Neural Control and Coordination

The resting and action potential: an unequal ion trade

A neuron at rest maintains a resting membrane potential of roughly -70 millivolts, the inside of the cell held negative relative to the outside, and this voltage difference is actively maintained by the sodium-potassium pump, which continuously trades ions across the membrane in a deliberately unequal ratio: for every cycle, it pumps 3 sodium ions (Na+) out of the cell while bringing only 2 potassium ions (K+) in, a net outward movement of positive charge that keeps the cell interior negative. When a stimulus arrives, the membrane's permeability to sodium suddenly increases, and sodium rushes back into the cell, rapidly reversing the membrane potential from negative to positive, a process called depolarisation; this rapid, temporary voltage spike is the action potential, the electrical signal that then propagates along the length of the neuron's axon.

The synapse: converting an electrical signal into a chemical one

Where one neuron meets the next, at a synapse, the signal has to cross a physical gap (the synaptic cleft) that an electrical impulse cannot simply jump across, so the signal is converted into a chemical one. The arriving action potential triggers the release of a neurotransmitter into the synaptic cleft, which diffuses across and binds to specific receptors on the next (postsynaptic) neuron's membrane, either exciting or inhibiting it depending on the specific neurotransmitter and receptor involved, before the signal reconverts back into an electrical impulse in that next neuron if the stimulus is strong enough.

The brain: three regions, three broad roles

The brain is organised into three major divisions, each with a broadly distinct functional role. The forebrain, dominated by the cerebrum (the largest part of the human brain, responsible for higher cognitive functions, sensory perception and voluntary motor control) along with structures like the thalamus and hypothalamus, handles the most complex processing. The midbrain functions chiefly as a relay and reflex centre, notably for visual and auditory reflexes. The hindbrain comprises the cerebellum (responsible for fine motor coordination and balance), the pons, and the medulla oblongata, which controls essential involuntary functions, including, as already covered in this site's own Breathing and Exchange of Gases note, the respiratory rhythm centre that regulates breathing.

The reflex arc: a shortcut that bypasses the brain entirely

A reflex arc is the specific neural pathway behind an involuntary, rapid, protective response (pulling a hand back from a hot surface, for instance), and its defining feature is that the response is generated at the level of the spinal cord, not the brain: a signal travels from a receptor, along a sensory (afferent) neuron, into the spinal cord, where it connects directly to a motor (efferent) neuron that immediately triggers the effector's (usually a muscle's) response, without needing to travel all the way up to the brain and back first. This shortcut is precisely what makes a reflex faster than a consciously willed action, even though the same sensory information is also relayed onward to the brain in parallel, which is why a person becomes consciously aware of having reacted only after the reflex response has already occurred.

Quick revision points

• Resting potential: about -70 mV, maintained by the sodium-potassium pump (3 Na+ out, 2 K+ in per cycle), keeping the cell interior negative.
• Action potential: a stimulus increases membrane sodium permeability, sodium rushes in, reversing the potential from negative to positive (depolarisation); this signal then propagates along the axon.
• Synapse: converts the electrical signal into a chemical one via neurotransmitter release across the synaptic cleft, which then either excites or inhibits the next neuron.
• Brain divisions: forebrain (cerebrum, higher cognition, sensory/motor control), midbrain (relay, visual/auditory reflexes), hindbrain (cerebellum for coordination/balance; medulla oblongata for involuntary functions including breathing regulation).
• Reflex arc: receptor to sensory neuron to spinal cord to motor neuron to effector, bypassing the brain entirely for speed, even though the same signal is relayed to the brain in parallel for conscious awareness afterward.