Detailed Notes on Graded Potentials, Action Potentials, and the Autonomic Nervous System

Definition: Graded potentials are changes in membrane potential that vary in size and depend on the strength of the stimulus. They are typically involved in initiating action potentials.
Types of Graded Potentials:
- Excitatory Postsynaptic Potentials (EPSPs):
- Make the inside of the neuron more positive (depolarization).
- Drive the membrane potential towards threshold (approx. -55mV).
- Achieved by:
  - Opening sodium ( $Na^+$ ) or calcium ( $Ca^{2+}$ ) channels.
  - Closing potassium ( $K^+$ ) or chloride ( $Cl^-$ ) channels.
- Inhibitory Postsynaptic Potentials (IPSPs):
- Make the inside of the neuron more negative (hyperpolarization).
- Drive the membrane potential away from threshold.
- Achieved by:
  - Closing sodium ( $Na^+$ ) or potassium ( $K^+$ ) channels.
  - Opening potassium ( $K^+$ ) or chloride ( $Cl^-$ ) channels.

Channel Dynamics:
- Closing an already open channel can create an inhibitory signal.
- Temporal Summation:
- Occurs when multiple subthreshold stimuli occur in rapid succession, preventing the first from returning to baseline. This can increase the chance of reaching the action potential threshold.
- Spatial Summation:
- Multiple signals from different locations can add together to produce a larger graded potential, similar to pooling money to buy candy.

All-or-None Principle:
- Action potentials are either initiated when threshold is reached or they do not occur at all.
- If firing occurs, it will reach a peak of +30mV regardless of the strength of the stimulus that triggered it.
Phases of Action Potentials:
1. Resting Phase: Membrane potential typically around -70mV (no graded potentials present).
2. Threshold Reached: Sodium channels open, leading to depolarization.
3. Peak Phase: Sodium influx causes membrane potential to reach +30mV.
4. Repolarization: Sodium channels close, potassium channels open, membrane potential begins to decrease back toward resting state.
5. Refractory Periods:
- Absolute Refractory Period: Occurs during repolarization, where a second action potential cannot be initiated.
- Relative Refractory Period: Occurs during hyperpolarization when a stronger-than-normal stimulus can initiate another action potential.

Size of Response:
- Action potentials are always the same size regardless of stimulus strength.
- Graded potentials vary in size; larger stimuli produce larger responses.
Decay:
- Action potentials do not decay and transmit signals over long distances.
- Graded potentials decay with time and distance from the stimulus origin.
Location:
- Action potentials occur solely on axons.
- Graded potentials can occur on dendrites, cell bodies, or axons.
Encoding Intensity:
- Action potentials encode stimulus intensity through frequency coding (more action potentials = higher stimulus intensity).
- Graded potentials encode intensity through amplitude (larger stimulus = larger graded potential).

Mechanisms:
- Saltatory Conduction: Occurs in myelinated axons, where action potentials jump from node of Ranvier to node, allowing faster transmission.
- Continuous Conduction: Occurs in unmyelinated fibers, where action potentials must generate continuously along the entire length of the axon, resulting in slower transmission.
Factors Influencing Speed of Conduction:
- Myelination: More myelin allows for faster conduction.
- Axon Diameter: Wider diameter reduces resistance, allowing for faster signal propagation.
- Temperature: Higher temperature increases speed due to enhanced ion diffusion.

Divisions:
- Sympathetic Nervous System: Known as the thoracolumbar system. Prepares the body for the fight-or-flight response. Key features include:
- Short preganglionic fibers, long postganglionic fibers
- Uses norepinephrine at target organs.
- Receptors: Adrenergic (alpha and beta types).
- Parasympathetic Nervous System: Known as the craniosacral system. Responsible for rest and digest functions. Key features include:
- Long preganglionic fibers, short postganglionic fibers
- Uses acetylcholine at target organs.
- Receptors: Cholinergic (nicotinic and muscarinic types).

Acetylcholine (ACh):
- Acts on nicotinic receptors (excitatory) at ganglionic synapses and on muscarinic receptors at target organs (inhibitory or excitatory depending on location).
Norepinephrine (NE):
- Binds to adrenergic receptors which can either stimulate or inhibit target organs.
- Alpha Receptors: Predominantly cause vasoconstriction in blood vessels.
- Beta Receptors: In the heart, increase heart rate (beta-1) and strength of contraction (positive inotropic effect).

Reflex Arcs: Comprised of sensory neurons (afferent pathways) and motor neurons (efferent pathways) that provide quick feedback and reaction to stimuli.
Importance of Coordination: The sympathetic and parasympathetic systems usually have antagonistic roles but can also work together (dual innervation) for proper physiological responses.
- Example: Heart rate modulation is controlled by both: sympathetic stimulation increases heart rate, while parasympathetic stimulation decreases it.