Neurons, Neurotransmission, and the Nervous System

Think of the neuron as a firearm rather than a dimmer switch. Pulling the trigger harder does not make the bullet faster or bigger; it fires at full power or not at all. The nervous system therefore cannot code a louder sound with a bigger spike, so it codes it with more spikes per second and more neurons recruited. Every all-or-none question is testing whether you know that intensity lives in the rate, not in the size.

For the two autonomic branches, ask what a body would need in that moment. Facing a threat, you need fuel and oxygen at the muscles: heart rate up, pupils wide, digestion suspended — sympathetic. Safe and full after a meal, you need the opposite: heart rate down, digestion running — parasympathetic. You can derive nearly every item on an autonomic question from that one question about need.

Neurotransmitters and hormones both carry messages, but the delivery differs: a neurotransmitter is hand-delivered across a synapse a few nanometers wide, while a hormone is mailed through the bloodstream to every address in the body. Hand delivery is fast and precise and brief; mail is slow, broad, and long-lasting. That difference alone answers most comparison items.

The neuron is the unit the exam builds from. An unfired neuron holds a resting potential of roughly -70 mV, more negative inside than outside. Incoming signals push that value up or down as graded changes, and if depolarization reaches the threshold of excitation near -55 mV, voltage-gated sodium channels open, sodium floods in, and the cell briefly reverses polarity. Potassium then leaves and the membrane repolarizes, with a refractory period during which the neuron cannot immediately fire again. Because the spike is all-or-none, a stronger stimulus cannot make it larger; it produces more frequent firing and recruits more neurons. Myelin insulates the axon and lets the impulse jump between the gaps in the sheath, which is why demyelination slows conduction dramatically.

Communication between neurons is chemical. The impulse arrives at the terminal buttons, vesicles release neurotransmitter into the synaptic cleft, and the molecules bind receptor sites on the postsynaptic membrane in a lock-and-key fit. What is left in the cleft is either taken back into the sending neuron by reuptake or broken down by enzymes. This is exactly where drugs act, and the exam expects the two labels: an agonist enhances a transmitter's effect — by mimicking it, by increasing its release, or by blocking reuptake so more stays in the cleft — while an antagonist reduces it, usually by occupying the receptor without activating it. Learn a short roster of transmitters with their headline associations: acetylcholine with muscle action and memory, dopamine with reward and movement, serotonin with mood and sleep, GABA as the main inhibitory transmitter, glutamate as the main excitatory one, and norepinephrine with arousal.

Above the cell sits the architecture. The central nervous system is the brain and spinal cord; everything else is peripheral. The peripheral system splits into the somatic branch, which carries sensory information in and voluntary commands out to skeletal muscle, and the autonomic branch, which runs the organs without conscious control. The autonomic branch splits again: the sympathetic branch produces fight-or-flight arousal (heart rate up, pupils dilated, digestion suspended), and the parasympathetic branch restores rest-and-digest. Reflex arcs are worth knowing as a case where the spinal cord acts before the brain is involved.

Finally, the endocrine system runs a parallel messaging service. Glands secrete hormones into the bloodstream, so signals travel more slowly, reach the whole body, and persist far longer than a synaptic event. The pituitary, directed by the hypothalamus, is the master gland that regulates other glands; the adrenal glands sit atop the kidneys and release epinephrine and cortisol under stress; the thyroid governs metabolism; the pancreas regulates blood glucose. The link back to behavior is the stress response, where the hypothalamus drives pituitary and adrenal activity and cortisol stays elevated long after the sympathetic surge has passed.