Neurophysiology: Plasma Membranes and Graded Potentials Study Notes

Leak Channels
- Always open, allowing continuous diffusion of specific ions.
- Function: Facilitate movement from higher concentration to lower concentration (concentration gradient).
- Specificity: Each leak channel permits only one specific type of ion (e.g., potassium, sodium).
Gated Channels
- Chemically Gated Channels
  - Normally closed; open temporarily in response to neurotransmitter binding.
  - Permit a specific type(s) of ion to diffuse across the membrane.
  - Similar in function to calcium-gated channels found in muscle cells.
- Voltage-Gated Channels
  - Normally closed; may open in response to changes in electrical charge across the plasma membrane.
  - Specific to particular ions with a unique gating mechanism.
  - Sodium Channels have two gates: Activation gate and Inactivation gate.
  - Resting State: Inactivation gate open, activation gate closed.
  - Activation State: Both gates open; sodium ions can flow across.
  - Inactivation State: Activation gate remains open while inactivation gate closes, stopping sodium flow.
- Modality-Gated Channels
  - Located on dendritic endings of sensory neurons.
  - Open and close in response to specific stimuli (e.g., pressure, light, temperature).

Entire Neuron
- Leak channels (sodium and potassium) distributed throughout the cell body, dendrites, and axon.
Receptive Segment (Dendrites)
- Contains chemically gated channels (potassium, chloride, and sodium).
- More sodium ions enter than potassium ions exit during activation.
Initial Segment (Axon Hillock)
- Composed of voltage-gated sodium and potassium channels.
Conductive Segment (Axon)
- Contains voltage-gated sodium and potassium channels.
Transmissive Segment (Synaptic Bulb)
- Contains voltage-gated calcium channels and calcium pumps critical for neurotransmitter release.

Analogies likened: Nervous system as the electrical system of a house.
Other systems analogies:
- Skeletal system as frame of the house.
- Vascular system as plumbing (minus genitourinary plumbing).
- Endocrine system as a smart home.

Definition: Direct proportionality between voltage, current, and resistance.
Key Components:
- Voltage (V): Difference in electrical charge; source of potential energy; measured in volts (V) or millivolts (mV).
  - Not directly lethal.
- Current (I): Movement of charged particles; represents kinetic energy; measured in amps (A) or milliamps (mA).
  - Current of 100-200 mA can be lethal; typical American outlet provides 15 A.
- Resistance (R): Opposition to the movement of charged particles; inversely related to current.
Formula: $I = \frac{V}{R}$
Conceptual analogy: Like a kinking garden hose; more resistance leads to less current flow.

Neurons utilize charged ions (potassium, sodium, chloride) to generate voltage across the plasma membrane.
Resting Membrane Potential:
- Typical value: -70 mV, maintained primarily by potassium leak channels.
- Intracellular Factors:
- Negative charge inside due to inorganic phosphate and negatively charged proteins.
- Potassium predominantly inside; its diffusion establishes resting potential.
- Equilibrium for Potassium:
- Achieved when electrical and concentration gradients balance.

Segments:
1. Receptive Segment: Where neurotransmitters bind, initiating graded potentials.
2. Initial Segment: Site for summation of graded potentials; axon hillock starts action potential.
3. Conductive Segment: Axon; action potentials propagate down this segment.
4. Transmissive Segment: Releases neurotransmitters into the synaptic cleft.

Definition: Short-lived changes in the resting membrane potential caused by ion movement.
Characteristics:
- Established at chemically gated channels.
- Strength diminishes with distance from origin (local potentials).
- Can vary direction and magnitude; do not propagate long distances.
Types of Graded Potentials:
1. Depolarizing Potentials
- Occur when sodium channels open, increasing the membrane's positive charge (Excitatory Post-Synaptic Potential or EPSP).
1. Hyperpolarizing Potentials
- Caused by open potassium or chloride channels, making the inside more negative (Inhibitory Post-Synaptic Potential or IPSP).

EPSP:
- Signal received from presynaptic neuron via neurotransmitters binding to receptors.
- Sodium influx decreases negativity of the interior; blip on the graph indicates transmission to axon hillock.
IPSP:
- Signals originate similarly; potassium channels remove positive charge from the cell.
- Causes further negativity, prevents signal transmittance.

Purpose: To determine whether an action potential occurs.
Balance of EPSPs and IPSPs:
- Spatial Summation: Multiple incoming signals (EPSPs and IPSPs) from various locations.
- Temporal Summation: Repeated, rapid firing from a single presynaptic neuron.
Overall summation determines whether threshold is reached at axon hillock, triggering action potential.

Definition: Minimal potential change required to trigger an action potential (approximately -55 mV).
Upon reaching this threshold, a significant influx of sodium ions occurs, leading to depolarization and propagation of the action potential along the axon.

Transitioning to the next section regarding action potentials, further elucidating how graded potentials contribute to neuronal signaling and synaptic transmission.