Nervous System Physiology Essentials 2 (2024-2025) PDF

Nervous System Physiology Essentials 2 Physiology CHAPTER 4  Cellular communication Cellular communication allows cells to coordinate with each other and create order in the organism. 1. Intracrine: Cells communicate with itself in intracellulary 2....

Nervous System Physiology Essentials 2 Physiology CHAPTER 4  Cellular communication Cellular communication allows cells to coordinate with each other and create order in the organism. 1. Intracrine: Cells communicate with itself in intracellulary 2. Juxtacrine: Cells communicate through direct contact. 3. Autocrine: The cell responds to signals it secretes. 4. Paracrine: Sends short-distance signals to nearby cells. 5. Neurocrine: Nerve cells transmit signals from synapses with neurotransmitters. 6. Endocrine (Hormonal): Hormones are carried to target cells by blood over long distances. In these ways, cells effectively provide order in the body at close or long distances.  Difference between Autocrine and Intracrine communication Autocrine Communication: The cell secretes a signal molecule, which goes out of the cell and then binds to surface receptors on the same cell to show its effect. In other words, the signal goes out of the cell and returns to the cell. Intracrine Communication: The signal molecule is synthesized inside the cell and shows its effect by binding directly to receptors inside the cell without leaving the cell. The signal molecule does not leave the cell.  Receptor types Cells use receptors to perceive and communicate with external stimuli. Receptors respond to different types of stimuli and initiate biochemical processes that create a response in the cell. Receptors located in the cell membrane receive incoming signals. And most importantly, they specifically receive these signals and direct the cell in the right direction to respond. There are 4 basic types of receptors: AND THEY ARE DIVIDED INTO 2 DEPENDING IN THEIR LOCALIZATION: MEMBRANE/INTRACELLULAR 1. G-Protein Coupled Receptors, (in the membrane) 2. Ligand Gated Ion Channels, (in the membrane) 3. Enzyme-Linked Receptors (in the membrane) 4. Nucleus Receptors (in the nucleus) Receptor Localization Intracellular Receptors Cell Surface Receptors Steroid hormones, Protein, peptide, or amino acid Thyroid hormones, derivative hormones Vitamin D Lipophilic Hydrophilic - Intracellular - Cell membrane - Hormone-receptor complex - Second messengers - Gene transcription - Protein phosphorylation Hours/days Seconds/minutes/hours The main properties of receptors: 1. Specificity: Each receptor recognizes and is specific to a specific ligand molecule. That is, it responds only to specific signals. 2. Affinity: The strength of the binding between the receptor and the ligand. High-affinity receptors bind strongly to their ligands even at low concentrations. 3. Saturation: Since receptors have a limited number of binding sites, receptors reach their maximum binding level when a certain amount of ligand is reached. 4. Competition: Different molecules can compete for the same receptor. Competition plays an important role in regulating cell signaling. 5. Hypersensitivity (Up-regulation): An increase in the number of receptors. When the ligand decreases or there is a continuous stimulus, the cell creates more receptors to increase the response. 6. Insensitivity (Down-regulation): A decrease in the number of receptors as a result of continuous stimulus. The cell reduces the number of receptors to limit the response. 7. Agonists: Molecules that bind to the receptor and activate it; they provide signal transmission. 8. Antagonists: Molecules that bind to the receptor and inhibit signal transmission; they block the receptor response.  Membrane potentials and types Understanding membrane potentials is critical to understanding how cells communicate, respond to environmental stimuli, and process information. Excitable cells, such as nerve and muscle cells, use these potential changes to receive, integrate, and respond to environmental signals. 1. Resting Membrane Potential represents the basic electrical state of the cell when it is not stimulated and prepares the cell for stimulation. 2. Threshold Membrane Potential Threshold and action potentials provide rapid information transmission, especially in nerve cells 3. Action Potential Threshold and action potentials provide rapid information transmission, especially in nerve cells 4. Graded Potentials Graded potentials collect signals and can initiate an action potential by allowing the cell to reach its threshold of stimulation. NOT A REAL ACTION POTENTIAL BUT TRIGGERS THE FORMING A REAL ACTION POTENTIAL Resting Membrane Potential The electrical potential difference when the cell is not stimulated, in most cells it is around -70 mV. This potential is caused by the ion concentration differences inside and outside the cell. In particular, the exit of K⁺ ions from the cell membrane and the inability of Na⁺ ions to enter create a negative internal environment. The resting potential prepares the cell for stimulation. Contributors to the Resting Membrane Potential: Na⁺/K⁺ Pump: Maintains balance between Na⁺ and K⁺ ions within the cell. Keeps the cell negatively charged by taking 3 Na⁺ ions out and 2 K⁺ ions in with each use of ATP. K⁺ Leak Channels: K⁺ ions leak out of the cell, which helps keep the cell negative. The outflow of K⁺ ions is the main determinant of the resting potential. Negatively Charged Molecules: Negatively charged proteins and phosphates found within the cell cannot be distributed evenly across the membrane, creating a negative internal environment. The combination of these factors creates the resting membrane potential, which is usually around -70 mV, and keeps the cell ready for stimulation. Threshold potential The minimum potential required for the cell to initiate an action potential, usually around -55 mV. When this potential is reached, voltage-gated Na⁺ channels open rapidly, causing the action potential to occur. A stimulus below the threshold value cannot initiate an action potential, so it works on the "all or nothing" principle.  Graded potentials 1. Receptor (Generator) potentials Sensory receptors respond to stimuli from mechanoreceptors, thermoreceptors, nociceptors (pain), chemoreceptors, and electromagnetic receptors (vision) Graded potential from stimuli is called receptor potential If graded potential reached threshold an action potential is generated and sensory information is sent to the spinal cord and brain 2. Pacemaker potential - heart Specialized coronary muscle cells in the cardiac pacemaker region (SA node) have “leaky” ion channels graded potentials can potentially induce a true cardiac action potential Graded potential is responsible for cardiac automaticity 3. Postsynaptic membrane potentials (EPSP_IPSP) Graded potentials that develop on the postsynaptic membrane during synaptic transmission (stimuli from other nerves - can be stimulatory or inhibitory) If graded potentials reach threshold action potential develops 4. End Plate Potential (EPP) Post synaptic graded potential that develops at the neuromuscular junction. Postsynaptic membrane potentials are important in AP generation in nerve to nerve and nerve to muscle communication. FEATURES OF THE GRADED POTENTIALS 1. Proportional to the Stimulus: Strong stimuli create larger potential changes. 2. Decreases with Distance: Its effect weakens as it moves away. 3. Summation: Different signals can accumulate and become a larger potential. 4. Does not fit the content of "all or nothing": There is no need to exceed the threshold value to initiate a graded potential.  Action potential An action potential is a sudden and large electrical change that occurs in the cell membrane. It provides information transmission in excitable cells such as nerve cells and muscle cells.  Action potential Characteristics of action potentials 1. Requires specific voltage- gated ion channels 2. AP are the result of rapid changes in ion conductance 3. AP occur only on regions of cell membranes that are electrically excitable 4. AP generally are a standard size and shape for a specific cell type 5. All or none - when membrane reaches threshold an AP is generated  Formation of Action Potential: Resting State: There is a resting potential of around -70 mV in the cell membrane. During this time, Na⁺ and K⁺ channels are closed. Reaching Threshold Value: If there is sufficient amount of stimulus reaching the cell and raises the membrane potential to the threshold value of approximately -55 mV. This initiates the action potential. Depolarization: When the threshold value is exceeded, voltage-gated Na⁺ channels open and Na⁺ ions rapidly enter the cell. The cell interior becomes positive and the potential rises rapidly (up to approximately +30 mV). Repolarization: Na⁺ channels close and K⁺ channels open. K⁺ ions exit the cell, making the membrane potential negative again. Hyperpolarization: K⁺ channels close slowly, so the membrane potential drops below the resting potential (approx -75, -90 mV). This short-term hyperpolarization signals the end of the action potential. Return to Resting Potential: The Na⁺/K⁺ pump returns the cell to its resting potential by transporting ions to their initial locations. Refractory period in Action Potential There are two specific periods during which the cell membrane’s re-excitability is temporarily reduced following an action potential: 1. Absolute Refractory Period 2. Relative Refractory Period. These periods ensure that the action potential is controlled and that signals travel in one direction. These two periods regulate signal transmission, ensure that action potentials occur sequentially. The Absolute Refractory Period (ARP) This period It covers the two-thirds period of repolarization. During this period, voltage-gated Na⁺ channels are inactive and cannot be reopened. No matter how strong a stimulus the cell receives, a new action potential cannot be initiated. ARP ensures that the action potential travels in one direction and prevents signals from returning. The Relative Refractory Period (RRP) This period continues from the remaining 1/3 of the repolarization period. During this stage, Na⁺ channels begin to reactivate, but since K⁺ channels are still open, the intracellular potential is more negative than normal. Since the cell is below its resting potential, a new action potential can occur, but this requires a stronger stimulus than normal. RRP allows the cell to gradually return to its resting potential and become fully excitable again.  Propagation of the stimulus in Action Potential If the nerve is unmyelinated, depolarization spreads to nearby areas and in this way the AP is continued. This is called conduction according to the theory of currents. If the nerve is myelinated, only depolarization occurs in the nodes of Ranvier. Thus, ion loss is prevented and the form of AP propagation is called Saltatory conduction. There are about 2000 extra high voltage channel Na+ channels in the nodes of Ranvier compared to the normal nerve axon membrane. Thus, the advancing signal gains extra ions in each node of Ranvier and there is no loss of power in AP propagation.