physiology / endocrinology

10/16/22 Dr Saba Butt (Zool 303) 1

NEUROTRANSMITTERSNeurotransmitter is a chemical:I. it must be released from presynaptic

terminal,ii. must elicit the normal depolarization,iii. The effect of the substance must be

blocked by the same agents that block synaptic transmission.

Neuromodulators alter or modify the functioning of synapses.

Agonists are the chemicals that have the same effect as a neurotransmitter.

Anatagonists are the chemicals that reduce or prevent the synaptic transmission.


Table 4-1. Selected Neurortransmitters and Neuromodulators

Compound Site of Action of Neurons

Acetylcholine Neuromuscular Junction, autonomic endings, autonomic ganglia, sweat glands, brain, retina, GI tract

Biogenic amines Epinephrine Norephinephrine Dopamine Serotonin Histamine

Brain, spinal cordSympathetic endings, brain, spinal cord, GI tractBrain, Sympathetic ganglia, retinaBrain, spinal cord, retina, GI tractBrain, GI tract


Compound Site of Action of Neurons

Amino acids GABA Glutamate Aspartate Glycine

Brain, retinaBrainSpinal cord, brain?Spinal cord, brain, retina

Prines/Purine nucleotides Adenosine ATP

BrainAutonomic ganglia, brain

Gas Nitric oxide

Brain, spinal cord, GI tract


Compound Site of Action of NeuronsPeptidesActivinsAngiotensin IIAtrial natriuretic peptideCalcitonin gene-related peptideCholecystokininβ-EndorphinesEnkephalinsEndothelinsFMRF amideGalanin

BrainBrain, spinal cordBrain

Spinal cord, brain,

Brain, RetinaBrain, retina, GI tractBrain, retina, GI tractBrain, pituitary glandBrainBrain, spinal cord


Compound Site of Action of NeuronsPeptidesGastrinGastrin-releasing peptideGonadotropin-relating hormone InhibinsMotilinNeuropeptide YNeurotensinOxytocin

BrainBrain

Brain, autonomic ganglia, retina

BrainBrain, pituitary glandBrain, autonomic nervous systemBrain, retinaPituitary gland, brain, spinal cord


Compound Site of Action of NeuronsPeptidesSecretinSomatostatinVasoactive intestinal polypeptide

BrainBrain, retina, GI tractAutonomic nervous system, spinal cord, brain, retina, GI tract


Synthesis and metabolism of Neurotransmitters

Synthesis and recycling of neurotransmitters is very important phenomenon.Let us take One Example

ACETYLCHOLINEAcetylcholine is the principal neurotransmitter at neuromuscular junctions. The neurons that synthesize and release acetylcholine are called “cholinergic neurons”.


Active uptake


SYNAPTIC PLASTICITY AND LEARNING

• Depending on history of discharge at a synapse, short term or long-term changes can occur at a synapse. • These are very important as these are the bases for learning and memory.


POSTTETANIC POTENTIATION

• It is the production of enhanced postsynaptic response to an incoming stimulus (coming again and again). • It acts through accumulation of Ca++ in the terminals. • It lasts for 60 seconds and occur after a brief period of stimulus at persynaptic neuron.


Stimulus

Neuron

More Ca++

More neurotransmitter release

More response

Ca++ ions


HABITUATION

When a stimulus is benign (have no effect) and is repeated over and over, response to it gradually disappears. This is called habituation. This inactivation is due to decreased response of Ca++ channel and resulting decrease of intracellular Ca++. It could be short-term or long-term.


SENSITIZATION• Sensitizaion is a the prolonged occurrence of augmented post-synaptic responses after a stimulus to which an animal become habituated is paired once or several times with a noxious stimulus.

• It may be short-term and long-term.• In case of short term, it occur by Ca++ mediated change in adenylyl cyclase that leads to greater production of cAMP and hence more response.


• Long-term potentaiton, however, also involve protein synthesis and growth of pre- and postsynaptic neurons and their connections.


LONG-TERM POTENTIATION (LTP)

LTP is the enhancement of the response of a postynaptic neuron to a presynaptic stimulation.

This occur after a repeated stimulus of the presynaptic neuron for a brief period.

It is due to increased Ca++ conductance in postsynaptic neurons. It occurs at many places in brain.


LONG TERM DEPRESSION• It is opposite response to the LTP.

• It is characterized by synaptic decrease in many ways.

• It is produced by slower stimulation of presynaptic neurons and is associated with a smaller rise in intracellular Ca++.


EPSP and IPSP• Excitatory Postsynaptic Potentials are those which cause excitation of the terminal.

• Inhibitory postsynaptic Potentials are those which cause a reverse potential or hyperpolarization to stop or inactivate a depolarizing current.

physiology / endocrinology

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