• Both the hormonal and nervous systems play important roles in homeostasis by detecting and responding to changes
• Nervous System


  Consists of two parts which serve to co-ordinate and regulate bodily functions: Central Nervous System (brain and spinal cord) and Peripheral Nervous System (cranial nerves, spinal nerves and sense organs)
• Stimulus


  Change in internal or external environment that elicits a response from an organism
• Voluntary action


  A deliberate action elicited by nerve impulses generated by the brain under conscious control
• Reflex action


  An immediate response to a specific stimulus, without conscious control
• Comparing voluntary and reflex action

  • Both involve an effector to carry out the action
  • Voluntary action involves transmission of nerve impulses within the nervous system, needs a stimulus, and is under conscious control
  • Reflex action involves generation of nerve impulses by receptors in sense organs, has no conscious control
• Reflex arc


  1. Receptors detect stimulus and generate nerve impulses

  2. Nerve impulses transmitted along sensory neurone to CNS

  3. Nerve impulses pass to relay neurone and then motor neurone

  4. Motor neurone transmits nerve impulses from CNS to effector
• Receptors, sensory neurones, relay neurones, motor neurones and effectors work together to produce a co-ordinated response in a reflex action as a result of a specific stimulus
• Thermoregulation reflex


  1. Thermoreceptors in skin detect increase in temperature and generate nerve impulses

  2. Nerve impulses transmitted to spinal cord and then hypothalamus

  3. Hypothalamus elicits thermoregulation corrective mechanism

  4. Nerve impulses transmitted to effector tissues in skin

  5. Arterioles vasodilate and sweat glands secrete more to decrease temperature

  6. Decreased skin temperature stimulates hypothalamus to reduce intensity of corrective mechanism through negative feedback

  
• Hormone


  A chemical substance produced by an endocrine gland and secreted directly into the bloodstream, transported to target organs where it exerts its effect, and eventually destroyed in the liver
• Endocrine gland


  A ductless gland that secretes hormones directly into the bloodstream
• Diabetes mellitus


  A disease in which the body is unable to regulate its blood glucose concentration within normal limits, resulting in excess glucose being excreted in the urine

  
• Types of diabetes


  • Type I: Genetic condition, early-onset, pancreas unable to produce sufficient insulin, treated with insulin injections
  • Type II: Genetic predisposition worsened by diet and lack of exercise, late-onset, target cells insensitive to insulin, treated by controlling blood sugar through diet and exercise

  
• Comparing nervous and hormonal control

  • Nervous control uses nerve impulses, is fast, localised, and can be voluntary or involuntary
  • Hormonal control uses chemical signals, is slower, widespread, and always involuntary
• Parts of the eye

  • Eyelid, eyelashes, tear gland, conjunctiva, sclera, cornea, iris, pupil, aqueous chamber, lens, suspensory ligament, ciliary body, retina, fovea, blind spot, optic nerve, choroid, vitreous chamber

  
• Pupil reflex


  1. Photoreceptors in retina detect change in light intensity and generate nerve impulses

  2. Nerve impulses transmitted to brain

  3. Brain sends nerve impulses to iris muscles to constrict or dilate pupil
• Accommodation


  1. Blur image stimulates photoreceptors in retina to generate nerve impulses

  2. Nerve impulses transmitted to brain

  3. Brain sends nerve impulses to ciliary muscles

  4. Ciliary muscles contract/relax, lens becomes more/less convex, focal length decreases/increases, allowing sharp focus on retina