Synapses and the nervous system

• Action potential


  Creates a new action potential adjacent to it through depolarisation, therefore the signal does not diminish in strength

  
• Action potential in unmyelinated neuron

  Conduction

  
• Synapses


  • Many presynaptic axon terminals converge on a single postsynaptic cell
  • Signal is transferred from the presynaptic neuron to the postsynaptic neuron

  
• Convergence & divergence


  Signal is transferred from the presynaptic neuron to the postsynaptic neuron

  
• Chemical synapse


  • The membrane of the axon (presynaptic membrane) is separated from the dendrite or soma of postsynaptic neuron by the synaptic cleft
  • The postsynaptic (or subsynaptic) membrane contains receptors for neurotransmitters
  • Synaptic knob at axon terminal contains vesicles packed with chemical neurotransmitters
  • Adjacent cells are not physically connected, information travels by means of a chemical intermediate (neurotransmitter) that diffuses from one cell to another

  
• Excitatory & Inhibitory postsynaptic potentials


  • A variety of compounds act as neurotransmitters
  • Some synapses are excitatory (cause Na+ channels to open, depolarizing postsynaptic neuron and start action potential) - excitatory postsynaptic potential, EPSP. Graded potential.
  • Other synapses are inhibitory (open K+ or Cl- channels which hyperpolarize the membrane, moving it further from the threshold) - inhibitory postsynaptic potential, IPSP. Graded potential.
  • Different neurotransmitters cause different permeability changes to postsynaptic neuron. Whether the synapse is excitatory or inhibitory is determined by the kind of the neurotransmitter receptors and not the neurotransmitter itself.
  • Neurotransmitters in the cleft must be removed in order that the neurons be responsive to further stimulation.

  
• Summation


  1. EPSPs and IPSPs of the postsynaptic membrane are summed

  2. If the threshold is reached an action potential is generated

  3. Triggering of an action potential thus depends on the number of excitatory & inhibitory synapses

  
• Neurons of the nervous system


  • NS function: fast-acting whole-body regulation through the integration of sensory information into an adaptive response
  • If response requires one or more skeletal muscles: behaviour
  • Sensory: convert physical properties of the environment (e.g. light, heat) or internal state (homeostasis) into neural responses
  • Interneurons: neural 'highways' that take information from one part of the body to another
  • Motor neurons: connect to muscles and coordinate behavioural acts

  
• Reflex arc


  Simplest behavioural response. Often bypasses the brain as this would take too long for an adaptive response. Sometimes straight sensory motor, sometimes interneuron involved to integrate information from more than one neuron.

  
• Vertebrate nervous system


  • Central nervous system (CNS): brain & spinal cord - essentially controls behaviour
  • Peripheral nervous system (PNS): Nerves that carry sensory messages to the CNS and motor commands from CNS to muscles & glands ('effector organs')
  • Neural Information is 'afferent' (incoming) or 'efferent' (outgoing)

  
• Organisation of vertebrate nervous system


  • CNS
  • Brain
  • Spinal chord
  • Sympathetic 'fight or flight'
  • Parasympathetic 'rest & digest'
  • Somatic

  
• Autonomic nervous system


  • Efferent division: carrying information away from CNS
  • Sympathetic 'fight or flight'
  • Parasympathetic 'rest & digest'
  • Visceral organs
  • Sympathetic: prepares the body for action; Parasympathetic: prepares the body to rest & conserve energy
  • Organs innervated by sympathetic & parasympathetic fibres. Allows for precise control
  • Tend to have opposing effects. Consider adaptive value of each system

  
• Somatic pathways


  • Two neurons from CNS to effector organ
  • Effector organ: visceral organs
  • Neuron stimulation has opposing effects
  • Usually involuntary control
  • One neuron from CNS to effector organ - motor neuron
  • Effector organ: skeletal muscle
  • Neuron only stimulates (inhibition possible through IPSPs)
  • Subject to voluntary control

  
• Adrenal gland releases hormone filled with neurotransmitters directly into blood instead of having a postganglionic fibre. The 'adrenal medulla' is a modified sympathetic ganglion.
• Concepts & reading


  • Conduction
  • Chemical synapses
  • Summation
  • EPSPs & IPSPs
  • Vertebrate nervous system
• Key reading (not only reading): Animal Physiology textbook. Chapter 4: Pages 123-130; Chapter 5: 140-142, 147-154, 156, 163-186.