Neurotransmitters and how it performs its functions Flashcards
How does acetylcholine work
Acetylcholinesterase (AChE): The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft into acetate and choline, terminating the signal. Choline is taken back up by the motor neuron and reused to synthesize new ACh molecules.
1. Signal Initiation:
Nerve Impulse: A motor neuron receives a signal from the nervous system, creating an action potential (an electrical impulse) that travels down the axon of the neuron.
2. Synaptic Transmission:
Arrival at Neuromuscular Junction: The action potential reaches the end of the motor neuron at the neuromuscular junction (the synapse between the motor neuron and the muscle fiber).
Release of Acetylcholine (ACh): The depolarization of the axon terminal causes voltage-gated calcium channels to open, allowing calcium ions to enter the terminal. This influx of calcium triggers synaptic vesicles to release ACh into the synaptic cleft via exocytosis.
3. Activation of Muscle Fiber:
Binding to Receptors: ACh diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the muscle fiber’s motor end plate.
Depolarization: The binding of ACh opens ligand-gated ion channels, allowing sodium ions (Na+) to flow into the muscle cell and potassium ions (K+) to flow out, resulting in a localized depolarization called the end plate potential (EPP).
Action Potential in Muscle Fiber: If the EPP is strong enough, it triggers an action potential that propagates along the sarcolemma (muscle cell membrane) and into the muscle fiber via T-tubules.
4. Muscle Contraction:
Release of Calcium Ions: The action potential in the T-tubules triggers the release of calcium ions from the sarcoplasmic reticulum (a specialized organelle that stores calcium) into the cytoplasm of the muscle fiber.
Interaction with Contractile Proteins: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from actin-binding sites. This allows myosin heads to attach to actin filaments and initiate the cross-bridge cycling process.
Sliding Filament Mechanism: Using ATP, myosin heads pull actin filaments towards the center of the sarcomere, shortening the muscle fiber and producing contraction.
5. Termination of Signal:
Acetylcholine and Cognitive Function
1. Signal Initiation:
Neuronal Communication: Acetylcholine (ACh) is synthesized in cholinergic neurons in the brain. These neurons are found in areas like the basal forebrain, which projects to various brain regions involved in cognition, such as the cerebral cortex and hippocampus.
2. Release and Binding:
Release of ACh: When a cholinergic neuron is activated, an action potential travels down the axon to the synaptic terminal, where it causes the release of ACh into the synaptic cleft.
Binding to Receptors: ACh binds to two types of receptors in the brain: nicotinic acetylcholine receptors (nAChRs) and muscarinic acetylcholine receptors (mAChRs). These receptors are located on the post-synaptic neurons.
3. Modulation of Cognitive Processes:
Attention and Arousal: ACh in the brain enhances arousal and attention by increasing neuronal activity in the cortex and other related brain regions.
Learning and Memory: ACh is critical for learning and memory formation, particularly in the hippocampus. It modulates synaptic plasticity, the ability of synapses to strengthen or weaken over time, which is fundamental for memory encoding.
Synaptic Plasticity: ACh influences long-term potentiation (LTP) and long-term depression (LTD), which are processes that underlie learning and memory by altering the strength of synaptic connections.
4. Signal Termination:
Degradation by Acetylcholinesterase (AChE): ACh in the synaptic cleft is quickly broken down by the enzyme acetylcholinesterase, terminating its action. The choline produced by this breakdown is taken up by the presynaptic neuron for the synthesis of new ACh.
5. Feedback and Regulation:
Autoreceptors: Cholinergic neurons have autoreceptors on their presynaptic terminals that detect ACh levels and regulate further release, maintaining optimal levels of neurotransmission.
Summary
Acetylcholine plays a crucial role in both muscle contraction and cognitive functions by acting as a chemical messenger. In muscle contraction, it enables the transmission of signals from motor neurons to muscle fibers, leading to muscle movement. In cognitive functions, ACh modulates attention, learning, and memory by affecting synaptic plasticity and neuronal activity in the brain. The processes are tightly regulated by enzymes like acetylcholinesterase and feedback mechanisms to ensure proper functioning.
