M5:S1 Communication and Homeostatsis

• Homeostasis


  The maintenance of a constant internal environment
• Homeostatic systems


  • Involve receptors, a communication system and effectors
  • Receptors detect when a level is too high or too low
  • The information is communicated via the nervous system or the hormonal system to effectors
  • The effectors respond to counteract the change, bringing the level back to normal
• Negative feedback mechanism


  The mechanism that restores the level to normal
• Negative feedback keeps things around the normal level, e.g. body temperature is usually kept within 0.5 °C above or below 37 °C
• Positive feedback mechanism


  Amplifies the change from the normal level
• Positive feedback isn't involved in homeostasis because it doesn't keep your internal environment constant
• Stimulus


  Any change in the internal or external environment
• Types of effector

  • Muscle cells
  • Cells found in glands, e.g. the pancreas
• Cell signalling


  Communication that makes sure the activities of different organs are coordinated to keep the organism working effectively
• Cell signalling can occur between adjacent (nearby) cells or between distant cells
• The nervous system and hormonal system are communication systems
• Sensory neurones


  Transmit nerve impulses from receptors to the central nervous system
• Motor neurones


  Transmit nerve impulses from the central nervous system to effectors
• Relay neurones


  Transmit nerve impulses between sensory neurones and motor neurones
• Nerve impulses

  Electrical impulses, also called action potentials
• In a neurone's resting state, the outside of the membrane is positively charged compared to the inside, creating a resting potential of about -70 mV
• Sodium-potassium pump

  Moves sodium ions out of the neurone and potassium ions into the neurone, creating the resting potential
• Potassium ion channels

  Allow potassium ions to diffuse out of the neurone, contributing to the resting potential
• Sodium-potassium pump

  Uses active transport to move three sodium ions (Na+) out of the neurone for every two potassium ions (K+) moved in
• Potassium ion channel

  Allows facilitated diffusion of potassium ions (K+) out of the neurone, down their concentration gradient
• Action potential

  Electrical impulse, nerve impulse
• Action potential sequence of events

  1. Stimulus excites neurone cell membrane, causing sodium ion channels to open

  2. Membrane becomes more permeable to sodium, so sodium ions diffuse into neurone down sodium ion electrochemical gradient

  3. Depolarisation if potential difference reaches threshold, voltage-gated sodium ion channels open, more sodium ions diffuse into neurone

  4. Repolarisation at around +30 mV, sodium ion channels close and voltage-gated potassium ion channels open, potassium ions diffuse out of neurone down potassium ion concentration gradient
• Action potentials

  • Once threshold is reached, action potential will always fire with the same change in voltage, no matter how big the stimulus is
  • If threshold isn't reached, action potential won't fire
  • A bigger stimulus won't cause a bigger action potential, but it will cause them to fire more frequently
• Myelinated neurones


  • Have a myelin sheath, an electrical insulator
  • Depolarisation only happens at the nodes of Ranvier where sodium ions can get through the membrane
  • Neurone's cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse 'jumps' from node to node (saltatory conduction)
• Synapse


  • Junction between a neurone and another neurone, or between a neurone and an effector cell (e.g. muscle or gland cell)
  • Tiny gap between the cells at a synapse is called the synaptic cleft
  • Presynaptic neurone has a synaptic knob containing synaptic vesicles filled with neurotransmitters
  • When an action potential reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft, they diffuse across and bind to receptors on the postsynaptic membrane
• Neurotransmitter transmission at a synapse

  1. Action potential triggers calcium influx into synaptic knob

  2. Calcium influx causes synaptic vesicles to fuse with presynaptic membrane and release neurotransmitters into synaptic cleft

  3. Neurotransmitters diffuse across cleft and bind to receptors on postsynaptic membrane, causing depolarisation and potentially an action potential
• Excitatory synapse


  Neurotransmitters depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached
• Inhibitory synapse


  Neurotransmitters hyperpolarise the postsynaptic membrane, preventing an action potential from being fired
• Synaptic divergence


  When one neurone connects to many neurones, information can be dispersed to different parts of the body
• Synaptic convergence


  When many neurones connect to one neurone, information can be amplified (made stronger)
• Spatial summation


  Small amount of neurotransmitter from each of many converging neurones can be enough altogether to reach the threshold in the postsynaptic neurone and trigger an action potential
• Temporal summation


  Two or more nerve impulses arriving in quick succession from the same presynaptic neurone make an action potential more likely because more neurotransmitter is released into the synaptic cleft
• Receptors for neurotransmitters are only on the postsynaptic membranes, so synapses make sure impulses can only travel in one direction
• Neurotransmitter release into synaptic cleft

  1. Many neurones

  2. Release excitatory neurotransmitters

  3. Action potential
• High frequency of weak impulses

  Action potential
• Both types of summation mean synapses accurately process information, finely tuning the response
• Synapses Make Sure Impulses are Transmitted One Way
• The hormonal system is also called the endocrine system
• Endocrine glands

  Groups of cells that are specialised to secrete hormones
• Hormones


  Chemical messengers, many are proteins or peptides, some are steroids