A neuron consists of a cell body (soma) containing the nucleus, multiple short, branching dendrites that receive incoming signals, and a single, typically long axon that transmits signals away from the cell body towards other neurons or effector organs.
Many axons are covered by a myelin sheath — a fatty, insulating layer formed by Schwann cells (in the peripheral nervous system), with small gaps called nodes of Ranvier between successive Schwann cells. This sheath significantly increases the speed of nerve impulse conduction.
Explain why myelinated neurons conduct nerve impulses considerably faster than unmyelinated neurons.
In a myelinated neuron, the myelin sheath acts as an electrical insulator along most of the axon's length, preventing continuous, uniform impulse propagation along the entire membrane. Instead, the nerve impulse effectively "jumps" from one node of Ranvier directly to the next (a process called saltatory conduction), skipping over the insulated segments entirely. This jumping mechanism is considerably faster than the continuous, node-by-node propagation required along an unmyelinated axon, where the impulse must regenerate at every single point along the membrane rather than skipping ahead — explaining the significant speed advantage of myelinated neurons.
In an inactive (resting) neuron, the inside of the membrane is maintained at a negative potential relative to the outside (typically around $-70\ \text{mV}$) — maintained primarily by the sodium-potassium pump, which actively transports 3 Na⁺ ions out for every 2 K⁺ ions brought in, along with the differential permeability of the membrane to these ions.
When a stimulus is strong enough to reach the threshold, voltage-gated sodium channels open rapidly, causing a sudden influx of Na⁺ ions and a rapid depolarisation (the inside becomes positive relative to outside) — this is followed by the opening of voltage-gated potassium channels, causing K⁺ to flow out and repolarise the membrane back towards its resting state.
Explain why the nerve impulse (action potential), once initiated, always travels in one direction along the axon, rather than spreading equally in both directions.
Immediately after an action potential passes through a particular segment of the axon membrane, that segment enters a brief refractory period, during which its sodium channels are temporarily inactivated and cannot be triggered again right away. Since the region just "behind" the currently active point on the axon (the direction the impulse just came from) is still in this refractory state, the impulse cannot travel backward into that already-activated region — it can only propagate forward into the next, still-unstimulated segment of the axon, which is precisely why nerve impulses reliably travel in one consistent direction along the axon.
A synapse is the junction between two neurons (or between a neuron and an effector cell), where signals are transmitted from one cell to the next. Most synapses in the nervous system are chemical synapses, using neurotransmitter molecules to relay the signal across a small gap called the synaptic cleft.
When an action potential reaches the axon terminal of the presynaptic neuron, it triggers the release of neurotransmitter molecules (stored in synaptic vesicles) into the synaptic cleft. These neurotransmitters diffuse across the gap and bind to specific receptors on the postsynaptic neuron's membrane, triggering a new electrical signal in that neuron.
Explain why chemical synapses only allow signal transmission in one direction, from the presynaptic to the postsynaptic neuron.
Neurotransmitter-containing synaptic vesicles are located specifically within the presynaptic neuron's axon terminal, and the corresponding neurotransmitter receptors are located specifically on the postsynaptic neuron's membrane, on the receiving side of the synaptic cleft. Since only the presynaptic side has the machinery to release neurotransmitter, and only the postsynaptic side has the receptors to receive and respond to it, signal transmission across a chemical synapse can only proceed in this one, fixed direction — the necessary molecular components for transmission simply aren't present in the reverse arrangement.
Not all synapses promote further signalling — excitatory synapses make the postsynaptic neuron more likely to fire an action potential (typically via neurotransmitters like acetylcholine or glutamate), while inhibitory synapses make it less likely to fire (typically via neurotransmitters like GABA). A neuron's overall response depends on the combined, summed effect of all the excitatory and inhibitory signals it receives simultaneously.
Explain why a single neuron receiving both excitatory and inhibitory synaptic inputs simultaneously does not simply respond to whichever signal arrives first.
A neuron's decision to fire an action potential depends on the overall, summed effect of all synaptic inputs it receives across its many synapses, integrated together at any given moment, rather than simply reacting to a single isolated input in the order it arrives. If the combined excitatory inputs are strong enough to push the membrane potential past the firing threshold, despite any simultaneous inhibitory input pulling in the opposite direction, the neuron will fire; if the inhibitory inputs are strong enough to counteract the excitatory ones, the neuron will not fire, regardless of arrival order — this integrative summing process allows neurons to process complex, weighted combinations of many different simultaneous signals rather than simply responding to the first one received.
A reflex action is a rapid, involuntary, automatic response to a specific stimulus, occurring without direct, conscious involvement of the brain — instead processed primarily through the spinal cord, allowing for a much faster response than would be possible if the signal had to travel all the way to the brain and back.
The pathway of a reflex action, called the reflex arc, typically follows this sequence: receptor (detects stimulus) → sensory (afferent) neuron (carries signal to spinal cord) → interneuron (in spinal cord, processes and relays signal) → motor (efferent) neuron (carries signal to effector) → effector (muscle or gland that produces the response).
Explain why withdrawing your hand quickly from a hot object is classified as a reflex action, and why this response is faster than if it required conscious brain processing.
This response follows the classic reflex arc pathway — heat/pain receptors in the skin detect the dangerous stimulus, sensory neurons carry this signal directly to the spinal cord, where an interneuron processes and relays the signal directly to motor neurons, which immediately trigger the arm/hand muscles to contract and withdraw the hand — all without the signal needing to first travel all the way up to the brain for conscious processing and decision-making. Since the spinal cord is physically much closer to the site of the stimulus than the brain, and processing occurs directly there without waiting for conscious deliberation, this pathway allows for a significantly faster protective response than if the signal had to travel further to the brain and back before triggering the muscle response.
The human brain is broadly divided into the forebrain, midbrain, and hindbrain. The forebrain, the largest and most developed region in humans, includes the cerebrum, thalamus, and hypothalamus.
The cerebrum, divided into left and right cerebral hemispheres connected by the corpus callosum, is responsible for higher cognitive functions — including voluntary movement, sensory perception, reasoning, memory, language, and conscious thought. Its outer layer, the cerebral cortex, is highly folded (into gyri and sulci), significantly increasing its surface area.
The extensive folding of the cerebral cortex dramatically increases the total surface area of grey matter (where most neuronal cell bodies, and hence most information processing, occurs) that can fit within the limited space of the skull — allowing for a much greater capacity for complex information processing than a smooth, unfolded brain surface of the same skull volume could support.
Explain the functional significance of the extensive folding (gyri and sulci) of the human cerebral cortex.
The cerebral cortex is the primary site of complex information processing — more surface area of cortical tissue generally corresponds to greater processing capacity. However, the skull provides only a limited, fixed volume of space to accommodate the brain. By folding extensively into ridges (gyri) and grooves (sulci), the cerebral cortex packs a much larger total surface area into this limited cranial volume than would be possible with a smooth, unfolded surface — this significant increase in available cortical surface area is believed to be closely linked to humans' advanced cognitive capabilities, compared to species with smoother, less-folded brains.
A relatively small region connecting the forebrain and hindbrain, involved in visual and auditory reflexes, and relaying certain sensory and motor signals between different brain regions.
Part of the hindbrain, the cerebellum is primarily responsible for coordinating voluntary movements, maintaining posture and balance, and fine-tuning motor precision — it does not initiate movements itself, but refines and coordinates movements initiated by the cerebrum, ensuring they are smooth, accurate, and well-balanced.
Also part of the hindbrain, the medulla oblongata controls essential, largely involuntary autonomic functions — including heart rate, breathing rate, blood pressure, and reflexes like swallowing, coughing, and vomiting.
Explain why damage to the cerebellum typically results in impaired coordination and balance, rather than complete paralysis or loss of voluntary movement altogether.
The cerebellum's primary role is to coordinate, fine-tune, and refine movements that are actually initiated and controlled by the cerebrum (specifically the motor cortex) — it doesn't generate the fundamental commands for voluntary movement itself. Because of this specific coordinating (rather than initiating) role, damage to the cerebellum typically results in movements that are still possible but become uncoordinated, jerky, imprecise, or poorly balanced (a condition known as ataxia) — rather than complete paralysis, which would instead result from damage to the motor cortex or the descending motor pathways that carry the actual movement-initiating signals from the cerebrum.
The spinal cord extends from the base of the brain (medulla oblongata) down through the vertebral column, protected by the surrounding bony vertebrae and cushioning cerebrospinal fluid. In cross-section, it shows a distinctive butterfly-shaped region of grey matter (neuron cell bodies) surrounded by white matter (myelinated axon tracts).
The spinal cord serves two primary functions: (1) acting as the main pathway for relaying sensory information up to the brain, and motor commands down from the brain to the body, and (2) independently processing and coordinating many reflex actions without requiring direct involvement of the brain.
Explain why an injury to the spinal cord can result in loss of both sensation and voluntary movement below the site of injury, even if the brain itself remains completely undamaged.
The spinal cord serves as the essential physical communication pathway connecting the brain to the rest of the body below the neck — sensory information from the body must travel up through the spinal cord to reach the brain, and motor commands from the brain must travel back down through the spinal cord to reach the muscles. If the spinal cord is severed or severely damaged at a particular point, this critical communication pathway is disrupted at that location — even though the brain itself may be completely intact and functional, its commands can no longer reach the muscles below the injury site, and sensory information from below that point can no longer reach the brain, resulting in loss of both voluntary movement and sensation in the affected regions.
Light enters through the transparent cornea, passes through the adjustable pupil (whose size is controlled by the surrounding iris), is further focused by the lens, and finally forms an image on the retina at the back of the eye, which contains the light-sensitive photoreceptor cells.
The retina contains two types of photoreceptor cells: rods, which are highly sensitive to light (enabling vision in dim conditions) but cannot distinguish colour, and cones, which require brighter light but enable detailed, colour vision. The retina also contains the fovea (a region densely packed with cones, providing the sharpest, most detailed vision) and the blind spot (where the optic nerve exits, containing no photoreceptors at all).
Explain why human vision becomes poor at distinguishing colours in very dim light (like at night), even though basic vision is still possible.
Cone cells, responsible for detailed colour vision, require relatively bright light to function effectively and are not sensitive enough to respond reliably in very dim conditions. Rod cells, by contrast, are much more sensitive to low light levels and remain functional even in dim conditions — allowing basic vision to continue — but rods cannot distinguish between different colours (only varying levels of brightness/darkness). This is why, in very dim lighting, human vision continues to function via the more light-sensitive rods, but appears to lose most of its colour discrimination, since the colour-sensitive cones simply aren't receiving enough light to function properly.
The eye's lens can change its shape (curvature) through the action of the ciliary muscles, a process called accommodation — becoming more rounded/curved to focus on nearby objects, and flatter to focus on distant objects, allowing the eye to maintain sharp focus across a wide range of viewing distances.
Explain why the ciliary muscles need to contract when focusing on a nearby object, compared to a distant one.
Focusing light from a nearby object onto the retina requires the lens to bend light more strongly, which requires the lens to become more rounded and curved. Contraction of the ciliary muscles relaxes the tension on the suspensory ligaments holding the lens in place, allowing the lens's own natural elasticity to make it become more rounded — this increased curvature provides the additional focusing power needed for near vision. For distant objects, the ciliary muscles relax, increasing tension on the suspensory ligaments, which flattens the lens and reduces its focusing power appropriately for distant vision.
The outer ear (pinna and ear canal) collects and channels sound waves towards the tympanic membrane (eardrum). The middle ear contains three small bones (the ossicles: malleus, incus, stapes) that mechanically amplify and transmit these vibrations to the inner ear.
The cochlea, a spiral-shaped, fluid-filled structure in the inner ear, contains specialised hair cells that convert mechanical vibrations into electrical nerve signals, which are then transmitted to the brain via the auditory nerve for interpretation as sound.
Also located in the inner ear, the vestibular apparatus (including the semicircular canals) is responsible for detecting changes in head position and movement, playing a crucial role in maintaining balance and spatial orientation.
Explain the specific role of the three small ossicle bones in the middle ear, in terms of sound transmission.
Sound waves arriving at the eardrum cause it to vibrate, but these vibrations, if transmitted directly into the fluid-filled inner ear without amplification, would be significantly weakened due to the greater resistance of fluid compared to air. The three ossicle bones (malleus, incus, and stapes) form a small mechanical lever system that amplifies the force of these vibrations as they pass from the eardrum to the inner ear's oval window — this amplification compensates for the loss of vibrational energy that would otherwise occur when transmitting sound from the air-filled outer/middle ear into the fluid-filled inner ear, ensuring sound signals remain strong enough to be effectively detected by the cochlea's hair cells.