Bio FA4

• Organisms must be able to monitor their environment – Both internal and external
• Organisms can survive internal and external changes through anatomical, physiological, and behavioural adaptations
• Homeostasis is maintenance of a constant internal environment, mediated by a feedback systems
• the role of homeostasis is to ensure that all metabolic reactions in the body can occur at a rate that ensures the survival of the organism
• §Irritability: the ability to detect and respond to changes in the environment. It is a characteristic of all living matter
• §Coordination occurs when different body parts work together to maintain homeostasis
• §A stimulus (input) is a detectable change in the environment, which is either physically or chemically
• §The cells must be able to detect the change and is done by molecules that are able to change due to the stimulus
• §Some cells specialised to receive external stimuli, others are to send/receive signals to coordinate body functions. Others determine if a change to cellular activities is required in order to maintain homeostasis
• §Input involves specialised cells found in a sensory organs - Receptor
• §A chemical message is transmitted to a coordinating centre (the regulator) that determines if a change is required
• §If so, another chemical message brings about a response (output) within an effector organ
• §In plants, a chemical transmitter (hormone) is released by the cell when an appropriate stimulus acts on it
• §The efficiency of the animal control system is measured in terms of the displacement from the optimal level or set point and the speed at which the optimal level is restored
• §Fluctuations from the set point (the optimal level) activate the control system, which returns the particular factor towards its optimal level
• §There are two types of feedback:Positive feedback - the same response will continueØNegative feedback - the response is reversed or negated
• §There are two types of feedback:

  • Positive feedback - the same response will continue
  • Negative feedback - the response is reversed or negated
• §In most stimulus-response systems, the response by the effector results in removing the original stimulus
• §There are two coordinating systems in multicellular animals:

  • The nervous system
  • The hormonal or endocrine system
• §Hormones are chemical messengers that are produced in very small quantities by cells to regulate the activity of cells and organs
• §There are three ways a hormone may effect a change

  • Autocrine hormones - Bind to receptors in the same cell   that produced them
  • Paracrine hormones - Bind to receptors on neighbouring cells
  • Endocrine hormones - Travel long distances through the blood   supply to reach the target cells
• §Pheromones - Hormones that act over a distance
• §A stimulus causes production and secretion of a hormone appropriate to that stimulus:

  • Input from the external environment is relayed to hormone excreting cells via the nervous system in most animals
  • Hormone secreting cells respond directly to changes in their immediate internal environment, often via a circulating fluid or diffusion
• §For a cell to respond to a stimulus, a signal transduction pathway must be activated, for example:

  • When a hormone binds to its specific receptor, the receptor rearranges its shape
  • This change in conformation can signal to other molecules in the cell to change their function

  §The secondary messenger molecules cause changes in the chemical pathways throughout the cell until these result in a final response to the hormone message

  §These responses include:

  Activating or Inactivating enzymes

  Exocytosis of molecules

  The switching on or off genes

  Opening or closing protein channels
• §The types of receptors found on the cell’s surface is determined by DNA. There are a great many different types of cell receptors present at any one time, each responding to a different signal. The cell constantly monitors what signals are coming in to its receptors and adjusts the quantities of each type of receptor accordingly
• §Down-regulation: When membrane receptors on cells are removed or recycled fewer receptors are present to detect signals and so there is a reduction of the cell’s sensitivity to that message
• Up-regulation: When a weak signal is received and more receptors on cells are synthesised to increase the cell’s sensitivity to the message
• §Structures specialised to respond to Stimuli:

  • Nerve Endings (Dendrites of Nerves)
  • Sense Organs
• • Mechanoreceptors – Respond to touch, pressure, stretch and itch
  • Thermoreceptors – Respond to changes in Temperature
  • Photoreceptor – Respond to light
  • Chemoreceptors – Respond to chemicals
  • Nociceptors – Respond to pain
• •In mammals, hormones are produced in ductless endocrine glands which are directly secreted into the blood vessels. The endocrine organs do not work in isolation with many hormones interacting to bring about a particular response
• •The stimuli controlling the release of any particular hormone include:

  • The presence of a specific metabolite in the blood
  • The presence of another hormone in the blood
  • Stimulation by the autonomic nervous system
• There are several ways a hormone can influence a target cell:

  • Change the permeability of the membrane to particular chemicals
  • Influence enzymes located in the membrane
  • Influence cell organelles
  • Activate genes to bring about specific protein production
• •The hypothalamus, collects information from other parts of the brain, as well as monitoring levels of hormones
• The pituitary gland is formed from two tissues:

  •A glandular portion - that produces and releases hormones

  •A neural portion - that releases the neurohormones that were   produced in the hypothalamus
• The adenohypophysis produces seven hormones which promotes growth, pigmentation and the tropic hormones

  •Tropic hormones: hormones which bring about changes in growth patterns

  •Hormones produced by the nerve cells in the hypothalamus act as either releasing hormones or release-inhibiting hormones to the adenohypophysis
• • The nervous system – is the second coordinating system
  • It links receptors, which detect stimuli to effectors such as muscles or glands that bring about a response
  • Nerves carry information from the receptors to regulator areas and effectors
  • A typical nerve cell, or neuron , consists of a cell body (soma) which contains the nucleus and from which extend fibres (dendrites or axons)
  • Information is carried from cell to cell by electrical impulses (called action potentials) that are generated by changes in ions across the neuron membrane
• The nucleus of a neuron is contained in the soma
• There are two main types of fibres:

  Axons - long and rarely branched, except at the ends

  Dendrites - shorter and have many branches
• Dendrites - receive input from other neurons
• Axons are the main pathway for the conduction of impulses