Nervous Tissue - Study Notes

Neurons, like muscle fibers, exhibit electrical excitability.
They communicate using two types of electrical signals:
- Graded Potentials: Used for short-distance communication only.
- Action Potentials: Allow communication over long distances within the body.

The production of electrical signals in neurons depends on the presence of a resting membrane potential (RMP).
- The RMP is analogous to the electrical potential of a 9V battery, which has a voltage gradient.
- The RMP is established by ion gradients and a variety of ion channels that react to specific stimuli.
- The lipid bilayer of the plasma membrane serves as an effective insulator, necessitating ion flow through channels.

Ion channels are present in the plasma membrane of all cells but are particularly prominent in the nervous system.
Neurons and all body cells expend much energy to maintain a net negative charge inside compared to the outside.
When ion channels open, they allow specific ions to cross the plasma membrane, following their electrochemical gradient:
- Ions migrate from areas of higher concentration to lower concentration (the chemical gradient).
- Positively charged cations move toward negative areas, and negatively charged anions move toward positive areas (the electrical gradient).

There are three main types of active gated channels:
- Ligand-gated channels: Respond to neurotransmitters; concentrated at synapses.
- Voltage-gated channels: React to changes in transmembrane electrical potential; located along neuronal axons.
- Mechanically-gated channels: Respond to physical deformation, such as applied pressure.
Leakage channels: Open and close randomly; they also influence RMP.

The RMP is measured at rest, when the neuron is not conducting an impulse.
It exists due to the buildup of negative ions within the cytosol near the membrane and positive ions in the extracellular fluid
This charge buildup occurs very close to the membrane; the rest of the cytosol remains neutral.
The RMP is typically around -70 mV, indicating a negative internal environment compared to the outside.
The RMP is maintained because:
- Potassium (K+): Leakage channels favor K+ moving out, more than Sodium (Na+) moving in, due to a higher number of K+ channels.
- Large negatively charged proteins present in the cytosol remain there, contributing to the negative charge.
- Sodium-Potassium Pumps (Na+/K+ ATPases) counteract Na+ inward leakage by pumping Na+ out to maintain the RMP.

A neuron exhibiting an RMP is polarized, making it ready to generate an action potential.
To generate an action potential, graded potentials must first depolarize the cell to threshold.
Graded potentials occur when ion flux through mechanically gated or ligand-gated channels creates a localized current.
- This current spreads short distances before dissipating within a few millimeters of its origin.
A depolarizing graded potential reduces the negativity of the cell relative to the extracellular fluid, while a hyperpolarizing graded potential increases it.

Graded potentials can be differentiated by the stimulus type and location of occurrence.
They are variable in amplitude, allowing for summation (addition) or cancellation of effects.
Graded potentials mainly occur in the dendrites and cell bodies of neurons and do not propagate down the axon.

An action potential (AP) or impulse is a full-length signal traveling through the neuron.
During an AP, the membrane potential reverses and returns to resting state.
If a neuron receives a threshold (liminal) stimulus, a full-strength nerve impulse is generated; if below threshold, no impulse occurs.
AP proceeds through two main phases:
- Depolarizing phase
- Repolarizing phase

Graded potentials that cause depolarization from -70 mV to threshold (~-55 mV) trigger rapid events:
- Voltage-gated Na+ channels open, allowing Na+ influx, causing the interior to become increasingly positive (up to +30 mV).
- Around 20,000 Na+ ions enter each area of the membrane during this event.
During the repolarization phase, K+ channels open, and K+ flows outward, returning the cell to a more negative state until RMP (-70 mV) is restored.

An after-hyperpolarizing phase may occur while voltage-gated K+ channels are open, causing the membrane potential to dip further (around -90 mV).
Afterward, the membrane potential returns to resting level (-70 mV).

The all-or-none principle dictates that the AP generated follows a similar strength regardless of the intensity of the stimulus beyond threshold; a stronger stimulus does not produce a larger impulse.

Absolute Refractory Period: Time during which no additional AP can be generated regardless of stimulus strength; coincides with Na+ channel activation and inactivation.
- Establishes an upper limit of 10–1000 nerve impulses per second, depending on neuron type.
Relative Refractory Period: A subsequent phase during which a second AP can occur but only with a stronger-than-normal stimulus; coincides with continued activity of voltage-gated K+ channels.
Graded potentials do not display a refractory period.

Propagation begins at the trigger zone near the axon hillock:
- Continuous Conduction: Occurs in unmyelinated axons.
- Saltatory Conduction: Occurs in myelinated axons, where AP jumps from one node (Node of Ranvier) to the next, enhancing speed.

AP speed is influenced by:
- Axon diameter: Larger diameters lead to faster conduction.
- Myelination: Increased insulation enhances speed.
- Temperature: Higher temperatures can increase conduction rates.
Frequency of AP also plays a crucial role in the perception of stimuli and response extent, alongside the number of neurons activated.