Neurons and synaptic transmission

• Neurotransmitter:

  • Chemical substances that relay nerve impulses across a synapse.
• Synapse:

  • Gap between the end of the axon of one neuron and the dendrite or cell body of another
• Sensory Neuron:

  • Carry nerve impulses from sensory receptors to CNS
• Motor Neurons:

  • form synapses with muscles and control their contractions
• Relay Neurons:

  • these neurons are the most common type of neuron in the CNS. They allow sensory and motor neurons to communicate with each others
• Synaptic Transmission:

  • refers to the process by which a nerve impulse passes across the synaptic cleft from one neuron to another
• What is the purpose of Neurons?:

  • Neurons - specialised cells whose function is to move electrical impulses to and from the central nervous system.
  • The average human brain contains somewhere in the region of 100 billion neurons and, on average, each neuron is connected to 1,000 other neurons.
  • This creates highly complex neural networks that give the brain its impressive processing capabilities.
• The Structure & Function of Neurons:

  • Neurons are cells that are specialised to carry neural information throughout the body. Neurons can be one of three types: sensory neurons, relay neurons or motor neurons.
  • Neurons typically consist of a cell body, dendrites and an axon. Dendrites at one end of the neuron receive signals from other neurons or from sensory receptors
• Neuron structure:

  • Dendrites are connected to the cell body, the control centre of the neuron. From the cell body the impulse is carried along the axon, where it terminates at the axon terminal.
  • In many nerves, including those in the brain and spinal cord, there is an insulating layer that forms around the axon - the myelin sheath.
  • This allows nerve impulses to transmit more rapidly along the axon. If the myelin sheath is damaged, impulses slow down. The length of a neuron can vary from a few millimetres up to one metre.
• Motor neurone

  
• Relay neurone

  
• Sensory neurones

  
• Loss of myelin leads to a variety of symptoms:

  • If myelin sheath surrounding nerve fibres is damaged/destroyed, transmission of nerve impulses is slowed of blocked. The impulse now has to flow continuously along the whole nerve fibre, a process that is much slower than jumping from node to node.
  • Loss of myelin can lead to 'short circuiting' of nerve impulses
  • An area where myelin has been destroyed is called a lesion or plaque
• Loss of myelin:

  • The slowing and 'short circuiting' of nerve impulses by lesions lead to a variety of symptoms related to nervous system activity
  • Symptoms include: sensory impairment (blurred vision, difficulties controlling movement, bodily functions problems such as failure to control urination)
• Sensory Neurons:

  • Sensory Neurons carry nerve impulses from sensory receptors to the spinal cord and the brain.
  • Sensory neurons convert information from these sensory receptors into neural impulses.
  • When these impulses reach the brain, they are translated into sensations of, for example, visual input or heat so the organism can react appropriately.
  • Not all sensory information travels as far as the brain, with some neurons terminating in the spinal cord. This allows reflex actions to occur quickly without delay of sending impulses to the brain.
• Relay neurones:

  • Most neurons are neither sensory nor motor, but lie somewhere between the sensory input and the motor output.
  • Relay neurons allows sensory and motor neurons to communicate with each other.
  • These relay neurons lie wholly within the brain and spinal cord.
• Motor neurons:

  • The term motor neuron refers to neurons located where axons are protected.
  • Motor neurons form synapses with muscles or glands and control their contraction/secretions..
  • When stimulated, the motor neuron releases neurotransmitters that bind to receptors on the muscle and triggers a response which leads to muscle movement.
  • When the axon of a motor neuron fires, the muscle with which it has formed synapses with contracts.
  • The strength of the muscle contraction depends on the rate of firing of the axons of motor neurons that control it.
• Axon Hillock - end of the cell body before the impulse is released down the axon.
• Schwann Cells - create the myelin sheath (located inside myelin sheath)
• Neuron parts

  
• Synaptic transmission diagram

  
• Synaptic transmission (1):

  • action potential reaches terminal button at axon end, it needs to be transferred to another neuron or to tissue.
  • it must cross a gap between the pre- and post- synaptic neurons
  • This area is the synapse - includes presynaptic neuron ending, the membrane of the postsynaptic neuron and the gap in between
  • physical gap between the pre- and postsynaptic cell membranes is synaptic gap
• Synaptic transmission (2):

  • At the end of the axon of the nerve cells are a number of sacs known as synaptic vesicles
  • vesicles contain the chemical messengers that assist in transfering impulses, neurotransmitters
  • action potential reaches the synaptic vesicles, it causes them to release their contents through exocytosis
• Postsynaptic Neuron Action Potential

  • In order for an action potential to be generated on the postsynaptic neuron there must be enough neurotransmitter binding to the receptors to reach the threshold.
  • It will receive two types of input, excitatory or inhibitory
  • The postsynaptic neuron will ‘sum up’ the type of inputs it is receiving i.e., how may excitatory or inhibitory. Adding up these inputs will determine the postsynaptic neurons likelihood of having its own action potential.
• Excitatory Neurotransmitters - make the postsynaptic cell more likely to fire by increasing the positive charge of the cell, i.e., noradrenaline. ‘Switch on’
• Inhibitory Neurotransmitters - make the postsynaptic cell less likely to fire by increasing the negative charge of the cell i.e., serotonin. ‘Switch off’
• Summation

  • In order for an action potential to be generated on the postsynaptic neuron there must be enough neurotransmitter binding to the receptors to reach the threshold, the likelihood can be increased in one of two ways:
  • Spatial and temporal summation
  • The neurons will still only generate a new action potential if they reach the threshold after summing up the amount of excitatory and inhibitory signals.
• Spatial summation - Neurotransmitters are released from more than one presynaptic neuron, which all connect to the same postsynaptic neuron.

  
• Temporal summation - A series of action potentials are delivered in quick succession along the one presynaptic neuron.

  
• 3 types of neuron:

  • Sensory (carry impulses from sensory receptors to CNS)
  • relay (allow motor and sensory neurons to communicate)
  • motor (conducts signals from CNS to effectors like muscles)
• • Neurons have a cell body, dendrites and an axon.
  • Dendrites receive nerve impulses, cell body is control centre, impulse terminates at axon terminal
• • Action potential (nerve impulses) has to cross synapse in synaptic transmission (between pre- and post- synaptic neuron)
  • Synaptic vesicles release neurotransmitters in exocytosis
  • Neurotransmitters (NTM) bind to specialised receptor molecules
• • Excitatory NTMs  increase the potential (EPSP) of excitatory signal being sent back to postsynaptic cell, increasing the chance of it firing again, and vice versa for inhibitory NTMs (IPSP)
• • EPSPs and IPSPs can be received by the same nerve cell. 
  • Net result determines if the cell fires.
  •  EPSPs strength determined by spatial, lots of same signal from different nerves sent to one at the same time, or temporal, lots of signals from same cell