Synaptic Transmission: Lecture Notes - Synapse and Myasthenia Gravis

# SYNAPTIC TRANSMISSION By Dr. Adeyomoye O.I ## Learning outlines 1. Definition 2. Types of Synapse 3. Electrical synapse 4. Chemical synapse 5. Neuromuscular transmission 6. Myasthenia gravis ## Synapse A synapse is the junction or connection between two neurons, or between a neuron and an effect...

# SYNAPTIC TRANSMISSION By Dr. Adeyomoye O.I ## Learning outlines 1. Definition 2. Types of Synapse 3. Electrical synapse 4. Chemical synapse 5. Neuromuscular transmission 6. Myasthenia gravis ## Synapse A synapse is the junction or connection between two neurons, or between a neuron and an effector cell (e.g., muscle or gland), where information is transmitted through electrical or chemical signals. Function: Synapses allow communication within the nervous system, enabling the transmission of signals that regulate body functions, sensory processing, and coordination of responses to stimuli. Types: There are two main types of synapses: Electrical Synapse: Direct transmission of electrical signals via gap junctions. Chemical Synapse: Indirect transmission using neurotransmitters across a synaptic cleft. ## Two types of synapse 1. Electrical 2. Chemical * Both types of synapses relay information, but do so by very different mechanisms. * Much more is known about chemical than about electrical synapses. ## Electrical synapse * Bidirectional transfer of information, but can be unidirectional. * Pre and postsynaptic cell membranes are in close apposition to each other, separated by gap junctions. * lons can flow through these gap junctions, providing low-resistance pathway for ion flow between cells without leakage to the extracellular space. * Instantaneous, fast transfer from one cell to the next (< 0.3 msec), unlike the delay seen with The image shows a diagram of electrical synapses between two cells, labeled Cell 1 and Cell 2. The cells are connected by gap junctions, which facilitate direct communication between them. The image shows a detailed diagram of gap junctions, with labels for presynaptic cell membrane, connexons, postsynaptic cell membrane, and pores connecting the cytoplasm of two neurons. The dimensions 3.5 nm and 20 nm are also indicated in the diagram. The image shows a diagram of a chemical synapse with labels for the presynaptic neuron, synaptic vesicle, postsynaptic neuron, neurotransmitter released, presynaptic membrane, postsynaptic neurotransmitter receptor, and postsynaptic membrane where ions flow through postsynaptic channels. | Feature (Differences) | Electrical Synapse | Chemical Synapse | | :------------------------- | :--------------------------------------- | :------------------------------------------ | | Mode of Signal Transmission | Direct electrical signal via gap junctions | Indirect chemical signal via neurotransmitters | | Speed of Transmission | Very fast | Relatively slower | | Direction of Signal | Usually bidirectional | Unidirectional | | Structure | Gap junctions with connexons | Synaptic cleft and vesicles with neurotransmitters | | Energy Requirement | Minimal (passive transmission) | Requires energy for neurotransmitter release | | Signal Amplification | No amplification | Can amplify signals via receptor activation | | Plasticity (Learning/ Memory) | Limited plasticity | High plasticity | | Occurrence | Found in reflex pathways, heart, and retina | Common in the central and peripheral nervous system | | Type of Signal | Electrical impulses (ion flow) | Chemical signals (neurotransmitters) | | Delay | Virtually no delay | Synaptic delay (~0.5 ms or more) | The image shows a diagram of neuromuscular transmission, showing the axon terminal with terminal button and synaptic vesicles containing acetylcholine. The diagram also features the voltage-gated calcium channel, voltage-gated sodium channel, neurotransmitter-gated channel, motor end plate, plasma membrane of muscle fiber, acetylcholine receptor site, acetylcholinesterase, and muscle fiber. ## Clinical correlate - Myasthenia Gravis * **Overview:** Chronic autoimmune neuromuscular disease and a long term disease * **Occurrence:** Any ethnic group and in both genders common in women under the age of 40years and in men under the age of 60 years. Newborn can have neonatal MG if the mother has the disease and symptoms disappear 3 months after birth. MG is not inherited or contagious. * **Signs and symptoms:** Muscles that control swallowing, facial expression and eye are most commonly affected. Blurred or double vision, weakness in arms and legs, unable to hold a steady gaze, changes in facial expression, difficult breathing if it affects breathing muscles. * **Causes:** Unknown * **Role of the Thymus gland:** Thymus gland tumour (Thymoma) * **Diagnosis:** Medical history, Physical examination of the muscle and eye, blood test showing presence of acetylcholine receptor antibodies, CT or MRI scan, nerve conduction study, * **Treatment:** Avoid stress, medications: prednisone, Azathioprine, Cyclosporin, mycophenulate, Mofetil, Tracrolimus all suppress the immune system, plasma replacement, Thymosectomy (Neostigmine, Physostigmine, Pyridostigmine -Cholinesterase inhibitors). * **Prognosis:** With proper treatment, signs and symptoms could reduce, does not affect life expectancy, Women with MG can have children The image shows a black and white photograph illustrating Myasthenia Gravis, showing the drooping eyelid of a patient. The image is sourced "From Principles of Neural science, 3rd edition, by E. Kandel, J. Schwartz and T. Jessel." ## Action potential in skeletal muscle cell * Action potential arrives at the neuromuscular junction. * Release of acetylcholine (ACh) into the synaptic cleft. * Binding of ACh to receptors → $Na^+$ influx → depolarization. * Propagation through T-tubules → activation of $Ca^{2+}$ release from the sarcoplasmic reticulum. ## Action potential in skeletal muscle cell * Skeletal Muscle Action Potential * Key Features: * Resting membrane potential: ~ -90 mV * Depolarization: Voltage-gated $Na^+$ channels open → Rapid $Na^+$ influx * Repolarization: Voltage-gated $K^+$ channels open → $K^+$ efflux * No plateau phase (short duration: ~2-5 ms) * Visual Aid: Diagram of a skeletal muscle action potential. ## Action potential in cardiac muscle cell * Key Features: * Resting membrane potential: ~ -85 mV * Phases: * Rapid depolarization: $Na^+$ influx (voltage-gated $Na^+$ channels). * Plateau phase: Slow $Ca^{2+}$ influx (L-type $Ca^{2+}$ channels) → prolonged depolarization. * Repolarization: $K^+$ efflux (voltage-gated $K^+$ channels). * Long duration: ~200-400 ms (prevents tetany). * Visual Aid: Action potential diagram highlighting the plateau phase. ## Action potential in smooth muscle cells * Key Features: * Resting membrane potential: ~ -50 to -60 mV (variable). * Depolarization: $Ca^{2+}$ influx (via voltage-gated $Ca^{2+}$ channels) rather than $Na^+$. * Repolarization: $K^+$ efflux. * Slow-wave potentials: Oscillations in membrane potential can trigger action potentials. * No true refractory period. * Visual Aid: Smooth muscle action potential vs. slow waves. | Feature | Skeletal Muscle | Cardiac Muscle | Smooth Muscle | | :-------------- | :-------------- | :------------- | :-------------- | | Resting Potential | ~-90 mV | ~ -85 mV | ~-50 to -60 mV | | Depolarizing Ion | $Na^+$ | $Na^+$, $Ca^{2+}$ | $Ca^{2+}$ | | Plateau Phase | No | Yes | No | | Duration (ms) | 2-5 | 200-400 | Variable | | Refractory Period | Short | Long (prevents tetany) | Minimal/none | Thank you