BIO NMDCAT

Multiple sclerosis is an autoimmune disorder characterized by the progressive demyelination of axons in the CNS, which disrupts saltatory conduction and leads to severe neurological deficits.

Temporal summation occurs when a single presynaptic neuron fires multiple times in rapid succession, allowing the resulting graded potentials to add together before the previous ones dissipate.

GABA is the major inhibitory neurotransmitter in the brain, functioning primarily by opening chloride channels on postsynaptic neurons to induce hyperpolarization. Glutamate is the major excitatory neurotransmitter.

Contralateral reflex arcs (e.g., the crossed extensor reflex) involve interneurons that cross the spinal cord midline to produce a motor response on the side of the body opposite to the stimulus.

Reciprocal inhibition ensures coordinated movement by using inhibitory interneurons to relax antagonistic muscles while excitatory pathways stimulate the agonist muscles.

At rest, sodium leak channels are very rare compared to abundant potassium leak channels, making the resting membrane highly impermeable to Na+ and highly permeable to K+.

The arrival of an action potential at the synaptic knob opens voltage-gated Ca²⁺ channels. The resulting Ca²⁺ influx causes synaptic vesicles to fuse with the presynaptic membrane and release their contents.

Acetylcholine (ACh) is the primary neurotransmitter of the parasympathetic nervous system (both pre- and postganglionic), while the sympathetic system predominantly relies on norepinephrine for postganglionic signaling.

Neurotransmitters bind to specific ligand-gated receptors on the postsynaptic membrane, which undergo a conformational change to open ion channels and generate graded potentials.

An EPSP involves the opening of cation channels (usually Na⁺), leading to a localized, graded depolarization that makes the postsynaptic neuron more likely to fire an action potential.

Opening Cl⁻ channels (allowing Cl⁻ influx) or K⁺ channels (allowing K⁺ efflux) leads to hyperpolarization, moving the membrane potential further away from the threshold and inhibiting the neuron.

Spatial summation is the additive effect of graded potentials occurring at different physical locations (synapses) on the neuron at the same time. Temporal summation occurs at a single synapse over time.

Acetylcholinesterase (AChE) is the enzyme located in the synaptic cleft that rapidly hydrolyzes ACh into acetate and choline, terminating the synaptic signal to prevent continuous stimulation.

A nerve fiber consists of an axon (axis cylinder) along with its surrounding myelin sheath and neurilemma (Schwann cell sheath). The term does not typically include the cell body or dendrites.

Once the threshold voltage is reached, the action potential occurs with a constant, maximum amplitude independent of stimulus strength. Stronger stimuli increase the frequency of impulses, not their amplitude.

The gate control theory suggests that non-painful tactile stimuli (rubbing) carried by fast, myelinated A-beta fibers activate inhibitory interneurons in the spinal cord, "closing the gate" to pain signals carried by slower C fibers.

Calcium influx through voltage-gated calcium channels is absolutely essential for the exocytosis of neurotransmitter vesicles. Low extracellular calcium directly impairs this release mechanism.

Visual information from the retinas travels via the optic nerve, optic chiasm, and optic tracts to the lateral geniculate nucleus of the thalamus, and finally projects to the primary visual cortex in the occipital lobe.

The sensory homunculus is a distorted topographical map of the body in the postcentral gyrus. Areas with high receptor density (lips, fingers) are represented by much larger areas of the cortex than regions with low density (back).