writing prompts - exam two

1. Diffusion of Ions and All-or-None Action Potentials

An action potential happens when ions move across the neuron’s membrane, changing its charge. At rest, the neuron has a negative charge inside (-70 mV) due to the sodium-potassium pump maintaining more Na+ outside and more K+ inside. When a stimulus reaches the threshold (-55 mV), voltage-gated sodium (Na+) channels open, allowing Na+ to rush in, making the inside more positive (+30 mV). This depolarization triggers potassium (K+) channels to open, allowing K+ to exit, restoring the negative charge (repolarization). If the initial stimulus is strong enough to reach the threshold, the action potential occurs completely; if not, it does not happen at all—this is the all-or-none principle.

2. Graded Potentials and Their Influence on Action Potentials

Graded potentials are small, local changes in membrane potential caused by ligand-gated or mechanically-gated ion channels. These can be excitatory (making the inside more positive) or inhibitory (making it more negative). The strength of graded potentials depends on the stimulus and decreases as it moves along the neuron. If multiple excitatory graded potentials add up (summation) and reach the threshold (-55 mV), an action potential starts. However, if inhibitory signals outweigh excitatory ones, the neuron stays below threshold, preventing an action potential.

3. Saltatory Conduction and Action Potential Propagation

Saltatory conduction speeds up action potentials along myelinated axons. Myelin, made by Schwann cells, insulates the axon and prevents ion leakage. Action potentials only happen at small unmyelinated gaps called nodes of Ranvier, where voltage-gated sodium channels are located. The signal "jumps" from node to node instead of moving continuously, allowing faster transmission. This is essential for quick reflexes and efficient nerve signaling.

4. Signal Transmission at a Synapse

When an action potential reaches the end of a neuron (axon terminal), voltage-gated calcium (Ca²⁺) channels open, letting Ca²⁺ enter. This causes vesicles to release neurotransmitters into the synaptic cleft. The neurotransmitters bind to receptors on the next neuron, triggering an electrical or chemical response. The signal stops when neurotransmitters are either reabsorbed by the neuron (reuptake), broken down by enzymes, or diffuse away from the synapse.

5. Ligand-Gated Receptors vs. G-Protein Coupled Receptors (GPCRs)

Ligand-gated receptors work fast. When a ligand (such as a neurotransmitter) binds, the receptor directly opens an ion channel, causing immediate changes in the cell.

G-protein coupled receptors (GPCRs) work slower but create longer-lasting effects. When a ligand binds, it activates a G-protein, which then triggers an enzyme (adenylate cyclase) to produce cAMP. cAMP acts as a second messenger, starting a chain reaction that affects cellular functions, like opening ion channels or altering gene expression.

6. Endocrine and Sympathetic Nervous System Influence on Heart Rate

The sympathetic nervous system increases heart rate using norepinephrine, which binds to beta-adrenergic receptors on heart cells, making them contract faster and stronger. This happens quickly but fades fast when stimulation stops.

The endocrine system uses hormones like epinephrine (adrenaline), which travels through the bloodstream and binds to the same beta-adrenergic receptors. This effect is slower to start but lasts longer than neural stimulation. Together, these systems ensure the heart responds appropriately to different situations, like stress or exercise.

7. Photoreceptor Response to Light and Darkness

In darkness, photoreceptors (rods and cones) are active, allowing Na⁺ and Ca²⁺ to flow in, which keeps the cell depolarized and continuously releasing glutamate, an inhibitory neurotransmitter that prevents signal transmission.

In light, a chemical reaction inside the photoreceptor closes Na⁺ and Ca²⁺ channels, causing hyperpolarization (the cell becomes more negative). This stops glutamate release, allowing bipolar cells to activate ganglion cells, which send visual signals to the brain through the optic nerve. This process helps the brain detect changes in light and process vision.

8. Sound Perception by Hair Cells

1. Sound waves enter the ear and cause the tympanic membrane (eardrum) to vibrate. These vibrations move through the ossicles (tiny bones in the middle ear: malleus, incus, and stapes).

2. The vibrations reach the cochlea, where they create waves in the basilar membrane. Different parts of this membrane respond to different sound frequencies.

3. The movement bends hair cells in the cochlea, opening mechanically-gated ion channels. This causes depolarization, leading to the release of neurotransmitters.

4. The signal is sent through the auditory nerve to the brain, where it is interpreted as sound.