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Basic units of the nervous system<br> Receive, integrate, and transmit information<br> Operate through electrical impulses<br> Communicate with other neurons u000b through chemical signals<br>
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Neurons • Basic units of the nervous system • Receive, integrate, and transmit information • Operate through electrical impulses • Communicate with other neurons through chemical signals
Structure of neuron 1. 3. 2. Main functions of Neurons : Reception, Conduction, Transmission
Mechanism of nerve transmission • Axonal transmission: Voltage gated cation channels generate action potentials in electrically excitable cells. • Synaptic transmission: Transmitter gated ion channels convert chemical signals into electrical ones at chemical synapses.
Axonal transmission • A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation. • When the axon is stimulated the resting potential changes. • It changes from -70mV inside the membrane to +40mV. • For a very brief period the inside of the axon becomes positive and the outside negative. • This change in potential is called action potential and lasts about 3 milliseconds.
Cont… • Cell membrane permeability must be changed for a nerve impulse to travel down a neuron. • Na⁺- K⁺ ATPase pumps K⁺ into and Na⁺ out of the neuron. a) A cell is normally polarized when a neuron is at rest – called it resting potential
b) A portion of the neuron becomes depolarized – Na⁺ moves into the cell c) Cell repolarization - K⁺ moves out of the cell.
Cont… • The axon may be myelinated or non-myelinated Saltatory conduction: It is the word used to describe the “jumping” of the action potential from node to node. - Provides rapid conduction of impulses - conserves energy for the cell Multiple sclerosis is the disease in which the myelin sheath is destroyed and associated with poor muscle coordination.
Synaptic transmission • Neuronal signals are transmitted from cell to cell at specialized site known as synapses. • It may be of 2 types: a) Electrical synapse: synaptic cleft gap is 20Å and direct action potential b) Chemical synapse: synaptic cleft gap is >200Å and action potential is by neurotransmitters. • A change in electric potential in the pre-synaptic cell triggers it to release small signal molecules known as Neurotransmitters, which are stored in synaptic vesicles and released by exocytosis.
Neurotransmitters • Chemical messengers that traverse the synaptic gaps between neurons • When released by the sending neuron, neurotransmitters travel across the synapse and bind to receptor sites on the receiving neuron, thereby influencing whether it will generate a neural impulse
Types of neurotransmitters • Excitatory (EPSP) • usually when receptor channels admit sodium Ex: Acetyl choline, glutamate, adrenaline • Inhibitory (IPSP) • usually when receptor channels admit chloride/potassium Ex: Dopamine, glycine, GABA
Acetylcholine • 1st substance identified as NT • Links motor neurons and muscles (contract or relax) • Also involved in memory, learning, sleep, dreaming. • It is called as cholinergic synapse, it has 2 receptors: a) Nicotinic receptor b) Muscarinic receptor • It is a ligand gated cation channel & degraded by acetylcholineesterase.
Neuromuscular transmission by acetylcholine It involves the sequential activation of gated ion channels 1. Arrival of an action potential at the terminus of a pre-synaptic motor neuron induces opening of voltage gated Ca²⁺ channel 2. Subsequent release of acetylcholine, which triggers opening of ligand gated acetylcholine receptors in the muscle plasma membrane 3. The resulting influx of Na⁺ produces a localized depolarization of the membrane leading to opening voltage gated Na⁺ channels and generation of an action potential
Cont… 4. When the spreading depolarization reaches the T tubules, it is sensed by voltage gated Ca²⁺ channels in the plasma membrane. This leads to opening of Ca²⁺ release channels in the sarcoplasmic reticulum membrane releasing stored Ca²⁺ into the cytosol. The resulting rise in cytosolic Ca²⁺ causes the muscle contraction.
