Pain sensation Pain receptors are also called as nociceptors They are located at the ends of small 'C unmyelinated or myelinated A delta fibers a) Definition Pain sensation is unpleasant but protective sensation aroused by noxious stimuli that damage or can damage body tissues b) Physiology (properties & reaction) Purpose or importance: Protective Stimulus: noxious (chemicals like- Ach, bradykinin, serotonin, hydrogen ions, potassium ions, prostaglandins or mechanical or thermal) Receptors: free nerve endings (polymodal receptors) Adaptation : non or slow adapting receptors Nerve fibers: fast pain is carried by A-delta nerve fibers while slow pain by 'C' type. Neurotransmitters: glutamic acid (at spinal cord) for fast pain, substance P (at spinal cord) for slow pain and Lewis P factor for muscle pain, Pathway: lateral spinothalamic (specific neo spinothalamic for fast pain and diffuse and non specific paleo spinothalamic for slow pain) Reaction : pain is a
Definition of synapse
Synapse is junction between two neurons through which impulse passes from one neuron (called presynaptic neuron) to the other (called postsynaptic neuron)
Apart from transmission impulses are also manipulated in many ways at synapse like summation, block, inhibition, direction change etc.
Anatomical classification
1. Axosomatic: Synapse between axon of one neuron and cell body of the next neuron. e.g Motor neurons in spinal cord, cerebrum and cerebellum.
2. Axodendritic: Synapse between axon of one neuron and dendrite of the next neuron. Most of the synapses are of this type. e.g. Motor neurons in spinal cord, autonomic ganglia, cerebellum.
3. Axoaxonic: Synapse between axon of one neuron and axon of other neuron. e.g. spinal cord.
4. Dendrodendritic: Synapse between dendrite of one neuron and dendrites of second neuron. Very rare. e.g. in olfactory bulb
Physiological classification
2. Electrical synapse (diagram-B): Here there are open fluid channels which conduct electricity from one cell to another. These open channels are called as gap junctions or low resistance bridges. Impulse can be transmitted in both directions. Such synapses are present in invertebrates and in smooth / cardiac muscles in vertebrates
3. Conjoint synapse (diagram-C):Synapses in which transmission is both chemical and electrical.
Mechanism of synaptic transmission
1. Arrival of action potential (AP) at presynaptic terminal 2. Opening of voltage gated Calcium (Ca++) channels at pre presynaptic membrane (active zone).
3. Entry of large amount of Ca+ in to pre presynaptic terminal. Ca++ via phospokinase 1 enzyme leads to phosphorylation of the synopsin in the wall of vesicle
4. This leads to free mobility of vesicles towards pre presynaptic membrane (called release sites)
5. Collected vesicles at release sites fuses with pre synaptic membrane and release the synaptic transmitter into presynaptic cleft via exocytosis.
6. Quantity of synaptic transmitter released is proportional to the amount of Catt ions entered into pre presynaptic terminal.
7. Synaptic transmitter release in to presynaptic cleft bind to receptors on postsynaptic membrane. This binding causes change in the resting membrane potential of postsynaptic membrane. (EPSP or IPSP)
8. If the EPSP is more than threshold of excitation, AP develops at initial segment of axon (axon hillock) mainly due to opening of voltage gated Na channels. This AP propagates along the axon to the axon terminal.
For example- if excitatory synaptic transmitter is Ach, it will bind to specific ligand gated Ach channels present on postsynaptic membrane to develop EPSP and soon degraded by cholinesterase enzyme present at basal lamina in synaptic cleft.
Electrical property - EPSP IPSP
- All the potentials, whether excitatory or inhibitory, get algebraically summated in the cell body of the neuron. If the algebric sum reaches the firing level, action potential is produced at the axon hillock and if the algebric sum does not reach the firing level, action potential is not produced
- There are basically two types of synaptic potentials produced on the postsynaptic membrane: excitatory or inhibitory post synaptic potentials depending upon the neurotransmitter released at the synaptic cleft.
EPSP (Excitatory post synaptic potential)-
When excitatory synaptic transmitter bind to post synaptic membrane depolarizes (its RMP- resting potential is decreased) due to 1- Main cause is opening of Na+ channels These lead to Na+ influx into post synaptic membrane.
2- Decrease Cl influx and decrease K+ efflux also contribute3- Increases No. of excitatory receptors of postsynaptic membrane and excitation of cellular metabolism also play a part.
All these changes change the RMP in the positive direction. This change is called EPSP
When EPSP is grater than the threshold of excitation AP develops at initial segment of axon (as this part has maximum no. of Na+ channels so most excitable part) and travel along the axon. EPSP has following characteristics-
It is graded and localizes potential change. 2. It does not follow all or none law. 3. It is mono phasic 4. It is fore runner of AP.
IPSP (Inhibitory postsynaptic potential)
When the synaptic transmitter is inhibitory, its binding with post synaptic receptors on postsynaptic membrane results hyper polarization of membrane due to
Main cause is opening of voltage gated Cl channels. This leads to Cl influx into post synaptic membrane
1
2. Increased K+ efflux also contributes
3. Decrease Na+ and Ca++ influx
All these changes change the RMP in the negative direction. This change or increase in negativity of RMP is called IPSP.
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