Neuron Ultrastructure

Neuron Ultrastructure

1. The reflex arc

   • Monosynaptic
   • 1. Sensory neuron detects muscle stretch
   • 2. SN signals to motor neuron to make relevant muscle contract
   • 3. Inhibitory interneurone switches off input to agonistic muscle
   • Systems don't work against each other

2. Sensory neuron structure

   • Branched dendrites myelinated by Schwann cells
   • Pseudo-unipolar soma
   • Few synapses
   • Surrounded by satellite cells
   • Branches in spinal cord and innervates motor neurons directly and indirectly (via interneurons)

3. Motor neurone structure

   • Large dendritic fields
   • Receive excitatory synapses from sensory neurons
   • Receive inhibitory synapses from interneurons
   • Also receives input from descending upper motor neurons
   • Designed to collect large number of signals (convergence) and to react rapidly (reflex arc)

4. Cerebellar circuitry

   • Input from motor cortex via pons
   • Input from vestibular nucleus, inferior olive and spinal cord
   • 3 cell layers
     • Outer layer: Molecular layer with stellate cells
     • Middle layer: Purkinje cells + basket cells
     • Inner layer: Granule cells + Golgi cells
   • Excitatory input modulated by balance of inhibition from Purkinje cells
   • Purkinje cells "learn" which modifiers to provide"
     • Electrical activity: small calcium spikes + larger sodium spikes
   • Basic pattern
     • 1. Climbing fibre - Purkinje synapse = excitatory
       • 1 Action Potential in climbing fibre sets off burst of APs in Purkinje = Complex Strike
     • 2. Mossy fibre - Granule cell synapse = excitatory
       • MFs in white matter. Multiple synaptic swellings (rosette) onto GC dendrite
     • 3. Granule - Purkinje synapse = excitatory
       • Very weak but multiple mossy fibres cause many granule cells to fire together: Purkinje cell exhibits Single Strike
     • 4. Golgi - Granule cell synapse = excitatory
       • Golgi cells inhibit mossy fibre - granule cell synapse = feedback inhibition
     • 5. Granule cell - basket cell synapse = excitatory
       • Basket cells inhibit Purkinje cells = feedforward inhibition

5. Ribbon synapses

   • Present in hair cells & photoreceptors
   • 3 groups of synaptic vesicles
     • Rapidly releasable (Near membrane)
     • Readily releasable
     • Filling pool

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