Nervous System and Action Potential
Brain Regions and Functions
Cerebrum (Cerebral Cortex):
The largest and most important region of the brain, responsible for cognition.
Processes thought, language, emotion, and voluntary movement.
Composed of gray matter (outer layer) and white matter (deeper). In contrast, the spinal cord has gray matter in the center and white matter on the outside.
Clinical Relevance: Targeted by drugs like antipsychotics and stimulants.
Cerebral Lobes: Each lobe has a primary function but is interconnected with others for integrated brain function and homeostasis.
Frontal Lobe:
Controls voluntary movement and decision-making.
Broca's Area: Located in the frontal lobe, controls the motor aspects of speech.
Damage (e.g., from stroke) can cause expressive aphasia (inability or difficulty to form words).
Parietal Lobe:
Primary somatosensory center: Processes sensory inputs from the body.
Interprets touch, temperature, pressure, and pain.
Damage: Can cause neglect or impaired body awareness (e.g., inability to sense pain if damaged, like touching a flame without feeling burning).
Temporal Lobe:
Damage: Can lead to receptive aphasia or dysphasia (difficulty understanding spoken language).
Occipital Lobe:
Primary center for vision.
Processes shape, color, and movement perception.
Damage: May impair vision.
Insula Lobe: A hidden lobe, mentioned but not detailed in this section.
Limbic System: A group of interconnected structures crucial for emotion, motivation, and memory.
Key Structures: Hippocampus, Amygdala, Cingulate Gyrus.
Hippocampus: Critical for long-term memory formation.
Amygdala: Regulates fear, aggression, and emotional learning.
Processes threatening stimuli and initiates protective responses.
Connected with the hypothalamus.
Dysfunction: Can contribute to aggressive behavior.
Basal Ganglia Related Structures:
Includes structures like the Globus Pallidus, Caudate Nucleus, and Putamen (implied by 'calendata' and 'glutamate'). These are functionally related to the basal ganglia.
Diencephalon and Limbic Connection:
Diencephalon: Connects the cerebrum with the limbic system.
Components: Thalamus and Hypothalamus.
Connects with limbic structures (hippocampus, amygdala).
Relates emotion, memory, and autonomic regulation.
Clinical Relevance: Disconnection between the diencephalon and limbic system is linked to mood and anxiety disorders.
Cerebellum:
Main function: Motor control.
Fine-tunes voluntary movement for precision (e.g., pointing a finger to the nose).
Integrates input from the spinal cord to the cerebral cortex.
Clinical Correlation: Cerebellar disorders impair coordination and precision of movement.
Action Potential
Cell Communication: The body's cells communicate via two main systems: the nervous system (fastest) and the endocrine system.
Definition: An action potential is a rapid electrical event or signal that occurs when neurons reach a critical threshold voltage.
Function: Transmits electrical signals (information) rapidly along neurons from sensory organs to the brain for interpretation, and from the brain to muscles and glands.
Mechanism: Generated by the movement of ions across the cell membrane through selective ion channels (voltage-gated channels) that open and close in response to voltage changes.
Clinical Significance: Understanding action potential mechanisms is essential in clinical pharmacy, as many drugs directly target these processes.
Resting State Membrane Potential
Voltage: Approximately -70 millivolts (mV) during the resting state.
Maintenance Factors:
Sodium-Potassium Pump ( \text{Na}^+\text{/K}^+- \text{ATPase} ): Actively pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell, maintaining an electrochemical gradient.
Potassium Leak Channels: Allow K+ ions to slowly leak out of the cell, contributing to the negative charge inside the membrane.
Large, Negatively Charged Proteins: Proteins within the cell cytoplasm carry a negative charge, making the inside of the cell more negative.
Clinical Relevance: Cardiac drugs like Digoxin can affect the stability of the resting potential. Albumin can block the sodium-potassium pump, disrupting the resting potential.
All-or-None Principle
Action potentials are generated fully or not at all; there is no partial action potential.
The amplitude of the action potential does not decrease with distance along the axon.
An action potential is only generated if the membrane potential reaches a specific threshold.
Signals below the threshold cannot initiate an action potential.
Threshold
Voltage: Approximately -55 mV.
Represents the critical voltage at which a neuron will fire an action potential.
If the membrane potential reaches this level (becomes less negative), voltage-gated sodium channels open, initiating depolarization.
Clinical Relevance: Anesthetics can raise the threshold, thereby reducing neuron excitability.
Subthreshold Stimuli and Summation
Subthreshold Stimuli: Produce local, graded depolarizations that spread only a short distance and decrease in strength away from the source.
They are not strong enough to initiate an action potential.
Summation of Local Responses: Neurons integrate multiple excitatory and inhibitory signals.
Spatial Summation: Combines signals from different synapses simultaneously to enhance their strength.
Temporal Summation: Involves repeated inputs from a single synapse over time.
If the sum of these local responses reaches the threshold, an action potential begins.
Clinical Relevance: Anticonvulsant medications modulate summation to reduce the occurrence of seizures.
Phases of the Action Potential
Resting State:
Membrane Potential: -70 mV.
Channels: Both voltage-gated Na+ and K+ channels are closed.
Maintained by the Na+/K+ pump, K+ leak channels, and intracellular negative proteins.
Depolarization:
Trigger: Membrane potential reaches the threshold ( -55 mV).
Channels: Voltage-gated Na+ channels open rapidly and widely.
Ion Movement: Na+ ions rush into the cell due to concentration and electrical gradients.
Voltage Change: The inside of the cell becomes rapidly positive, rising to a peak potential of approximately +30 to +40 mV.
Repolarization:
Trigger: Occurs at the peak of depolarization ( +30 to +40 mV).
Channels: Voltage-gated Na+ channels inactivate and close. Voltage-gated K+ channels open (more slowly than Na+ channels).
Ion Movement: K+ ions flow out of the cell, making the inside of the cell more negative.
Voltage Change: The membrane potential returns towards the resting negative state.
Clinical Relevance: Potassium channel blockers can prolong repolarization, which is utilized in cardiac therapy.
Hyperpolarization (Undershoot/Refractory Period):
Trigger: K+ channels remain open for a short period after the membrane potential has returned to the resting state, leading to excessive K+ efflux.
Voltage Change: The membrane potential briefly drops below the resting potential ( -70 mV), becoming even more negative.
State: During this hyperpolarized state, the neuron is unable to respond to further stimuli (refractory period).
Clinical Relevance: Sedatives can enhance hyperpolarization to reduce neuronal activity.
Restoration Phase:
The sodium-potassium pump actively works to restore the original ion concentrations across the membrane, bringing the neuron back to its resting potential and preparing it for the next action potential.
Refractory Period
Absolute Refractory Period: During the depolarization and initial repolarization phases, the neuron cannot fire another action potential, regardless of stimulus strength. This is due to inactive Na+ channels.
Relative Refractory Period: During hyperpolarization, a stronger-than-normal stimulus is required to trigger a new action potential.
Importance:
Ensures that impulses travel in one direction only (unidirectional nerve conduction), preventing backward spread.
Limits how frequently nerves can fire, providing a crucial break between impulses.
Contributes to signal precision and temporal coding.
Protects the nervous system from uncontrolled overexcitation.
Clinical Relevance: Drugs (e.g., some anticonvulsants) that prolong the refractory period can prevent seizure activities by reducing neuronal hyperexcitability.