Big picture
Touching an unexpectedly hot utensil makes the hand withdraw before a detailed conscious decision is formed. Neural coordination provides rapid communication among receptors, processing centres and effectors. Electrical signals travel along neurons, while communication between cells often uses chemical messengers.
The nervous system does more than produce reflexes: it integrates sensory information, controls muscles, supports memory and coordinates internal organs. Trace information as receptor → neuron → integration → effector, then ask where the signal changes form. This approach connects microscopic membrane events with whole-body behaviour.
Learning goals
Describe neuron structure and nervous-system divisions.
Explain resting potential, action potential and synaptic transmission.
Identify major brain regions and protective structures.
Trace a reflex arc and distinguish sensory from motor pathways.
Explain basic eye and ear structure and function.
Core ideas
1. Neurons and supporting cells
A neuron has a cell body containing nucleus and Nissl substance, dendrites receiving input and an axon conducting signals away from the cell body toward terminals. Axonal terminals contain synaptic vesicles. Nissl substance is absent from the axon. Neuroglia support, protect and maintain the environment of neurons; nervous tissue is not made only of signal-carrying cells.
Neurons may be multipolar, bipolar or unipolar according to their processes. Motor neurons are commonly multipolar; bipolar neurons occur in specialised sensory pathways such as the retina. A myelin sheath insulates many axons. Schwann cells form myelin in peripheral nerves, while oligodendrocytes do so in the central nervous system. Gaps called nodes of Ranvier allow impulses to be regenerated at intervals. In a diagram, distinguish the axon membrane from the myelin surrounding it.
2. Divisions of the nervous system
The central nervous system, CNS, consists of brain and spinal cord. The peripheral nervous system, PNS, consists of nerves connecting the CNS to the body. Afferent pathways bring sensory information inward; efferent pathways carry commands outward. Human cranial nerves occur in twelve pairs and spinal nerves in thirty-one pairs.
The motor division includes somatic pathways supplying skeletal muscles and autonomic pathways regulating smooth muscle, cardiac muscle and glands. Autonomic sympathetic activity commonly prepares for increased activity, while parasympathetic activity commonly supports restoration and digestion. For example, sympathetic stimulation tends to increase heart rate, while parasympathetic stimulation tends to reduce it. These divisions coordinate rather than acting as universally opposed switches for every organ.
3. Resting membrane potential
A resting neuron maintains different ion concentrations across its membrane. Sodium concentration is higher outside, potassium higher inside, and large negatively charged intracellular molecules contribute to charge distribution. Selective membrane permeability, especially potassium leak, helps make the interior electrically negative relative to the outside.
The sodium–potassium pump uses ATP to move three sodium ions out and two potassium ions in per cycle, maintaining concentration gradients. The pump supports the conditions for excitability; it is not the immediate mechanism carrying each action potential down the axon. A typical resting potential is around −70 mV, although values vary between cells. Resting does not mean metabolically inactive: maintaining the gradients requires energy.
4. Action potentials and conduction
A sufficient stimulus reaching threshold opens voltage-gated sodium channels. Sodium enters, causing depolarisation and a brief reversal of membrane polarity. Sodium channels then inactivate and potassium channels permit potassium efflux, producing repolarisation. A brief undershoot can occur before the resting state is re-established. The refractory period limits immediate re-excitation and helps maintain effective one-way propagation along an activated pathway.
Local currents depolarise the adjacent axonal membrane, regenerating the action potential without progressive reduction in amplitude. In myelinated axons, regeneration occurs chiefly at nodes, called saltatory conduction, which is faster than continuous conduction along a comparable unmyelinated axon. Stronger stimuli usually increase impulse frequency or recruit more neurons; they do not indefinitely enlarge the amplitude of each action potential. This is the all-or-none principle for a stimulated excitable membrane.
5. Synaptic transmission
At a chemical synapse, the presynaptic terminal is separated from the postsynaptic membrane by a synaptic cleft. An arriving impulse opens calcium channels, calcium entry triggers vesicle fusion and a neurotransmitter is released. It diffuses across the cleft and binds postsynaptic receptors, changing ion conductance and the likelihood of a new impulse.
Transmission is usually one-way because transmitter release machinery and receptors occupy different sides. The postsynaptic effect can be excitatory or inhibitory. Acetylcholine at a neuromuscular junction provides an excitatory example. Electrical synapses allow direct ionic communication through cell connections and can transmit very rapidly. Do not claim that every synapse necessarily releases acetylcholine: different circuits use different transmitters.
6. Brain protection and forebrain
The brain is protected by the skull, three meninges and cerebrospinal fluid. From outside inward the meninges are dura mater, arachnoid and pia mater. Fluid cushions neural tissue and contributes a stable environment. A sagittal diagram should distinguish the brain's regions from its surrounding protective coverings.
The forebrain includes cerebrum, thalamus and hypothalamus. The cerebral hemispheres have an outer grey-matter cortex and deeper white matter. The corpus callosum connects the hemispheres. Cerebral regions support sensation, voluntary movement, language and higher functions, with association areas integrating information. The thalamus is an important sensory relay; the hypothalamus regulates temperature, hunger, thirst and endocrine coordination. Limbic structures participate in emotion, motivation and memory. Avoid assigning every complex behaviour to one isolated spot.
7. Midbrain, hindbrain and reflexes
The midbrain contains pathways and centres involved in visual and auditory reflexes. The hindbrain includes pons, cerebellum and medulla oblongata. The cerebellum coordinates precision, balance and posture; damage can impair coordination without directly causing loss of every muscle's ability to contract. The medulla contains centres important for respiration and cardiovascular regulation. The pons participates in communication and respiratory regulation.
A reflex arc includes receptor, sensory neuron, integration centre, motor neuron and effector. In withdrawal from heat, skin receptors detect a stimulus, sensory input enters the spinal cord, interneuronal circuits activate motor output and muscles withdraw the limb. Information also reaches the brain, producing awareness. A reflex can be rapid and involuntary while still involving CNS processing. Not all reflexes use the spinal cord; some are integrated in brainstem circuits.
8. Eye and vision
The eyeball has an outer sclera continuous with transparent cornea, a vascular choroid and an inner retina. The iris regulates pupil size. The lens focuses light, while ciliary muscles change its shape for accommodation. Aqueous humour lies anteriorly and vitreous humour behind the lens. Light passes through cornea, pupil and lens before reaching retinal photoreceptors.
Rods are sensitive in dim light and do not provide colour discrimination; cones support colour and high-acuity vision in brighter light. The fovea has densely packed cones and sharp vision. The optic disc is the blind spot because photoreceptors are absent where the optic nerve exits. Light causes changes in photopigments that alter photoreceptor electrical signalling, ultimately producing impulses in retinal ganglion-cell axons. Sensory transduction converts a physical stimulus into a neural signal; light does not travel down the optic nerve as a beam.
9. Ear, hearing and balance
The external ear collects sound into the auditory canal toward the tympanic membrane. Middle-ear ossicles—malleus, incus and stapes—transmit vibration to the oval window. The Eustachian tube helps equalise pressure across the eardrum. The inner ear contains the cochlea for hearing and vestibular structures for balance.
Within the cochlea, movement of fluid produces displacement of the basilar membrane. Hair cells of the organ of Corti bend relative to associated structures, converting mechanical displacement into electrical signalling. The auditory nerve carries the resulting information centrally. Semicircular canals detect angular acceleration, while utricle and saccule detect linear acceleration and head position relative to gravity. A labelled diagram should separate cochlea from semicircular canals rather than describing both merely as sound detectors. Eye and ear details are included as sensory-organ extension coverage where prescribed.
A reliable approach to questions
Trace the complete information route before naming a response.
For membrane events, state the ion, movement direction and electrical consequence.
Separate conduction within a neuron from transmission across a synapse.
Match brain regions with their characteristic functions without overgeneralising.
Explain sensory organs through stimulus, receptor and transmitted signal.
Connections that matter
Membrane transport supplies the gradients required for electrical excitability.
Muscle physiology explains how motor output produces contraction.
Endocrine coordination links hypothalamic neural processing with hormonal regulation.
Common mistakes
Reversing afferent and efferent confuses incoming sensory and outgoing motor signals.
Making the sodium pump the immediate action-potential current ignores voltage-gated channels.
Calling every reflex spinal ignores brainstem reflexes.
Treating rods as colour receptors confuses them with cones.
Revision checklist
I can label a neuron and explain myelin.
I can sequence action-potential events correctly.
I can explain chemical synaptic transmission.
I can compare major brain regions and trace a reflex.
I can connect eye and ear structures to sensory functions.