Neurophysiology Lecture 1 PDF

Neurophysiology Prof.Dr.Heba Shawky Lecture 1  NEUROPHYSIOLOGY Introduction: The nervous system along with the endocrine system, provide most of the control functions of the body. In general, the nervous system controls the rapid activities such as muscular contractions ,also it is...

Neurophysiology Prof.Dr.Heba Shawky Lecture 1  NEUROPHYSIOLOGY Introduction: The nervous system along with the endocrine system, provide most of the control functions of the body. In general, the nervous system controls the rapid activities such as muscular contractions ,also it is the center of all mental activity including thought, learning, and memory. The endocrine system, regulates the metabolic functions of the body. 1-Central Nervous System (CNS): includes the brain and spinal cord. It contains centers for integration of the input signals and for production of output signals. 2-Peripheral Nervous System (PNS): is further subdivided into: Sensory Division: (Afferent Division) Consists of the sensory receptors, axons and cell bodies of sensory neurons Motor Division: (Efferent Division): is further subdivided into: Somatic division: consists of the axons of motor neurons that innervate skeletal muscles. Autonomic division: consists of the axons of motor neurons that innervate the viscera, smooth muscles, cardiac muscles and glands. Sensory division of the nervous system  Synaptic Transmission Synaptic transmission means transmission of an impulse (action potential) from one neuron to another along a synapse. What are the types of synaptic transmission? 1- Electric transmission: the electric current pass from one neuron to the other directly through Gap junction channels. This type is present in few areas in human CNS, e.g. retina 2- Chemical transmission: occurs by release of a chemical substance (called a neurotransmitter) from the presynaptic neuron to act on receptors on the membrane of the postsynaptic neuron What is a synapse? A synapse is a junction between an axon terminal of one neuron (presynaptic neuron) and a second neuron (post synaptic neuron). Types of Synapses: 1. Axoaxonic: where the presynaptic neuron synapses with the axon of a postsynaptic neuron. This type is the most excitable. 2. Axosomatic: where the presynaptic neuron synapses with the cell body, (soma) of the postsynaptic neuron. 3. Axodendritic: where the presynaptic neuron synapses with the dendrites of the postsynaptic neuron. This type represents up to 80- 95% of all synapses. Functional Anatomy of a Synapse: 1. Presynaptic knob. 2. Synaptic cleft. 3. Postsynaptic membrane. The synapse is formed of 3 main components: 1) The Presynaptic Axon Terminal: The terminal end of presynaptic fiber becomes expanded forming synaptic knob or terminal knob. The synaptic knobs contain important internal structures: a. A large number of mitochondria which provides ATP needed for the synthesis and exocytosis of the synaptic transmitters. b. A large number of synaptic vesicles which contain the synaptic transmitters. There are three types of synaptic vesicles: - Small clear vesicles containing the rapidly acting chemical transmitters e.g. acetyl choline, glycine, glutamate and GABA. - Small granular vesicles containing catecholamines. - Large granular vesicles that contain slowly acting neuropeptides. Synaptic vesicles discharge their transmitters at areas of membrane thickening in presynaptic membrane called active zones. c. SNARE proteins: - V- SNARE (Synaptobrevin) on the synaptic vesicle. - T- SNARE (Syntaxin) on the presynaptic membrane.  2) The synaptic cleft: It is a definite space (30-50 nm width) containing extracellular fluid rich in Na+Cl- and poor in K+. 3) The postsynaptic membrane (PSM): Contains appropriate Receptors for the neurotransmitters. This receptors is ionotropic receptors: which affect neuronal activity directly by opening ligand-gated ion channels in receptor itself : i) Cation channels: e.g. Na+, K+ or Ca++ channels. ii) Anion channels: e.g. Chloride channels.  Mechanism of Synaptic Transmission 1. Release of chemical transmitter: - Arrival of action potential in the presynaptic nerve opens voltage-gated Ca++ channels predominant in this area. - Ca++ enters the knob according to concentration and electric gradients. - Influx of Ca++ triggers binding of the SNARE proteins then fusion of the synaptic vesicles with the presynaptic membrane then exocytosis of their chemical transmitter into the synaptic cleft. The amount of the transmitter released is proportional to amount of Ca++ entered.  2- Binding of chemical Transmitter with its Receptors: Interaction of the chemical transmitter with its specific receptors changes the permeability of the postsynaptic membrane to one or more ions. 3- Generation of Postsynaptic Potential: Change in ion fluxes through postsynaptic membrane leads to change in its resting membrane potential According to the type of neurotransmitter (NT) & its receptor. a. If excitatory NT: membrane becomes less negative causing excitatory postsynaptic potential. (EPSP). b. If inhibitory NT: membrane becomes more negative causing inhibitory postsynaptic potential. (IPSP).  The magnitude and duration of EPSP and IPSP depends on the amount of released Neurotransmitter. Therefore they are not All or None. 4- Removal of Neurotransmitters from the Synaptic Cleft: As long as the neurotransmitter remains in combination with the receptor sites, the alteration in membrane permeability responsible for the EPSP or IPSP continues. Thus, after combining with the postsynaptic receptor, the chemical transmitter must be removed in one of following ways: a- Passive diffusion of the transmitter away from the synaptic cleft. b- Inactivation of the transmitter by specific enzymes within the synaptic cleft, e.g. acetyl choline by choline esterase enzyme. c- Active reuptake of the transmitter into the axon terminals, e.g. catecholamines, dopamine, serotonin.