Nervous System - MCQs from Guyton & Hall Physiology

SYNAPSES & NEUROTRANSMISSION

• A) Both directions equally
• B) From postsynaptic to presynaptic neuron only
• C) From presynaptic to postsynaptic neuron only ✅
• D) Either direction depending on stimulus strength

Explanation: Chemical synapses always transmit signals in one direction - from the presynaptic neuron (which secretes the neurotransmitter) to the postsynaptic neuron. This one-way conduction allows signals to be directed toward specific goals. (Guyton & Hall, p.570)

• A) 5-20%
• B) 20-50%
• C) 50-75%
• D) 80-95% ✅

Explanation: As many as 10,000 to 200,000 synaptic knobs lie on the surfaces of a motor neuron, with about 80% to 95% on the dendrites and only 5% to 20% on the soma. (Guyton & Hall, p.569)

• A) Hyperpolarization of the postsynaptic membrane by K+ efflux
• B) Opening Cl- channels in the presynaptic terminal fibril via GABA ✅
• C) Activation of postsynaptic receptor enzymes
• D) Blockade of voltage-gated Ca2+ channels on the soma

Explanation: Presynaptic inhibition is caused by release of an inhibitory substance (usually GABA) onto the outsides of presynaptic nerve fibrils. GABA opens anion channels, allowing Cl- to diffuse into the terminal fibril. The negative charges cancel much of the excitatory effect of Na+ ions, inhibiting synaptic transmission. (Guyton & Hall, p.412)

• A) Na+ influx into the postsynaptic neuron
• B) Ca2+ influx into the postsynaptic neuron
• C) Cl- influx and/or K+ efflux across the postsynaptic membrane ✅
• D) Depolarization to -55 mV threshold

Explanation: Inhibitory synapses mainly open chloride channels (Cl- influx) and/or K+ channels (K+ efflux). Both actions make the interior membrane potential more negative (hyperpolarization). The IPSP shifts the potential from -65 mV (resting) toward -70 mV (Nernst potential for Cl-). (Guyton & Hall, p.393-395)

• A) 1-2 terminals
• B) 3-5 terminals
• C) ~40-80 terminals ✅
• D) >200 terminals

Explanation: Stimulation of a single presynaptic terminal almost never excites a postsynaptic neuron. Many presynaptic terminals from different fibers must fire simultaneously (spatial summation) to generate enough EPSPs to reach the excitatory threshold for firing. Typically around 40-80 or more concurrent inputs are needed depending on neuronal type. (Guyton & Hall, p.419-420)

Na+-K+ PUMP & MEMBRANE PHYSIOLOGY

• A) 2 Na+ out : 3 K+ in
• B) 3 Na+ out : 2 K+ in ✅
• C) 1 Na+ out : 1 K+ in
• D) 2 Na+ out : 2 K+ in

Explanation: The Na+-K+ pump has 3 binding sites for Na+ on the inside and 2 binding sites for K+ on the outside. Activation of its ATPase function extrudes 3 Na+ to the outside and moves 2 K+ to the inside per ATP molecule hydrolyzed. (Guyton & Hall, p.1793-1800)

• A) 10-20%
• B) 20-40%
• C) 40-50%
• D) 60-70% ✅

Explanation: For electrically active nerve cells, 60% to 70% of the cell's energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell via the Na+-K+ ATPase pump. (Guyton & Hall, p.1813)

SPINAL CORD REFLEXES

• A) 10-20 ms
• B) 50-100 ms
• C) 200-500 ms ✅
• D) 1-2 seconds

Explanation: About 0.2 to 0.5 second (200-500 ms) after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend (crossed extensor reflex). This latency is long because many interneurons are involved in crossing to the opposite side of the cord. (Guyton & Hall, p.22)

• A) Both agonist and antagonist muscles are excited simultaneously
• B) Excitation of one muscle group with simultaneous inhibition of its antagonist ✅
• C) Inhibition of all muscles when a painful stimulus is applied
• D) Inhibition of the ipsilateral limb when the contralateral limb extends

Explanation: When a stretch reflex excites one muscle, it often simultaneously inhibits the antagonist muscles. This is called reciprocal inhibition, mediated through the neuronal circuit of reciprocal innervation. (Guyton & Hall, p.32)

• A) Strong flexor spasms of skeletal muscles
• B) Evacuation of the bladder and colon
• C) Arterial pressure rising often above 200 mmHg systolic
• D) Selective activation of only upper limb musculature ✅

Explanation: The mass reflex involves large portions or all of the cord and includes: strong flexor spasms, bladder/colon evacuation, arterial hypertension (systolic >200 mmHg), and profuse sweating. It is NOT selective to one limb - it is a total cord event. (Guyton & Hall, p.112)

• A) Local inhibitory interneurons are hyperactivated
• B) Acute hemorrhage into the anterior horn destroys motor neurons
• C) Loss of tonic facilitatory input from reticulospinal, vestibulospinal, and corticospinal tracts ✅
• D) Rapid demyelination of dorsal column fibers

Explanation: Spinal shock occurs because normal activity of cord neurons depends greatly on continual tonic excitation from higher centers, particularly via reticulospinal, vestibulospinal, and corticospinal tracts. Loss of this input causes immediate total depression of cord reflexes. (Guyton & Hall, p.117-120)

MOTOR CORTEX & CORTICOSPINAL TRACT

• A) 10%
• B) 30% ✅
• C) 50%
• D) 70%

Explanation: The corticospinal tract originates about 30% from the primary motor cortex, 30% from the premotor and supplementary motor areas, and 40% from the somatosensory areas posterior to the central sulcus. (Guyton & Hall, p.281)

• A) Anterior limb of the internal capsule
• B) Posterior limb of the internal capsule ✅
• C) Genu of the internal capsule
• D) Corona radiata directly to the spinal cord

Explanation: After leaving the cortex, the corticospinal tract passes through the posterior limb of the internal capsule (between the caudate nucleus and the putamen of the basal ganglia), then through the brain stem forming the pyramids of the medulla, and most fibers cross in the lower medulla. (Guyton & Hall, p.282)

• A) Loss of ability to comprehend spoken language
• B) Complete inability to produce any sounds
• C) Inability to generate fluent speech while comprehension is preserved ✅
• D) Loss of reading ability only (alexia)

Explanation: Broca aphasia prevents fluent speech but the person can still comprehend spoken language. They may only produce an occasional simple word like "no" or "yes" depending on the severity of damage. (Guyton & Hall, p.256)

• A) Primary motor cortex (area 4)
• B) Head rotation area
• C) Voluntary eye movement field
• D) Hand skills area immediately anterior to the primary motor cortex for hands/fingers ✅

Explanation: There is a specialized "hand skills" premotor area immediately anterior to the primary motor cortex for hands and fingers. Destruction of this area causes hand movements to become uncoordinated and non-purposeful - a condition called motor apraxia. (Guyton & Hall, p.264)

CNS INFORMATION PROCESSING & MEMORY

• A) Decreased vesicle recycling after repeated stimulation
• B) Synaptic fatigue from repeated use
• C) Facilitation of synapses with repeated signal passage ✅
• D) Permanent destruction of inhibitory terminals

Explanation: Each time certain types of sensory signals pass through sequences of synapses, those synapses become more capable of transmitting the same type of signal the next time - a process called facilitation. After repeated use, signals generated within the brain itself can activate the same sequences even without external sensory input (memory). (Guyton & Hall, p.27)

• A) Are slower in signal transmission
• B) Can transmit signals bidirectionally ✅
• C) Always produce inhibitory potentials
• D) Require neurotransmitter release into the synaptic cleft

Explanation: Chemical synapses transmit in one direction only. Electrical synapses (gap junctions) can transmit signals in either direction (bidirectionally). They help coordinate large groups of neurons, are useful for synchronous firing, and are found in some hypothalamic hormone-secreting neurons. (Guyton & Hall, p.103)

• A) Are released from the neuronal soma by exocytosis
• B) Are recycled and reused repeatedly after release ✅
• C) Take hours to days to produce and are not recycled
• D) Act exclusively through G-protein coupled receptors

Explanation: Small-molecule transmitter vesicles are continually recycled. After fusing with the synaptic membrane and releasing their transmitter, the vesicle membrane invaginates back into the presynaptic terminal within seconds to minutes and pinches off to form a new vesicle (still containing appropriate enzyme/transport proteins). (Guyton & Hall, p.266)

LEVELS OF CNS FUNCTION

• A) Cortical level - Subcortical level - Spinal cord level
• B) Spinal cord level - Lower brain (subcortical) level - Higher brain (cortical) level ✅
• C) Spinal cord level - Cortical level - Subcortical level
• D) Peripheral nervous system - Brainstem - Cortex

Explanation: The three major levels of CNS function, from lowest to highest, are: (1) the spinal cord level, (2) the lower brain or subcortical level, and (3) the higher brain or cortical level. Each has specific functional characteristics inherited from evolutionary development. (Guyton & Hall, p.36)

• A) Excessive acetylcholine release at the neuromuscular junction causing sustained depolarization
• B) Positive feedback loop: metabolic irritation → pain signals → reflex contraction → more sensory stimulation ✅
• C) Acute ATP depletion causing rigor-like cross-bridge locking
• D) Inhibition of Golgi tendon organs causing loss of autogenic inhibition

Explanation: Local irritating factors (severe cold, lack of blood flow, overexercise) cause pain/sensory signals transmitted from the muscle to the spinal cord, which cause reflex contraction. This contraction stimulates the same sensory receptors even more, increasing the spinal cord contraction signal. Positive feedback then develops, escalating from mild irritation to a full-blown cramp. (Guyton & Hall, p.104)

Answer Key