What is the main difference between the** Reticular Theory **and the Neuron Theory?
Reticular Theory (Golgi) proposed the nervous system is a continuous network. Neuron Theory (Cajal) proved neurons are discrete individual cells.
DefineAfferent and Efferent neurons.
Afferent: Sensory neurons carrying signals toward the CNS. Efferent: Motor neurons carrying signals away from the CNS to effectors.
What are Interneurons and their two types?
Interneurons : connect neurons within the CNS.
* Projection interneurons have long axons;
* Local interneurons have short axons for local circuits.
Distinguish between the Central (CNS) and Peripheral Nerve System (PNS).
CNS: Brain and spinal cord (integration center). PNS: Nerves and ganglia outside the CNS (communication lines).
Define an ‘Excitable cell’ and the role of a ‘Semipermeable membrane’.
Excitable cells (neurons/muscles) can generate electrical signals.
The semipermeable membrane regulates ion flow to maintain potential.
Explain Donnan Equilibrium in neurons.
The equilibrium reached when non-diffusible intracellular anions (proteins) influence the distribution of diffusible ions (K+ Cl-).
Donnan equilibrium refers to the specific distribution of ion species between two ionic solutions that are separated by a semipermeable membrane. In the context of a neuron, this equilibrium is established because the cell membrane allows some ions to pass through while acting as a barrier to others.
Key features of Donnan Equilibrium in neurons include:
- Non-diffusible Ions: The primary driver of this equilibrium is the presence of negatively charged protein molecules (A-) located inside the neuron. These large molecules are unable to cross the semipermeable membrane.
- Unequal Ion Distribution: Because these internal proteins are trapped, the boundary layer maintains an unequal distribution of ionic solute concentrations. The membrane acts as a selective barrier, preventing all ion types from reaching equal concentrations on both sides.
- Governing Forces: The movement and eventual equilibrium of these ions are determined by two main factors: the concentration (chemical) gradient and the electrical gradient. Ions naturally tend to move toward areas of lower concentration and toward areas of opposite charge.
- Interdependence of Ions: If the movement of one ion species is blocked (such as the internal proteins), it prevents other types of diffusible ions from being evenly distributed as well.
- Contribution to Resting Potential: When the electrical and chemical forces for all permeable ions balance out, the resulting voltage difference across the membrane is known as the resting potential. In an unstimulated neuron, this potential is typically around –70 mV, meaning the inside is negative relative to the outside.
While Donnan equilibrium describes a passive distribution based on membrane permeability, the neuron also maintains a steady state through active mechanisms like the $Na^+$-$K^+$ pump, which uses energy to further regulate ion concentrations against their gradients.
What is the ‘Steady State’ in a resting neuron?
A stable condition where ion concentrations are maintained by active transport (Na+/K+ pump) despite constant leakage.
Define Equilibrium Potential and Resting Membrane Potential.
- Equilibrium Potential: The voltage where electrical and chemical gradients for a single ion balance.
- Resting Potential: voltage across th eplasma membrane of an excitable unstimulated cell (The overall charge (-70mV) of a cell at rest.)
List 4 types of gated ion channels.
1) Voltage-gated
2) Ligand-gated (ionotropic) : (Neurotransmitter = sensitive to hormone)
3) Phosphorylation-gated
4) Stretch-gated (mechanical).
5) Gap junction channel
What is the function of the ‘Ion Selectivity Filter’ and ‘Voltage Sensor’?
- Selectivity filter: Allows only specific ions (e.g. K+) to pass.
- Voltage sensor: Detects membrane potential changes to open/close the gate.
Describe the Activation and Inactivation gates of Na+ channels.
Activation gate: Opens at threshold to allow Na+ influx. Inactivation gate: Closes automatically after a millisecond to stop Na+ flow.
What is the stoichiometry of the Na+/K+ pump?
It uses ATP to pump 3 Na+ ions out of the cell and 2 K+ ions into the cell (Electrogenic).
The stoichiometry of the Na+/K+ pump (also known as the Na+/K+-ATPase) is the transport of three sodium ions ($Na^+$) out of the cell in exchange for two potassium ions ($K^+$) transported into the cell.
Key details of this stoichiometric process include:
- Ion Direction: The pump moves three $Na^+$ ions from the intracellular space to the extracellular fluid and puts two $K^+$ ions into the cytoplasm.
- Energy Consumption: This active transport process requires energy, which is provided by the hydrolysis of one ATP molecule per cycle.
- Electrogenic Nature: Because three positive charges leave the cell for every two that enter, the pump is electrogenic and contributes to maintaining the inside-negative resting potential of the cell.
- Biological Significance: This pump is found in virtually every cell and is essential for regulating cellular volume and maintaining the ionic gradients necessary for neural communication. In the brain, this pump can account for approximately 30% of the cell’s total energy consumption.
Explain the role of Na+/Ca2+ channels in neurons.
They regulate intracellular calcium levels which is crucial for neurotransmitter release and signaling.
Define Threshold Potential and the All-or-none Principle.
Threshold: The minimum depolarization needed to trigger an action potential. All-or-none: The signal is either full strength or not at all.
Contrast Depolarization; Repolarization; and Hyperpolarization.
Depolarization: Membrane potential becomes less negative (Na+ in). Repolarization: Returns to resting potential (K+ out). Hyperpolarization: Becomes more negative than resting.
What are Myelinated axons and the Nodes of Ranvier?
Myelinated axons are wrapped in insulating fat (myelin). Nodes of Ranvier are the uninsulated gaps where ion channels are concentrated.
Define Saltatory Conduction.
The rapid ‘jumping’ of the action potential from one Node of Ranvier to the next in myelinated axons.
- Speed incresing
- Protection from damage
Contrast Chemical Synapse and Electrical Synapse.
Chemical: Uses neurotransmitters across a cleft (slow/plastic). Electrical: Uses Gap Junctions for direct ion flow (fast/bidirectional).
What is an EPSP and an IPSP?
EPSP (Excitatory): Depolarizing local potential (often Na+). IPSP (Inhibitory): Hyperpolarizing local potential (often Cl- or K+).
Explain Spatio-temporal Summation.
Spatial: Summing signals from different locations. Temporal: Summing signals arriving at high frequency from one location.
Name the 5 classes of Neurotransmitters.
Give some examples
1) Biogen amines: Histamin, Serotonin, Dopamin, Norepinephrine, Adrenalin (define monoamines and catecholamine)
2)Amino Acids : Glutamate, Aspartate, glicine, GABA
3)Peptide: Substance P, enkephalins, endorphins, angiotensin, somatostatin, vasopressin
4)Acetylcholin: Mediate NS ⇨muscle
5)Other transmitters : NO, CO, ATP, H+
What are Catecholamines? List three examples.
A subgroup of monoamines derived from tyrosine: Dopamine; Norepinephrine; and Epinephrine.
Define GABA and its typical effect.
Gamma-aminobutyric acid; the primary inhibitory neurotransmitter in the brain (causes IPSP).
Which neurotransmitter is the largest amount? Which is the second largest?
- Glutamate
- Acetylcholine (→fist discovered)
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