Movement Homeostasis and Neuronal Communication Study Guide

Brain (Central Nervous System - CNS) * Perception and Processing: Responsible for interpreting sensory stimuli for both somatic and autonomic systems. * Motor Execution: Handles the execution of voluntary motor responses within the somatic system. * Homeostasis: Regulates homeostatic mechanisms via the autonomic system.
Spinal Cord (Central Nervous System - CNS) * Reflex Initiation: Initiates somatic reflexes from the ventral horn gray matter and autonomic reflexes from the lateral horn gray matter. * Communication Pathways: Functions as the primary pathway for sensory and motor functions traveling between the periphery and the brain for both somatic and autonomic systems.
Nerves (Peripheral Nervous System - PNS) * Composed of fibers belonging to sensory and motor neurons supporting both somatic and autonomic functions.
Ganglia (Peripheral Nervous System - PNS) * Sensory Reception: Receives sensory stimuli through dorsal root and cranial ganglia (somatic/autonomic). * Motor Relay: Relays visceral motor responses through autonomic ganglia.
Digestive Tract (Enteric Nervous System - ENS) * The ENS is located specifically in the digestive tract. * It is responsible for autonomous functions and possesses the unique ability to operate independently of the brain and spinal cord.

Electrical Communication: Neurons communicate using "electricity," which is technically the exchange of positive ions across the cell membrane.
Ion Movement: The ability to transmit electrical impulses is fundamentally due to the movement of Sodium ( $Na^+$ ) and Potassium ( $K^+$ ) ions.
The Resting Membrane Potential: * The standard resting potential is defined as $-70\,mV$ . * At this state, no signals are actively being sent. * The potential is established because there are more positively charged sodium molecules outside the cell than inside. * The presence of negatively charged molecules known as anions inside the cell further establishes this electrical potential.

Specific concentrations of ions across the neuronal membrane at equilibrium: * Sodium ( $Na^+$ ): Inside concentration is $15\,mM$ ; Outside concentration is $145\,mM$ . * Potassium ( $K^+$ ): Inside concentration is $125\,mM$ ; Outside concentration is $5\,mM$ . * Chloride ( $Cl^-$ ): Inside concentration is $13\,mM$ ; Outside concentration is $150\,mM$ .

Sodium-Potassium ATP Pumps: * These active transport mechanisms move potassium ions into the cell against their concentration gradient. * Function: This process is essential to keep neurons from becoming over-stimulated. * Operation: The transporter pumps $K^+$ ions into the cell while pumping $Na^+$ ions out.
State of Channels at Rest: * At the resting potential ( $-70\,mV$ ), all voltage-gated $Na^+$ channels are closed. * Most voltage-gated $K^+$ channels are also closed.

Polarization: This refers to the state at rest where there is a significant difference in charge across the membrane.
Depolarization: * Definition: A reduction in the charge difference across the membrane. * Mechanism: Occurs when sodium channels open and $Na^+$ ions flow into the cell. * Context: This occurs rapidly when the neuron is being excited.
Repolarization and Hyperpolarization: * Once excitation has occurred, the membrane potential must decrease and reset to allow for subsequent signaling.

Initiation at the Axon Hillock: * Electrical signals are initiated at the axon hillock, which is the region located between the cell body and the axon. * Once the signal is initiated here, it flows down the axon toward the axon terminal where synapses are established.
Step-by-Step Sequence: 1. Stimulation: Stimulation of the axon hillock changes the voltage of the surrounding axon area. 2. Reaching Threshold: Voltage-gated channels only open once a specific target voltage, known as the action potential threshold, is reached. * For neurons, this threshold is $-55\,mV$ . 3. Depolarization Phase: * Upon reaching $-55\,mV$ , voltage-gated sodium channels open. * Sodium ions rapidly flow into the neuron. * The membrane potential continues to rise until it reaches approximately $+30\,mV$ . 4. End of Depolarization: * At $+30\,mV$ , the voltage-gated sodium channels close. 5. Repolarization Phase: * Potassium channels open. * Potassium ( $K^+$ ) flows out of the cell, causing the membrane potential to drop. 6. Hyperpolarization Phase: * Potassium channels respond more slowly to changes than sodium channels. * They remain open even after the threshold has been passed, causing the potential to drop below the resting state to approximately $-90\,mV$ . 7. Recovery: * The sodium-potassium ATP pump re-establishes the resting membrane potential of approximately $-70\,mV$ .

Mechanism of Spread: Action potentials spread as sodium ions diffuse along the interior of the axon.
Triggering Adjacent Areas: This internal diffusion causes neighboring areas of the membrane to reach the action potential threshold, effectively moving the signal down the line.

Definition: Periods during which the neuron is resistant to firing another action potential.
Dependency: A new action potential cannot be generated until the resting membrane potential of $-70\,mV$ is re-established.
Absolute Refractory Period: * During this phase, no action potential can be fired regardless of the stimulus strength. * Physiological state: Potassium ( $K^+$ ) channels are open and voltage-gated Sodium ( $Na^+$ ) channels are locked.
Relative Refractory Period: * During this phase, an action potential can be generated, but it requires a significantly stronger-than-normal stimulus to overcome the hyperpolarized state.