Difficulty: Intermediate to Advanced | Prerequisites: Cell structure (Study Notes 02), tissue types (Study Notes 03), neuromuscular junction basics (Study Notes 05).
The nervous system is the body's rapid communication network. It detects changes in the internal and external environment, processes that information, and coordinates responses, all within milliseconds. This section covers the cells of nervous tissue, the organisation of the central and peripheral nervous systems, the major structures and functions of the brain, the spinal cord, reflex arcs, and the sensory system (how we perceive the world). If the endocrine system is the body's postal service (slow, widespread, long-lasting), the nervous system is its electrical wiring (fast, targeted, brief).
Nervous tissue is made of neurons (signal conductors) and neuroglia (support cells). The CNS (brain and spinal cord) is the processing centre; the PNS (nerves and ganglia) relays signals to and from the body. The brain has four lobes with distinct functions, and 12 pairs of cranial nerves handle specialised tasks. The sensory system uses different receptor types to detect stimuli from the environment and the body itself.
Neuron
The functional cell of the nervous system. Generates and conducts electrical impulses (action potentials). Structurally composed of a cell body, dendrites, and an axon.
Neuroglia (glial cells)
Support cells of the nervous system. They do not conduct impulses but protect, insulate, and nourish neurons. Types include astrocytes, oligodendrocytes, microglia, and ependymal cells (CNS), and Schwann cells and satellite cells (PNS).
Cell body (soma)
The part of the neuron that contains the nucleus and most organelles. The metabolic centre of the cell.
Dendrites
Short, branching extensions of the neuron that receive signals from other neurons or from sensory receptors and carry them toward the cell body. Think of them as the neuron's antennae.
Axon
A single, long extension of the neuron that carries impulses away from the cell body toward other neurons, muscles, or glands. Think of it as the neuron's output cable.
Central nervous system (CNS)
The brain and spinal cord. The body's integration and command centre.
Peripheral nervous system (PNS)
All nerves and ganglia outside the CNS. Connects the CNS to the rest of the body. Divided into sensory (afferent) and motor (efferent) divisions.
Meninges
Three protective membranes surrounding the brain and spinal cord: dura mater (tough outer layer), arachnoid mater (middle, web-like layer), and pia mater (thin, innermost layer adhering to the brain surface).
Cerebrospinal fluid (CSF)
A clear fluid that fills the ventricles of the brain and the subarachnoid space. Cushions the CNS, delivers nutrients, and removes waste.
Reflex arc
The neural pathway that mediates a reflex action. Components: receptor (detects stimulus), sensory neuron (afferent, carries signal to CNS), integration centre (usually in spinal cord), motor neuron (efferent, carries response signal), and effector (muscle or gland that carries out the response). In simple terms, this is the shortcut circuit that lets you pull your hand off a hot surface before you consciously feel the pain.
Plexus
A network of intersecting nerves. Major plexuses include the cervical (C1-C5), brachial (C5-T1, serves the upper limb), lumbar (L1-L4), and sacral (L4-S4, serves the lower limb).
Cerebrum
The largest part of the brain. Responsible for higher-order functions: conscious thought, reasoning, memory, language, voluntary movement, and sensory perception. Divided into left and right hemispheres.
Cerebellum
Located at the posterior base of the brain. Coordinates voluntary movement, balance, posture, and motor learning. Does not initiate movement but fine-tunes it. In simple terms, the cerebellum is why you can walk without consciously thinking about each step.
Brainstem
Composed of the midbrain, pons, and medulla oblongata. Controls basic life functions: heart rate, breathing, blood pressure, sleep-wake cycles, and reflexes such as swallowing and coughing.
Frontal lobe
The anterior portion of the cerebrum. Functions include voluntary motor control (primary motor cortex), reasoning, planning, personality, speech production (Broca's area), and working memory.
Parietal lobe
Located posterior to the frontal lobe. Processes somatosensory information: touch, pressure, temperature, pain, and proprioception (primary somatosensory cortex).
Temporal lobe
Located on the lateral sides of the cerebrum. Involved in auditory processing, memory formation, language comprehension (Wernicke's area), and emotion.
Occipital lobe
Located at the posterior of the cerebrum. The primary visual processing centre (primary visual cortex).
Cranial nerves
Twelve pairs of nerves that arise directly from the brain (most from the brainstem). Each has specific sensory, motor, or mixed functions. Examples: olfactory (I, smell), optic (II, vision), vagus (X, parasympathetic control of heart, lungs, digestive tract).
Olfaction
The sense of smell. Chemoreceptors in the nasal cavity detect airborne chemical molecules.
Gustation
The sense of taste. Chemoreceptors on the tongue (taste buds) detect dissolved chemicals. Five basic tastes: sweet, salty, sour, bitter, umami.
Proprioception
The sense of body position and movement. Receptors in muscles, tendons, and joints provide continuous information about where the limbs are in space.
Equilibrium
The sense of balance. Maintained by the vestibular apparatus in the inner ear, along with visual and proprioceptive inputs.
Nociception
The detection of pain. Nociceptors respond to potentially damaging stimuli (thermal, mechanical, chemical).
Mechanoreceptors
Sensory receptors that respond to mechanical forces: touch, pressure, vibration, and stretch.
Thermoreceptors
Sensory receptors that detect changes in temperature.
Chemoreceptors
Sensory receptors that respond to chemical stimuli (e.g., taste, smell, blood oxygen levels).
Photoreceptors
Sensory receptors in the retina of the eye that respond to light. Rods (dim light, peripheral vision) and cones (colour, visual acuity).
Myopia
Nearsightedness. The eye focuses images in front of the retina rather than on it, making distant objects blurry. Corrected with concave lenses.
Presbyopia
Age-related farsightedness caused by loss of elasticity in the lens. Near objects become difficult to focus on. Corrected with convex lenses (reading glasses).
Ossicles
The three tiny bones of the middle ear: malleus (hammer), incus (anvil), and stapes (stirrup). They amplify and transmit sound vibrations from the tympanic membrane to the oval window of the inner ear.
Neurons have three functional parts:
Dendrites: receive incoming signals.
Cell body: integrates signals and maintains the cell.
Axon: transmits the action potential to the next neuron or to an effector (muscle/gland).
Neurons are classified by function:
Sensory (afferent): carry signals from receptors to the CNS.
Motor (efferent): carry signals from the CNS to effectors.
Interneurons: connect sensory and motor neurons within the CNS; responsible for integration.
Neuroglia outnumber neurons and perform vital support roles:
Astrocytes (CNS): regulate the blood-brain barrier, maintain ion balance, provide nutrients.
Oligodendrocytes (CNS): form myelin sheaths around axons in the CNS.
Schwann cells (PNS): form myelin sheaths around axons in the PNS.
Microglia (CNS): immune defence, phagocytose debris and pathogens.
Ependymal cells (CNS): line brain ventricles, help produce and circulate CSF.
Satellite cells (PNS): surround neuron cell bodies in ganglia, provide support.
CNS (brain and spinal cord): the processing and command centre. Protected by the skull and vertebral column, the meninges, and cerebrospinal fluid.
PNS (nerves and ganglia): the communication lines. The sensory (afferent) division carries information in; the motor (efferent) division carries commands out. The motor division is further divided into:
Somatic nervous system: voluntary control of skeletal muscles.
Autonomic nervous system: involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into sympathetic ("fight or flight") and parasympathetic ("rest and digest").
Meninges: three layers (dura mater, arachnoid mater, pia mater) encase the brain and spinal cord.
CSF: produced by the choroid plexus in the ventricles, circulates in the subarachnoid space. Provides buoyancy (the brain effectively "floats," reducing its functional weight), cushioning against impact, and a stable chemical environment.
A reflex arc is the simplest functional pathway: receptor, sensory neuron, integration centre, motor neuron, effector. Reflexes are rapid and involuntary.
The patellar (knee-jerk) reflex is a monosynaptic reflex (one synapse, no interneuron). Most reflexes are polysynaptic (involve interneurons in the spinal cord).
Plexuses (cervical, brachial, lumbar, sacral) are networks where spinal nerves merge and redistribute, ensuring that each limb receives fibres from multiple spinal segments (providing redundancy).
Cerebrum: largest region. Two hemispheres connected by the corpus callosum. The outer layer (cerebral cortex) is grey matter (neuron cell bodies); the inner mass is white matter (myelinated axons). Responsible for consciousness, thought, memory, sensory perception, and voluntary movement.
Cerebellum: coordinates and smooths voluntary movement, maintains balance and posture, involved in motor learning. Damage results in ataxia (uncoordinated movement), not paralysis.
Brainstem: midbrain (visual and auditory reflexes), pons (relay between cerebrum and cerebellum, breathing regulation), medulla oblongata (cardiac, respiratory, vasomotor centres, and reflexes like vomiting, coughing, sneezing).
Frontal lobe: motor cortex (precentral gyrus), Broca's area (speech production), prefrontal cortex (executive function, personality, judgement).
Parietal lobe: somatosensory cortex (postcentral gyrus), spatial awareness, sensory integration.
Temporal lobe: auditory cortex, Wernicke's area (language comprehension), hippocampus (memory formation).
Occipital lobe: primary visual cortex. Damage causes visual field deficits, not blindness in the eyes themselves (the eyes still work, but the brain cannot process the input).
12 pairs, numbered I to XII.
Each is sensory, motor, or mixed.
Commonly tested examples:
I (Olfactory): sensory, smell.
II (Optic): sensory, vision.
VII (Facial): mixed, facial expression, taste (anterior two-thirds of tongue).
X (Vagus): mixed, major parasympathetic nerve controlling heart rate, digestion, respiratory rate.
Sensory modalities:
General senses: touch, pressure, pain, temperature, proprioception. Receptors distributed throughout the body.
Special senses: sight, hearing, smell, taste, equilibrium. Receptors concentrated in specialised organs.
Receptor types by stimulus:
Mechanoreceptors: touch, pressure, vibration, stretch, hearing.
Thermoreceptors: temperature change.
Chemoreceptors: chemical stimuli (taste, smell, blood chemistry).
Photoreceptors: light (rods and cones in the retina).
Nociceptors: pain (thermal, mechanical, chemical damage).
Light enters through the cornea, passes through the pupil (regulated by the iris), is focused by the lens, and strikes the retina.
Rods: sensitive to dim light, responsible for peripheral and night vision, no colour.
Cones: colour vision, high acuity, concentrated in the fovea.
Myopia: eyeball too long or cornea too curved; image focuses in front of retina. Corrected with concave (diverging) lenses.
Presbyopia: lens loses elasticity with age, reducing ability to focus on near objects. Corrected with convex (converging) lenses.
Outer ear: pinna and external auditory canal. Funnels sound waves to the tympanic membrane (eardrum).
Middle ear: contains the ossicles (malleus, incus, stapes). Ossicles amplify vibrations and transmit them to the oval window of the inner ear.
Inner ear: cochlea (hearing, converts mechanical vibrations into nerve impulses) and vestibular apparatus (semicircular canals and otolith organs for balance and equilibrium).
Understanding reflex arcs is part of every neurological examination; a diminished patellar reflex can indicate nerve damage or spinal cord pathology. Knowledge of cerebral lobes and their functions is essential for interpreting stroke symptoms: weakness on one side of the body (contralateral motor cortex), speech difficulty (Broca's or Wernicke's area), or vision loss (occipital cortex). The vagus nerve (cranial nerve X) is a therapeutic target in conditions ranging from epilepsy to depression (vagus nerve stimulation). Myopia correction is one of the most common reasons people visit an optometrist.
Students often confuse the CNS and PNS. The CNS is only the brain and spinal cord. Everything else (all the nerves running through the body, the ganglia outside the spinal cord) belongs to the PNS.
Dendrites and axons are sometimes mixed up. Dendrites receive; axons send. Signals move from dendrite to cell body to axon.
The cerebellum is sometimes thought to initiate movement. It does not. The cerebrum initiates voluntary movement; the cerebellum coordinates and fine-tunes it.
Students sometimes think damage to the occipital lobe causes the eyes to stop working. The eyes function normally; it is the brain's ability to process visual information that is impaired.
⚠️ Know the parts of a neuron and the direction of signal flow (dendrite, cell body, axon).
⚠️ Be able to distinguish the CNS from the PNS and their subdivisions (somatic vs autonomic, sympathetic vs parasympathetic).
⚠️ The components of a reflex arc (receptor, sensory neuron, integration centre, motor neuron, effector) are a very common exam question.
⚠️ Associate each cerebral lobe with its primary function.
⚠️ Cranial nerves: at minimum, know the olfactory (I), optic (II), facial (VII), and vagus (X).
⚠️ Know the four receptor types (mechanoreceptors, thermoreceptors, chemoreceptors, photoreceptors) and give examples of each.
⚠️ Rods vs cones and myopia vs presbyopia are frequently tested.
⚠️ Ossicles (malleus, incus, stapes) and their function are standard exam material.
True or false: Neuroglia conduct electrical impulses.
The three protective layers surrounding the brain and spinal cord are collectively called the __________.
True or false: The parietal lobe is the primary visual processing area.
Fill in the blank: __________ are photoreceptors responsible for colour vision; __________ are photoreceptors responsible for dim-light and peripheral vision.
The three ossicles of the middle ear are the __________, __________, and __________.
(Answers: 1. False, neuroglia support neurons but do not conduct impulses. 2. Meninges. 3. False, the occipital lobe is the primary visual processing area; the parietal lobe handles somatosensory processing. 4. Cones; rods. 5. Malleus, incus, stapes.)
Q: Trace the pathway of a reflex arc, using the example of touching a hot surface.
A: A thermoreceptor/nociceptor in the skin (receptor) detects the painful heat. A sensory neuron carries the signal to the spinal cord (integration centre), where an interneuron relays it to a motor neuron. The motor neuron carries the response signal to the muscles of the arm (effector), which contract to pull the hand away. This occurs before the pain signal reaches the brain for conscious awareness.
Q: Compare the functions of the cerebrum, cerebellum, and brainstem.
A: The cerebrum handles higher-order functions: conscious thought, reasoning, voluntary movement, sensory perception, memory, and language. The cerebellum coordinates voluntary movement, maintains balance, and supports motor learning. The brainstem controls basic life-sustaining functions: heart rate, respiration, blood pressure, and protective reflexes.
Q: A patient has damage to the left frontal lobe near Broca's area. What symptom would you expect?
A: Expressive (Broca's) aphasia. The patient can understand language but has difficulty producing speech. Their words may be halting, effortful, or grammatically incomplete.
Q: Explain the difference between general and special senses, and give two examples of each.
A: General senses have receptors distributed throughout the body and include touch and pain (nociception). Special senses have receptors concentrated in specialised organs and include vision (photoreceptors in the retina) and hearing (mechanoreceptors in the cochlea).
Q: Why does presbyopia develop with age, and how is it corrected?
A: The lens of the eye gradually loses elasticity with age, reducing its ability to change shape (accommodate) for focusing on near objects. This makes close-up vision blurry. It is corrected with convex (converging) lenses, typically in reading glasses or bifocals.
The nervous system connects to the muscular system through the neuromuscular junction (Study Notes 05). It connects to the endocrine system through the hypothalamus, which links neural and hormonal control. Sensory input (this section) drives the motor output that produces movement via the skeletal and muscular systems (Study Notes 04 and 05). The autonomic nervous system controls smooth muscle in the cardiovascular, digestive, respiratory, and urinary systems. CSF production and circulation tie back to the integumentary concept of fluid balance and the meninges as a type of membrane (Study Notes 03).
neuron, neuroglia, glial cells, astrocyte, oligodendrocyte, Schwann cell, microglia, ependymal cell, satellite cell, dendrite, axon, cell body, soma, myelin sheath, action potential, synapse, CNS, PNS, central nervous system, peripheral nervous system, somatic nervous system, autonomic nervous system, sympathetic, parasympathetic, fight or flight, rest and digest, meninges, dura mater, arachnoid mater, pia mater, cerebrospinal fluid, CSF, reflex arc, sensory neuron, motor neuron, interneuron, afferent, efferent, plexus, brachial plexus, lumbar plexus, cerebrum, cerebral cortex, cerebellum, brainstem, medulla oblongata, pons, midbrain, frontal lobe, parietal lobe, temporal lobe, occipital lobe, Broca's area, Wernicke's area, cranial nerves, olfactory, optic, facial, vagus, sensory modalities, general senses, special senses, mechanoreceptor, thermoreceptor, chemoreceptor, photoreceptor, nociceptor, olfaction, gustation, proprioception, equilibrium, nociception, rods, cones, retina, myopia, presbyopia, ossicles, malleus, incus, stapes, cochlea, vestibular apparatus, anatomy and physiology, A&P