Synapse

SYNAPSE Synapse is the junction between two cells where, the axon/ dendrite of one cell terminate in the dendrite of another cell. Neurons which sends messages is called presynaptic cell. Whereas the neuron that receives messages is called the postsynaptic neuron.

SYNAPSE

Synaptic knob

Subsynaptic and Post synaptic membrane

Synaptic cleft

Composition of synapse z

z z

Synaptic knob- Terminal end of presynaptic axon. Consists of synaptic vesicles, release neuro transmitter Mitochondria Microtubules Presynaptic membrane Sub synaptic membraneSurface of cell membrane involved in synapse Post Synaptic membraneRemainder of the cell membrane of the motor neuron

Synaptic cleftSeparates the pre and post synaptic membrane Size 20-40 nm z

NOTE – Receptors are mainly located on subsynaptic membrane

TYPES AXONDENDRITIC Most common

BetweenAxon and dendrite LOCATION Motor neurons in spinal cord

AXOSOMATIC

AXOAXONAL

DENDRO DENDRITIC rare

Axon and soma

Axon and axon

Dendrite and dendrite

Motor neuron in Spinal cord spinal cord and autonomic ganglia

Olfactory pathways synapse

SYNAPSE divided into • Chemical • Electrical ELECTRICAL SYNAPSE 1. The transmission of impulse occurs due to the release of ions.

CHEMICAL SYNAPSE 1. The transmission of impulse occurs due to the release of chemical substance 2. Largely present in CNS 3. Synaptic cleft present 4. Sensitive to hypoxia

Present at some places in CNS Low resistance bridges replaces synaptic cleft Resistance to hypoxia

EPSP excitatory post synaptic potential Excitability of neuron to other stimuli is increased by initial depolarizing response produced by a single stimulus to the sensory nerve. Therefore it is called EPSP Single stimulus is not sufficient to generate a spike and it declines slowly.

EPSP need to be summated by each of the active knob to generate action potential. This summation may be temporal or spatial. EPSP is generated by the opening of Na+ channels in the subsynaptic membrane of post synaptic cell

TEMPORAL SUMMATION Stimulation of single afferent nerve for the second time before decline of first stimulation.

SPATIAL SUMMATION Simultaneous stimulation of two afferent nerve fibers

NOTE- This is due the increased release of chemical transmitter IPSP - Inhibitory post synaptic potential Inhibition of excitability of neurons to the stimuli. Therefore it is called IPSP. It is due to hyperpolarizing response initiated by the stimulation of presynaptic nerve fibers SPATIAL AND TEMPORAL summation of IPSP also occurs as with EPSP. INHIBITION AT SYNAPSES Two types Post synaptic inhibition Pre synaptic inhibition Post synaptic inhibition- Release of inhibitory chemical by axons that lead to hyperpolarisation of subsynaptic membranes. Subdivided into Direct and indirect inhibition.

DIRECT INHIBITION Due to the stimulation of afferent nerve fibers which passe directly to motor neurons. INDIRECT INHIBTION Due to effect of discharge of previous postsynaptic neuron. eg Renshaw cell inhibition Negative feed back inhibition of the neurons by themselves. Presynaptic inhibition Inhibition that occur at the pre synaptic terminals before the signals reach the synapse Due to the discharge of inhibitory neurons. NOTE Inhibitory transmitter release is GABA – At axo axonal synapse. PROPERTIES 1. 2. 3. 4. 5. 6. 7. 8.

CONVERGENCE AND DIVERGENCE SYNAPTIC FATIGUE SYNAPTIC DELAY ONE WAY CONDUCTION SUMMATION SYNAPTIC OCCLUSION SUBLIMINAL FRINGE SYNAPSE PLASTICITY