The nervous system is divided into the CNS and PNS. What does each consist of and how are they defined?
A: The CNS consists of the brain and spinal cord; the PNS consists of all neural tissue outside the CNS, including cranial nerves, spinal nerves, and their associated ganglia
B: The CNS consists of the brain only; the PNS consists of the spinal cord, cranial nerves, and spinal nerves
C: The CNS consists of voluntary motor pathways; the PNS consists of all sensory and autonomic pathways
D: CNS and PNS are alternative terms for the somatic and autonomic divisions
Correct: The CNS consists of the brain and spinal cord; the PNS consists of all neural tissue outside the CNS, including cranial nerves, spinal nerves, and their associated ganglia
The central nervous system (CNS) comprises the brain and spinal cord — the primary integration and processing centres — enclosed and protected by the skull and vertebral column and by the three meningeal layers. The peripheral nervous system (PNS) consists of all neural tissue outside the CNS: the 12 pairs of cranial nerves, 31 pairs of spinal nerves, and the ganglia (clusters of neuronal cell bodies) associated with them. The PNS carries information to the CNS via afferent (sensory) pathways and carries motor commands from the CNS via efferent pathways. The CNS/PNS distinction is anatomical — defined by location relative to the meninges and bony enclosure — not functional.
The PNS is functionally divided into somatic and autonomic divisions. What is the key distinction between them?
A: The somatic division controls voluntary skeletal muscle and carries somatic sensory information; the autonomic division controls involuntary smooth muscle, cardiac muscle, and glands
B: The somatic division governs all sensory input; the autonomic division governs all motor output
C: The somatic division is associated with the sympathetic system; the autonomic division with the parasympathetic system
D: The somatic division is part of the PNS; the autonomic division is part of the CNS
Correct: The somatic division controls voluntary skeletal muscle and carries somatic sensory information; the autonomic division controls involuntary smooth muscle, cardiac muscle, and glands
The somatic nervous system carries sensory information from somatic receptors (skin, muscles, joints, special senses) to the CNS, and motor commands from the CNS to skeletal muscle — enabling voluntary, conscious movement. The autonomic nervous system (ANS) regulates involuntary functions: cardiac muscle (heart rate, contractility), smooth muscle (blood vessels, airways, digestive tract, bladder), and exocrine glands. Although some autonomic responses can be consciously modulated (e.g., through biofeedback), their default operation is outside conscious awareness. Both the somatic and autonomic divisions contain afferent (sensory) and efferent (motor) fibres — the distinction is not purely sensory/motor but concerns the type of effector organ innervated.
The autonomic nervous system has two major divisions. What are their correct anatomical origins and primary functional effects?
A: Sympathetic: originates from cranial nerves and sacral cord ("craniosacral"), promotes rest-and-digest; Parasympathetic: originates from thoracic and lumbar cord ("thoracolumbar"), promotes fight-or-flight
B: Sympathetic: originates from thoracic and lumbar cord ("thoracolumbar"), promotes fight-or-flight; Parasympathetic: originates from cranial nerves and sacral cord ("craniosacral"), promotes rest-and-digest
C: Both divisions originate in the hypothalamus and produce opposite effects on the same effector organs
D: Sympathetic: exclusively activates smooth muscle; Parasympathetic: exclusively relaxes smooth muscle; both originate throughout the brainstem
Correct: Sympathetic: originates from thoracic and lumbar cord ("thoracolumbar"), promotes fight-or-flight; Parasympathetic: originates from cranial nerves and sacral cord ("craniosacral"), promotes rest-and-digest
The sympathetic division originates from preganglionic neurons in the thoracic (T1–T12) and upper lumbar (L1–L2) spinal segments — "thoracolumbar." It prepares the body for emergency: increases heart rate and cardiac output, dilates bronchioles, constricts blood vessels in skin and viscera (redirecting blood to skeletal muscle), dilates pupils, stimulates glycogenolysis, and inhibits digestion. The parasympathetic division originates from cranial nerves (III oculomotor, VII facial, IX glossopharyngeal, X vagus) and sacral spinal cord (S2–S4) — "craniosacral." The vagus nerve (CN X) mediates the large majority of parasympathetic outflow, reaching heart, lungs, and abdominal viscera. Parasympathetic activation promotes rest-and-digest: slows heart rate, stimulates gastrointestinal motility and secretion, constricts pupils, and promotes urination. Most visceral organs receive dual innervation and the two divisions often produce opposing effects.
In autonomic motor pathways, how many neurons connect the CNS to the effector organ, and how does this differ from somatic motor pathways?
A: Autonomic: one neuron (preganglionic) runs directly from spinal cord to effector; somatic: two neurons via an intermediate ganglion
B: Autonomic: two neurons — a preganglionic neuron synapses in an autonomic ganglion, and a postganglionic neuron continues to the effector; somatic: one lower motor neuron runs directly from spinal cord to skeletal muscle
C: Both autonomic and somatic pathways use two-neuron chains; the distinction is in neurotransmitter type only
D: Autonomic: three neurons (preganglionic, ganglionic, and peripheral relay); somatic: two neurons
Correct: Autonomic: two neurons — a preganglionic neuron synapses in an autonomic ganglion, and a postganglionic neuron continues to the effector; somatic: one lower motor neuron runs directly from spinal cord to skeletal muscle
Autonomic motor pathways always consist of a two-neuron chain. The preganglionic neuron has its cell body in the CNS (brainstem or spinal cord) and its axon exits to synapse in an autonomic ganglion located outside the CNS. The postganglionic neuron has its cell body in the ganglion and its axon travels to the effector organ. All preganglionic neurons release acetylcholine (ACh) and act on nicotinic receptors on postganglionic neurons. Most sympathetic postganglionic neurons release noradrenaline (acting on adrenergic receptors on effectors); parasympathetic postganglionic neurons release ACh (acting on muscarinic receptors). By contrast, somatic motor pathways consist of a single lower motor neuron that runs directly from the ventral horn of the spinal cord (or brainstem motor nuclei) to skeletal muscle — releasing ACh at the neuromuscular junction.
The terms "afferent" and "efferent" describe direction of signal flow in the nervous system. Which correctly describes each?
A: Afferent carries motor commands from CNS to periphery; efferent carries sensory information from periphery to CNS
B: Afferent (from Latin ad = toward) carries sensory information from receptors in the periphery toward the CNS; efferent (from Latin ex = away from) carries motor commands away from the CNS to effectors
C: Afferent refers to descending spinal pathways; efferent refers to ascending pathways
D: Afferent and efferent are synonyms — both describe bidirectional nerve fibres
Correct: Afferent (from Latin ad = toward) carries sensory information from receptors in the periphery toward the CNS; efferent (from Latin ex = away from) carries motor commands away from the CNS to effectors
Afferent and efferent describe direction of signal flow relative to the CNS, and apply to both somatic and autonomic divisions. Afferent (ad = toward) fibres carry sensory information from peripheral receptors toward the CNS: pain and temperature (Aδ and C fibres), touch and pressure (Aβ fibres), proprioception (Ia/Ib fibres), and visceral sensory signals. Efferent (ex = away from) fibres carry motor commands from the CNS to effectors. In spinal nerves, this anatomy is fixed: dorsal roots contain afferent fibres (their cell bodies in dorsal root ganglia), ventral roots contain efferent fibres (lower motor neuron axons from ventral horn). This principle — the Bell-Magendie law — was established by Charles Bell and François Magendie in the 1820s and represents one of the earliest neuroscientific discoveries about nervous system organisation.
The blood-brain barrier (BBB) is a selective permeability barrier between the blood and the CNS. What is its primary function and composition?
A: The BBB prevents all substances from entering the brain, including glucose and oxygen
B: The BBB is formed primarily by tight junctions between cerebral endothelial cells (with astrocyte end-feet and pericytes) and selectively regulates passage of substances from blood into the CNS — blocking most pathogens, toxins, and large hydrophilic molecules while allowing glucose, oxygen, and lipid-soluble molecules to cross
C: The BBB is formed by the meningeal layers and only prevents physical trauma, not chemical threats
D: The BBB is located in the choroid plexus and filters cerebrospinal fluid, not blood
Correct: The BBB is formed primarily by tight junctions between cerebral endothelial cells (with astrocyte end-feet and pericytes) and selectively regulates passage of substances from blood into the CNS — blocking most pathogens, toxins, and large hydrophilic molecules while allowing glucose, oxygen, and lipid-soluble molecules to cross
The blood-brain barrier is formed by the tight junctions (zonula occludens) between adjacent cerebral capillary endothelial cells — unlike peripheral capillaries, which have large inter-cellular gaps. Astrocyte end-feet wrap around capillaries and signal endothelial cells to maintain barrier properties; pericytes also contribute structural support and regulate blood flow. The BBB selectively permits: small lipid-soluble molecules (including many drugs, steroid hormones, O2, CO2); glucose via GLUT1 transporters; amino acids via specific carriers. It blocks most water-soluble molecules, large proteins, most immune cells, and pathogens. Efflux transporters (P-glycoprotein, MRP proteins) actively pump many drugs back out. The BBB is a critical challenge in CNS pharmacology: most drugs that work peripherally cannot reach the brain. Areas without a full BBB (circumventricular organs — area postrema, subfornical organ, etc.) allow the brain to sample blood composition directly.
The enteric nervous system (ENS) is sometimes described as the "second brain." What justifies this characterisation?
A: The ENS is structurally identical to the cerebral cortex, with the same number and types of neurons
B: The ENS contains 200–600 million neurons organised in two plexuses that can detect the chemical and mechanical state of the gut and coordinate peristalsis and secretion largely independently of the brain or spinal cord, communicating bidirectionally with the CNS via the vagus nerve
C: The ENS is part of the CNS and controls all autonomic functions including those of the heart, lungs, and bladder
D: The ENS produces the majority of the body's dopamine and is the origin of most reward-related signals
Correct: The ENS contains 200–600 million neurons organised in two plexuses that can detect the chemical and mechanical state of the gut and coordinate peristalsis and secretion largely independently of the brain or spinal cord, communicating bidirectionally with the CNS via the vagus nerve
The enteric nervous system (ENS), first described systematically by Langley (1900) and popularised by Gershon (1999) as "the second brain," comprises approximately 200–600 million neurons organised in two interconnected networks: the myenteric plexus (Auerbach's plexus, between muscle layers) controls gut motility, and the submucosal plexus (Meissner's plexus) regulates secretion and local blood flow. Unlike other peripheral autonomic networks, the ENS contains its own sensory neurons, interneurons, and motor neurons, enabling it to coordinate peristalsis, mixing movements, and secretion without requiring input from the brain or spinal cord (demonstrated by gut function continuing after vagotomy). The ENS communicates bidirectionally with the CNS via the vagus nerve (about 90% of signals travel afferent, from gut to brain) and sympathetic splanchnic nerves — forming the gut-brain axis. The ENS produces approximately 95% of the body's serotonin, which acts as a key local signal for gut motility.
Autonomic dysreflexia is a potentially life-threatening complication of spinal cord injury above T6. What mechanism underlies it?
A: The parasympathetic system is completely destroyed by the injury, leaving the sympathetic system unopposed at all times
B: Stimuli below the injury (e.g., a distended bladder) trigger massive, unmodulated sympathetic discharge below the lesion — causing severe hypertension — because supraspinal inhibitory control of the sympathetic preganglionic neurons is severed; the intact vagus nerve causes compensatory bradycardia above the lesion, but cannot lower blood pressure sufficiently
C: The vagus nerve is damaged at T6, removing all parasympathetic control of the heart
D: Somatic sensory pathways are severed, causing the brain to generate random autonomic signals
Correct: Stimuli below the injury (e.g., a distended bladder) trigger massive, unmodulated sympathetic discharge below the lesion — causing severe hypertension — because supraspinal inhibitory control of the sympathetic preganglionic neurons is severed; the intact vagus nerve causes compensatory bradycardia above the lesion, but cannot lower blood pressure sufficiently
Autonomic dysreflexia illustrates the principle that the ANS requires supraspinal regulation. The sympathetic outflow originates from T1–L2; injury at or above T6 leaves a large portion of the sympathetic system intact below the lesion but disconnected from descending inhibitory control from the brain. When a noxious or otherwise intense stimulus occurs below the injury level (distended bladder or bowel is the most common trigger), afferent signals reach the spinal cord and activate sympathetic preganglionic neurons below the lesion. Without supraspinal inhibition, a massive, unmodulated sympathetic discharge occurs: intense vasoconstriction below the lesion causes severe, potentially life-threatening hypertension (systolic BP can exceed 300 mmHg). Compensatory baroreceptor reflexes via the vagus (CN X, which is intact and above the lesion) cause bradycardia and vasodilation above the lesion (sweating, flushing, headache) — but cannot adequately counteract the systemic hypertension. Treatment is immediate removal of the triggering stimulus.
Nervous System Organisation
The nervous system is divided into the CNS and PNS. What does each consist of and how are they defined?
About this quiz
The nervous system is divided anatomically into the central nervous system (CNS) and peripheral nervous system (PNS), and functionally into somatic and autonomic divisions. Understanding this organisation is fundamental to all of biopsychology: it explains how the brain communicates with the body, how voluntary and involuntary functions are controlled, and why injury at specific sites produces predictable functional deficits.
This quiz covers the CNS/PNS division, the somatic and autonomic subdivisions, sympathetic and parasympathetic function, afferent/efferent pathways, the blood-brain barrier, and clinical implications of autonomic organisation. Key sources include Kandel et al. (2013) Principles of Neural Science and Bear, Connors & Paradiso (2016) Neuroscience: Exploring the Brain.