Structure of a Neuron PDF

STRUCTURE OF A NEURON (PSY 7) Synaptic Vesicles: Contain neurotransmitters that are released into the synapse. Cell Body (Soma) Function: The cell body is the neuron's control GENERATING ACTION POTENTIALS cente...

STRUCTURE OF A NEURON (PSY 7) Synaptic Vesicles: Contain neurotransmitters that are released into the synapse. Cell Body (Soma) Function: The cell body is the neuron's control GENERATING ACTION POTENTIALS center, containing the nucleus and other organelles. It processes incoming signals and lonic Composition of Intracellular and maintains the neuron's health. Extracellular Fluids Components: Intracellular Fluid (ICF): Nucleus: Contains the cell's genetic material The intracellular fluid is the fluid within cells. (DNA). Major ions in the ICF include: Cytoplasm: Includes various organelles, such as Potassium (K+): The predominant ion, with a mitochondria (energy production) and ribosomes high concentration inside the cell. (protein synthesis). Phosphate (HP04^2-) and proteins: These are Dendrites negatively charged and help balance the positive Function: Dendrites are branching extensions charge of K+. that receive signals from other neurons. They Sodium (Na+): Present in lower concentrations conduct electrical messages to the cell body. compared to the extracellular fluid. Structure: They have a large surface area to Extracellular Fluid (ECF): receive more signals through their many The extracellular fluid is the fluid outside cells, branches. including interstitial fluid and blood plasma. Major ions in the ECF include: Axon Sodium (Na+): The predominant ion, with a high Function: The axon carries electrical impulses concentration outside the cell. away from the cell body to other neurons, Chloride (Cl-): The main anion in the ECF. muscles, or glands. Calcium (Ca2+): Also present in significant Components: amounts. Axon Hillock: The cone-shaped region where the Potassium (K+): Present in lower concentrations axon joins the cell body. It is the site where action compared to the intracellular fluid. potentials are initiated. Myelin Sheath: A fatty layer that covers the axon Electrochemical Gradient in segments. It acts as an insulator, increasing the The concept of an electrochemical gradient is speed of signal transmission. crucial for understanding how ions move across Nodes of Ranvier: Gaps in the myelin sheath cell membranes, including in neurons where it where ion channels are concentrated, allowing the plays a pivotal role in generating and propagating action potential to jump from node to node action potentials. (saltatory conduction). Axon Terminals (Synaptic Boutons) An electrochemical gradient is a combination of Function: These are the endpoints of the axon two gradients: a chemical gradient and an where the neuron makes contact with other cells. electrical gradient. These gradients drive the They release neurotransmitters into the synaptic movement of ions across membranes. cleft to communicate with other neurons or Chemical Gradient (Concentration effector cells. Gradient): Components: Definition: This gradient is formed due to differences in the concentration of ions across the membrane. lons will naturally move from an area into the cell, maintaining the concentration of higher concentration to an area of lower gradients of these ions. concentration to reach equilibrium. Example: In a neuron, there is a higher The Resting Potential concentration of sodium ions (Na+) outside the The resting potential is the electrical potential cell and a higher concentration of potassium ions difference across the cell membrane when the cell (K+) inside the cell. is not actively sending signals. Electrical Gradient (Voltage Gradient): Typically, the resting potential is around -70 Definition: This gradient is formed due to mV, with the inside of the cell being negative differences in electrical charge across the relative to the outside. membrane. lons are attracted to areas of opposite Permeability charge. Meaning: permeability refers to the ability of the Example: The inside of a neuron is typically cell membrane to allow certain substances to pass more negatively charged compared to the outside, through it while blocking others. This is crucial creating an electrical gradient that influences the for the regulation of the cell's internal movement of positively charged ions like Na+ environment. This potential is primarily and K+. established by: The differential permeability of the cell Diffusion and lon Movement membrane to Na+ and K+ (more permeable to K+ Diffusion is the movement of particles from an due to more K+ leak channels). area of higher concentration to an area of lower The Na+/K+ pump, which maintains high K+ concentration. In the context of neurons and inside and high Na+ outside the cell. action potentials, it involves the movement of Negatively charged proteins and ions inside the ions (primarily sodium, Na+, and potassium, K+) cell that cannot cross the membrane. across the cell membrane through specific ion channels. The Action Potential An action potential is a rapid, temporary change The movement of ions across the cell membrane in the membrane potential that travels along the is crucial for generating action potentials and axon of a neuron. It involves several phases: maintaining the resting potential. This movement Resting State: occurs via: The neuron is at resting potential, and lon Channels: These are proteins that create voltage-gated Na+ and K+ channels are closed. specific pathways for charged ions to pass through the cell membrane. Depolarization Leak Channels: Always open, allowing ions to Depolarization refers to the process by which the move down their concentration gradient membrane potential of a neuron becomes less Gated Channels: Open or close in response to negative (more positive) than its resting potential. specific stimuli (voltage-gated, ligand-gated, or This is a critical step in the generation of an mechanically gated). action potential. lon Pumps: Active transport mechanisms that move ions against their concentration gradients How Depolarization Occurs: using ATP. 1. Stimulus: A neuron receives a stimulus strong Sodium-Potassium Pump (Na+/K+ ATPase): enough to cause a change in the membrane Pumps 3 Na+ ions out of the cell and 2 K+ ions potential. 2. Threshold: If the stimulus causes the How Hyperpolarization Occurs: membrane potential to reach a critical level, 1. Repolarization: After depolarization, Na+ called the threshold (around -55 mV for many channels close, and voltage-gated potassium (K+) neurons), voltage-gated sodium (Na+) channels channels open, allowing K+ to exit the neuron, open. which helps to restore the negative membrane 3. Na+ Influx: The opening of these channels potential. allows Na+ ions, which are in higher 2. K+ Efflux: The outflow of K+ ions makes the concentration outside the cell, to rush into the inside of the neuron more negative. neuron due to both the concentration gradient and 3. Overshoot: Often, the K+ channels remain the electrical gradient (inside of the cell is open slightly longer than necessary, causing an negative relative to outside). overshoot. This means the membrane potential 4. Membrane Potential Change: The influx of becomes even more negative than the typical positively charged Na+ ions makes the inside of resting potential (e.g., it might go down to -80 the neuron less negative (more positive), moving mV or more). the membrane potential from the resting state (-70 4. Return to Resting Potential: Eventually, the mV) towards zero and even beyond, up to around K+ channels close, and the Na+/K+ pump (which +30 mV. moves 3 Na+out and 2 K+in) helps restore the membrane potential back to the resting level (-70 Repolarization: repolarization occurs after an mV). action potential has been fired. During an action Return to Resting Potential: K+ channels close, potential, the neuron's membrane potential and the Na+/K+ pump restores the resting becomes more positive (depolarization) due to the potential by moving Na+ out and K+ back into influx of sodium ions (Na+) into the cell. the cell Repolarization restores the negative resting membrane potential by the efflux of potassium Refractory Periods Refractory periods play a ions (K+) out of the cell through potassium crucial role in maintaining the fidelity of neuronal channels. Voltage-gated Na+ channels signaling. The absolute refractory period prevents inactivate, stopping the influx of Na+. any possibility of a new action potential; ensuring Voltage-gated K+ channels open, allowing K+ discrete, unidirectional signals, while the relative to exit the cell, making the membrane potential refractory period allows for controlled, more negative frequency-limited firing. These mechanisms are essential for proper neuronal communication and Hyperpolarization overall nervous system function. Hyperpolarization refers to the process by which the membrane potential becomes more negative Absolute Refractory Period: than the resting potential. This usually follows Definition: This is the period immediately repolarization and occurs due to the movement of following the initiation of an action potential K+ ions out of the neuron. during which no stimulus, no matter how strong, Efflux: Refers to the process of a substance can trigger another action potential. flowing out of a cell or a compartment. In Mechanism: After the voltage-dependent biological contexts, it often describes the sodium (Na+) channels have opened and allowed movement of ions, molecules, or other substances Na+ ions to flow into the neuron, these channels from the inside of a cell to the outside, usually become inactivated. They cannot open again until across the cell membrane. the membrane potential returns to near resting axon. Once the action potential has passed a levels. segment of the axon, that segment cannot Duration: This period lasts for about 1-2 immediately re-fire, which prevents the signal milliseconds after the peak of the action potential. from traveling backward. This is to avoid Significance: The absolute refractory period backward propagation of an action potential that ensures that each action potential is a discrete, can disrupt normal neuronal signaling, interfere all-or-nothing event and enforces unidirectional with synaptic transmission, and potentially cause propagation of the action potential along the cellular damage. axon. It also limits the maximum frequency at Limits Firing Frequency: The refractory which action potentials can be generated. periods set an upper limit on how frequently a neuron can fire, contributing to the regulation of Relative Refractory Period: neuronal activity and preventing excessive firing Definition: Following the absolute refractory that could lead to conditions such as epilepsy. period, there is a phase during which a neuron can Facilitates Signal Coding: By influencing the fire another action potential, but only in response intervals between action potentials, refractory to a stronger-than-usual stimulus. periods help encode information in the timing and Mechanism: During this period, the neuron is frequency of action potentials, which is vital for hyperpolarized due to the continued efflux of various neural processes, including sensory potassium (K+) ions, which makes the inside of perception and motor control. the cell more negative than the resting membrane potential. Key Points in Propagating Action Potentials Duration: This period lasts until the membrane All-or-Nothing Principle: An action potential potential returns to the resting level, which can be either occurs fully or not at all. Once the several milliseconds longer than the absolute threshold is reached, the action potential will refractory period. propagate along the entire length of the axon Significance: During the relative refractory without decreasing in size. period, the Na+ channels gradually recover from Speed of Propagation: The speed of action inactivation, but not all are fully ready to open. potential propagation is influenced by the As a result, a larger than normal depolarization diameter of the axon (larger diameter axons (typically 10-15 mV compared to the usual 5 mV) conduct faster) and the presence of myelin is required to reach the threshold for triggering (myelinated axons conduct faster than another action potential. This period helps unmyelinated ones). regulate the timing and frequency of action Energy Efficiency: Myelination increases the potentials, ensuring that neurons do not become efficiency of action potential propagation by overstimulated. reducing the number of ions that need to be pumped back across the membrane, conserving Importance in Neuronal Function energy. Prevents Overlapping Action Potentials: Refractory periods prevent action potentials from Propagating action potentials is a crucial process overlapping, which is crucial for clear, discrete for neuronal communication, involving the signaling in neurons. initiation of an action potential at the axon Ensures Directional Propagation: The absolute hillock, the sequential depolarization and refractory period enforces unidirectional repolarization of the axon membrane, and the propagation of the action potential along the efficient transmission of the signal through myelinated or unmyelinated axons. This process ensures that electrical signals are transmitted quickly and accurately within the nervous system, allowing for complex functions such as sensation, movement, and cognition. Differences between Refractory Period and Propagation of Action Potentials Nature: The refractory period is a time-based property of neurons following an action potential, while propagation of action potentials is a spatial process describing the movement of the action potential along the axon. Function: The refractory period ensures discrete, unidirectional action potentials and limits firing frequency, while propagation of action potentials enables signal transmission along the neuron. Mechanism: The refractory period is determined by the state of voltage-gated ion channels, whereas propagation involves sequential depolarization and repolarization of the axon membrane.