Difficulty: Intermediate to Advanced | Prerequisites: Chapters 4 (tissue types, nervous tissue basics), 10 (neuromuscular junction concepts)
This is the densest block in the course. Chapter 12 introduces the nervous system's building blocks: neurons, neuroglia, and how they are organised into the CNS and PNS. Chapter 13 focuses on the brain and spinal cord, emphasising structural identification (the exam leans more on anatomy here than on function). Chapter 14 covers peripheral nerves, spinal nerve organisation, nerve plexuses, and the 12 cranial nerves. Chapter 15 wraps up with the autonomic nervous system (ANS), comparing the sympathetic and parasympathetic divisions. If you know the neuron basics from Chapter 12, everything else in this block builds on that foundation.
The nervous system splits into the CNS (brain and spinal cord) and the PNS (nerves and ganglia). Neurons are classified by structure and function, and supporting glial cells differ between the CNS and PNS. You need to identify brain structures by location, know all 12 cranial nerves, and distinguish sympathetic from parasympathetic responses.
Central nervous system (CNS)
The brain and spinal cord. The integrating and command centre.
Peripheral nervous system (PNS)
Everything outside the CNS: cranial nerves, spinal nerves, and ganglia. The communication lines between the CNS and the rest of the body.
Afferent (sensory)
Carrying information toward the CNS. In simple terms, sensory neurons are afferent: they bring signals in.
Efferent (motor)
Carrying information away from the CNS to effectors (muscles and glands). Motor neurons are efferent: they send commands out.
Effectors
The structures that carry out responses: muscles (which contract) and glands (which secrete).
Neuron
An excitable nerve cell specialised for transmitting electrical impulses.
Multipolar neuron
A neuron with one axon and multiple dendrites radiating from the cell body. The most common structural type, found throughout the CNS and as motor neurons.
Myelin
A fatty, insulating sheath that wraps around axons and speeds up nerve impulse conduction. Think of it as electrical insulation around a wire.
Oligodendrocytes
Glial cells in the CNS that form myelin sheaths. One oligodendrocyte can myelinate portions of several axons.
Schwann cells
Glial cells in the PNS that form myelin sheaths. Each Schwann cell wraps around a single segment of one axon.
White matter
Regions of the CNS composed mainly of myelinated axons. The myelin gives it its pale colour.
Grey matter
Regions of the CNS composed mainly of cell bodies, dendrites, and unmyelinated axons. Darker in appearance.
Nerve
A bundle of axons (nerve fibres) in the PNS, wrapped in connective tissue layers.
Tract
A bundle of axons within the CNS. The CNS equivalent of a nerve.
Ganglion (pl. ganglia)
A cluster of neuron cell bodies in the PNS.
Nucleus (neural context)
A cluster of neuron cell bodies within the CNS. The CNS equivalent of a ganglion.
Reflex arc
The neural pathway of a reflex. Five components: receptor, sensory (afferent) neuron, integration centre (often in the CNS), motor (efferent) neuron, effector.
Rostral
Toward the nose or front of the brain. Used when describing positions within the brain itself.
Caudal
Toward the tail end (inferior/posterior direction within the brain and spinal cord).
Meninges
Three protective membranes surrounding the brain and spinal cord: dura mater (outermost, tough), arachnoid mater (middle, web-like), pia mater (innermost, delicate, adheres directly to the brain/cord surface).
Preganglionic neuron
The first neuron in the autonomic pathway. Its cell body is in the CNS and its axon extends to a ganglion in the PNS.
Postganglionic neuron
The second neuron in the autonomic pathway. Its cell body is in the ganglion and its axon extends to the effector organ.
CNS versus PNS
CNS: brain + spinal cord; processes and integrates information
PNS: cranial nerves (12 pairs) + spinal nerves (31 pairs) + ganglia; carries signals to and from the CNS
Sensory (afferent) division of PNS: brings information in
Motor (efferent) division of PNS: sends commands out, subdivided into somatic (voluntary, to skeletal muscle) and autonomic (involuntary, to smooth muscle, cardiac muscle, glands)
Anatomy of a multipolar neuron
Cell body (soma): contains the nucleus and most organelles
Dendrites: branching extensions that receive signals from other neurons
Axon: a single long projection that conducts impulses away from the cell body
Axon hillock: the cone-shaped region where the axon originates
Axon terminals (synaptic knobs): the branched ends of the axon that release neurotransmitter
Structural classification of neurons
Multipolar: one axon, many dendrites (most common; motor neurons, interneurons)
Bipolar: one axon, one dendrite (found in special sense organs: retina, olfactory epithelium)
Unipolar (pseudounipolar): single process that branches into a peripheral and central process (most sensory neurons)
Functional classification of neurons
Sensory (afferent): carry impulses toward the CNS
Motor (efferent): carry impulses away from the CNS
Interneurons (association neurons): lie entirely within the CNS, connecting sensory and motor neurons
Myelinating cells
CNS: oligodendrocytes (one cell myelinates segments of multiple axons)
PNS: Schwann cells (one cell wraps one segment of one axon)
Myelin increases the speed of impulse conduction via saltatory conduction (impulses jump between nodes of Ranvier)
Nerve versus tract, nucleus versus ganglion
Nerve = bundle of axons in the PNS; tract = bundle of axons in the CNS
Ganglion = cluster of cell bodies in the PNS; nucleus = cluster of cell bodies in the CNS
White matter and grey matter
White matter: myelinated axons (tracts in the CNS)
Grey matter: cell bodies, dendrites, unmyelinated axons, synapses
Gross anatomy of a nerve (CT wrappings)
Epineurium: outermost, wraps the entire nerve
Perineurium: wraps each fascicle of nerve fibres
Endoneurium: wraps each individual nerve fibre (axon)
This mirrors the epi-, peri-, endo- pattern of skeletal muscle
Five components of the reflex arc
Receptor (detects stimulus)
Sensory (afferent) neuron (transmits impulse to CNS)
Integration centre (usually one or more interneurons in the CNS; processes the signal)
Motor (efferent) neuron (transmits impulse from CNS to effector)
Effector (muscle or gland that carries out the response)
Note: The exam emphasises structures and anatomical locations over detailed functional descriptions.
Cerebrum
Lobes: frontal, parietal, temporal, occipital
Major sulci/fissures: central sulcus (separates frontal and parietal lobes), lateral sulcus/fissure (separates temporal lobe from frontal and parietal), longitudinal fissure (separates left and right hemispheres)
Meninges
Dura mater: tough outer layer
Arachnoid mater: middle layer, web-like
Pia mater: innermost, delicate, directly on brain/cord surface
Cerebrospinal fluid (CSF) circulates in the subarachnoid space (between arachnoid and pia)
Ventricles
Right and left lateral ventricles: one in each cerebral hemisphere
Third ventricle: in the diencephalon (between the two halves of the thalamus)
Fourth ventricle: between the pons/medulla and the cerebellum
Ventricles are filled with CSF
Diencephalon
Thalamus: the relay station; nearly all sensory input passes through the thalamus on its way to the cerebral cortex
Hypothalamus: the master regulator of homeostasis; controls body temperature, hunger, thirst, circadian rhythms, and links the nervous and endocrine systems
Brainstem structures (be able to identify)
Midbrain: superior part of the brainstem
Pons: middle portion, bulges anteriorly
Medulla oblongata: most inferior part, continuous with the spinal cord; notable external landmarks include the pyramids (motor tracts) and olives
Cerebellum
Located posterior to the pons and medulla
Connected to the brainstem via cerebellar peduncles (the middle cerebellar peduncle is the largest)
Coordinates movement, balance, and posture
Spinal cord
Grey matter is internal (butterfly or H-shaped), white matter is external (the opposite arrangement from the brain)
Grey matter horns: dorsal (posterior) horns contain sensory interneurons; ventral (anterior) horns contain motor neuron cell bodies; lateral horns are present in the thoracic and upper lumbar regions and contain autonomic (sympathetic) neuron cell bodies
White matter columns: dorsal (posterior), lateral, and ventral (anterior) columns, each carrying ascending or descending tracts
Dorsal versus ventral roots
Dorsal roots carry sensory (afferent) information into the spinal cord; dorsal root ganglia contain sensory neuron cell bodies
Ventral roots carry motor (efferent) information out of the spinal cord
Where dorsal and ventral roots merge, they form a spinal nerve (which is therefore mixed: both sensory and motor)
Spinal nerves
31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
Each spinal nerve divides into rami after exiting the vertebral column
Dorsal ramus: supplies the posterior body trunk (skin and muscles of the back)
Ventral ramus: supplies the anterior and lateral trunk and the limbs
Ventral rami (except T2-T12) form nerve plexuses
Rami and plexuses
Ventral rami of most spinal nerves intermingle to form plexuses
Dorsal rami do not form plexuses
Ventral rami of T2-T12 do not form plexuses either; they run as intercostal nerves
Major nerve plexuses and selected nerves
Cervical plexus (C1-C5): phrenic nerve (diaphragm, breathing)
Brachial plexus (C5-T1): musculocutaneous nerve, median nerve, ulnar nerve, radial nerve, axillary nerve
Lumbar plexus (L1-L4): femoral nerve, obturator nerve
Sacral plexus (L4-S4): sciatic nerve (largest nerve in the body), tibial nerve, common fibular (peroneal) nerve
Cranial nerves (12 pairs)
I Olfactory: sensory (smell)
II Optic: sensory (vision)
III Oculomotor: motor (most eye movements, pupil constriction, lens shape)
IV Trochlear: motor (superior oblique muscle of the eye)
V Trigeminal: both (sensory from face, motor to muscles of mastication)
VI Abducens: motor (lateral rectus muscle of the eye)
VII Facial: both (motor to muscles of facial expression, sensory for taste from anterior 2/3 of tongue)
VIII Vestibulocochlear: sensory (hearing and equilibrium)
IX Glossopharyngeal: both (sensory from posterior 1/3 of tongue including taste, motor to pharyngeal muscles)
X Vagus: both (major parasympathetic nerve, sensory and motor to thoracic and abdominal viscera)
XI Accessory (spinal accessory): motor (trapezius and sternocleidomastoid muscles)
XII Hypoglossal: motor (tongue muscles)
Somatic motor versus ANS
Somatic: one motor neuron from CNS directly to skeletal muscle; voluntary; always excitatory (causes contraction)
ANS: two-neuron chain (preganglionic then postganglionic) to smooth muscle, cardiac muscle, or glands; involuntary; can be excitatory or inhibitory
Sympathetic versus parasympathetic
Sympathetic ("fight or flight"): prepares body for physical activity; increases heart rate, dilates pupils, diverts blood to skeletal muscles, inhibits digestion
Preganglionic neurons originate in the thoracolumbar region (T1-L2) of the spinal cord lateral horn
Short preganglionic fibres, long postganglionic fibres
Parasympathetic ("rest and digest"): conserves energy, promotes digestion, slows heart rate, constricts pupils
Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and the sacral spinal cord (S2-S4); hence "craniosacral"
Long preganglionic fibres, short postganglionic fibres
Lateral horn of the spinal cord
Found only in the thoracic and upper lumbar segments (T1-L2)
Contains the cell bodies of sympathetic preganglionic neurons
Autonomic tone
The hypothalamus is the primary brain region regulating autonomic tone (the balance between sympathetic and parasympathetic activity)
Classifying body responses
Heart rate increases: sympathetic
Pupils dilate: sympathetic
Digestion is stimulated: parasympathetic
Bronchioles dilate: sympathetic
Salivation increases: parasympathetic
"Fight or flight" responses = sympathetic; "rest and digest" responses = parasympathetic
Students mix up afferent and efferent. A useful cue: Afferent = Arriving (sensory input arriving at the CNS); Efferent = Exiting (motor commands exiting the CNS).
Oligodendrocytes and Schwann cells both make myelin, but they belong to different divisions. Oligodendrocytes are CNS only; Schwann cells are PNS only. They are never interchangeable.
Grey matter is not always on the outside. In the spinal cord, grey matter is internal and white matter is external, the reverse of the cerebral cortex arrangement.
Students often assume all cranial nerves are sensory. Several are purely motor (III, IV, VI, XI, XII), several are both sensory and motor (V, VII, IX, X), and only three are purely sensory (I, II, VIII).
⚠️ The CNS versus PNS chart (and the subdivisions within each) is foundational. Expect questions that test whether you can place a structure or function in the correct division.
⚠️ Cranial nerve identification, including names, numbers, and whether each is sensory, motor, or both, is heavily tested. Use a mnemonic if it helps. A common one for the names: "Oh, Oh, Oh, To Touch And Feel Very Green Vegetables, AH!" (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal). For function type: "Some Say Marry Money But My Brother Says Big Brains Matter More" (S = sensory, M = motor, B = both).
⚠️ For Chapter 13, focus on being able to identify structures on a diagram (lobes, sulci, ventricles, brainstem parts, cerebellum) rather than memorising detailed functional maps.
⚠️ The reflex arc's five components are a staple exam question.
⚠️ Sympathetic versus parasympathetic: know the origins (thoracolumbar vs. craniosacral), the fibre lengths (short pre/long post vs. long pre/short post), and be able to classify common body responses.
True or false: Afferent neurons carry signals away from the CNS.
Fill in the blank: In the CNS, myelin is produced by __________.
True or false: The lateral horn of the spinal cord is found at all levels of the cord.
Fill in the blank: The five components of a reflex arc are receptor, sensory neuron, integration centre, motor neuron, and __________.
True or false: The vagus nerve (CN X) is the major parasympathetic nerve.
Answers: 1. False (afferent = toward the CNS; efferent = away). 2. Oligodendrocytes. 3. False (lateral horns are found only in the thoracic and upper lumbar segments, T1-L2). 4. Effector. 5. True.
Q: Compare the structural and functional classifications of neurons.
A: Structural classification is based on the number of processes: multipolar (many dendrites, one axon), bipolar (one dendrite, one axon), unipolar (one process that splits). Functional classification is based on direction of impulse: sensory (afferent, toward CNS), motor (efferent, away from CNS), and interneurons (within the CNS).
Q: Distinguish between a nerve and a tract.
A: Both are bundles of axons. A nerve is in the PNS; a tract is in the CNS.
Q: What are the three meninges, from outermost to innermost?
A: Dura mater, arachnoid mater, pia mater.
Q: Name the four ventricles of the brain and their general locations.
A: Right and left lateral ventricles (in the cerebral hemispheres), third ventricle (in the diencephalon), fourth ventricle (between the pons/medulla and the cerebellum).
Q: How does the sympathetic division differ from the parasympathetic division in terms of origin and fibre length?
A: Sympathetic preganglionic neurons originate from the thoracolumbar cord (T1-L2) and have short preganglionic fibres and long postganglionic fibres. Parasympathetic preganglionic neurons originate from the brainstem and sacral cord (craniosacral) and have long preganglionic fibres and short postganglionic fibres.
Q: The glossopharyngeal nerve (CN IX) is sensory, motor, or both?
A: Both. It carries sensory information from the posterior third of the tongue (including taste) and motor signals to pharyngeal muscles.
The autonomic nervous system (Ch. 15) directly controls the cardiac conduction system (Ch. 19) and smooth muscle in blood vessel walls (Ch. 20), making it essential for understanding cardiovascular regulation.
Cranial nerves (Ch. 14) reappear in the respiratory system (vagus nerve innervates the larynx and bronchi, Ch. 22) and the digestive system (vagus nerve stimulates peristalsis and secretion, Ch. 23).
The reflex arc (Ch. 12) provides the template for understanding how the body responds to stimuli throughout all organ systems.
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