201. Two myelinated axons have intact terminal vesicles and functional post-synaptic receptors. Fibre P retains its myelin, while Fibre Q is experimentally demyelinated without cutting the axonal membrane. Proximal stimulation reaches Q’s terminal less efficiently, but direct terminal stimulation produces normal transmitter release. The evidence supports:
ⓐ. a primary receptor defect in the post-synaptic membrane
ⓑ. impaired axonal conduction caused by loss of myelin
ⓒ. failure of transmitter storage within synaptic vesicles
ⓓ. reversal of afferent and efferent pathway directions
Correct Answer: impaired axonal conduction caused by loss of myelin
Explanation: Direct stimulation of the terminal produces normal transmitter release, showing that the vesicles, pre-synaptic release machinery and post-synaptic receptors can function. The failure appears when excitation must travel along the experimentally demyelinated axon from the proximal site to the terminal. Myelin and nodes of Ranvier support rapid saltatory conduction, whereas loss of the sheath disrupts efficient node-to-node propagation. The axonal membrane has not been cut, but the normal structural basis of saltatory conduction has been removed. The experiment distinguishes conduction from synaptic transmission by stimulating at two locations. A proximal stimulus tests the complete axonal route, while direct terminal stimulation bypasses that route and tests release. The contrasting results localise the defect to myelin-dependent impulse conduction rather than to the chemical synapse. Normal release after direct terminal stimulation shows that vesicles, receptors and the chemical junction still work; the reduced proximal response must arise before the terminal.
202. Use the pathway described below. Structure P detects a peripheral change. Arrow Q carries an impulse towards the CNS. Within the CNS, terminal R releases a transmitter across a cleft. Arrow S then carries regulatory output to smooth muscle in an involuntary organ. The correct interpretation is:
ⓐ. Q is afferent, R is pre-synaptic and S is autonomic efferent
ⓑ. Q is autonomic efferent, R is post-synaptic and S is somatic afferent
ⓒ. Q is somatic efferent, R is a node and S is visceral afferent
ⓓ. Q is visceral efferent, R is a dendrite and S is somatic efferent
Correct Answer: Q is afferent, R is pre-synaptic and S is autonomic efferent
Explanation: Arrow Q travels from a peripheral detecting structure towards the central neural system, establishing the afferent direction. Terminal R releases neurotransmitter into a synaptic cleft, so it belongs to the transmitting or pre-synaptic side of a chemical synapse. Arrow S leaves the CNS and ends on smooth muscle in an involuntary organ. Its direction is efferent, and its target places it in the autonomic division. The three labels arise from different pieces of evidence: direction identifies Q, transmitter release identifies R, and direction plus target identifies S. None of these categories is interchangeable. A node of Ranvier lies between myelin segments on one axon, while a dendrite primarily carries incoming information towards a neuronal cell body. The described route integrates peripheral detection, central synaptic processing and involuntary output. The pathway is solved by assigning each label at its own level: directional fibre, transmitting synaptic terminal and target-defined motor output.
203. Extracellular \(\mathrm{Na^+}\) is greatly reduced around an afferent axon, while its myelin, potassium distribution and central synapses initially remain intact. An effective peripheral stimulus is then applied. The most likely linked outcome is:
ⓐ. increased sodium influx followed by stronger afferent signalling
ⓑ. normal depolarisation followed by accelerated potassium influx
ⓒ. unchanged afferent input followed by enhanced autonomic output
ⓓ. reduced depolarisation followed by weaker sensory input to the CNS
Correct Answer: reduced depolarisation followed by weaker sensory input to the CNS
Explanation: An effective stimulus normally produces a sharp increase in membrane permeability to \(\mathrm{Na^+}\). Sodium then moves inward from the extracellular fluid, where its concentration is normally high, and creates local depolarisation. Reducing extracellular sodium weakens the condition driving this influx. Even though the axon remains myelinated and its central synapses are initially intact, generation and renewal of the action potential along the afferent pathway become less effective. The CNS receives weaker or failed sensory input from the stimulated tissue. The change should not be attributed to receptor blockade or transmitter failure, since those parts are stated to remain intact. The reasoning connects an altered ion gradient with membrane excitation and then with the direction-specific function of an afferent nerve.
204. Select the symbolic relation that consistently links peripheral stimulation, axonal excitation, central chemical transmission and target response.
ⓐ. \(\text{Stimulus}\rightarrow\mathrm{K^+\ influx}\rightarrow\text{afferent impulse}\rightarrow\text{pre-synaptic receptor}\rightarrow\text{somatic input}\)
ⓑ. \(\text{Stimulus}\rightarrow\mathrm{Na^+\ efflux}\rightarrow\text{efferent impulse}\rightarrow\text{cleft current}\rightarrow\text{central receptor}\)
ⓒ. \(\text{Stimulus}\rightarrow\mathrm{Na^+\ influx}\rightarrow\text{afferent impulse}\rightarrow\text{transmitter release}\rightarrow\text{post-synaptic response}\)
ⓓ. \(\text{Stimulus}\rightarrow\mathrm{K^+\ efflux}\rightarrow\text{resting potential}\rightarrow\text{vesicle formation}\rightarrow\text{afferent target}\)
Correct Answer: \(\text{Stimulus}\rightarrow\mathrm{Na^+\ influx}\rightarrow\text{afferent impulse}\rightarrow\text{transmitter release}\rightarrow\text{post-synaptic response}\)
Explanation: A peripheral stimulus can initiate a temporary increase in sodium permeability, allowing \(\mathrm{Na^+}\) to enter the axon and produce depolarisation. If the information is travelling towards the CNS, the resulting impulse is carried through an afferent pathway. When the action potential reaches a pre-synaptic terminal, transmitter-filled vesicles release their contents into the chemical synaptic cleft. The neurotransmitter then binds receptors on the post-synaptic membrane and produces an excitatory or inhibitory potential. This symbolic chain keeps ion direction, pathway direction and synaptic orientation consistent. Potassium efflux belongs to repolarisation rather than initial excitation, and receptors are positioned on the post-synaptic side rather than serving as the releasing structure. Each arrow represents a necessary transition in one continuous information route. The relation also separates channel-mediated axonal excitation from transmitter-mediated intercellular transfer, preventing one mechanism from being substituted for the other.
205. A graph plots relative membrane potential against time at an axon terminal P and an adjacent post-synaptic membrane Q. P shows a depolarising rise first; Q shows a later potential change. When vesicle fusion is blocked, P’s curve remains, but Q’s change disappears. The strongest inference is:
ⓐ. P communicates with Q through a chemical synapse
ⓑ. P and Q are nodes on one continuously conducting axon
ⓒ. Q depolarises before any signal reaches terminal P
ⓓ. Q forms myelin around the pre-synaptic terminal
Correct Answer: P communicates with Q through a chemical synapse
Explanation: The axon terminal depolarises first, showing that an action potential reaches the transmitting side. A later change at Q indicates that the receiving membrane responds after the terminal event. Blocking vesicle fusion leaves the electrical event at P intact but removes the response at Q. This separates axonal excitation from transmission across the junction. Vesicle fusion is required for neurotransmitter release at a chemical synapse, so loss of the post-synaptic curve identifies a transmitter-dependent connection. Two nodes on one axon would not depend on synaptic vesicle fusion for conduction between them. The graph provides both timing and intervention evidence: P precedes Q, and a pre-synaptic release block selectively removes Q’s response. These observations support directional chemical transmission from P to Q. The preserved terminal curve confirms arrival of the action potential, while disappearance of Q only after fusion is blocked localises the missing link to transmitter-mediated transfer.
206. Signals from the semicircular canals enter the neural system normally, but extensive demyelination affects the fibres carrying that information towards an intact cerebellum. Individual skeletal muscles still contract when directly stimulated. The most appropriate prediction is:
ⓐ. impaired motor output to all skeletal muscles
ⓑ. impaired processing within an otherwise intact cerebellum
ⓒ. improved balance because conduction becomes continuous
ⓓ. impaired balance-signal conduction towards the cerebellum
Correct Answer: impaired balance-signal conduction towards the cerebellum
Explanation: The sensory structures continue to generate information, and the cerebellum itself remains intact. The altered component is the myelin surrounding fibres that carry balance-related input centrally. Myelin supports rapid saltatory conduction between nodes of Ranvier. Extensive demyelination reduces the efficiency of this conduction, so information from the semicircular canals may reach the cerebellum slowly, incompletely or unreliably. Direct muscular stimulation still causes contraction, demonstrating that basic muscle responsiveness remains. The predicted deficit concerns coordination rather than complete paralysis. An intact cerebellum cannot integrate information that is not delivered normally through its incoming pathways. The case combines peripheral sensory input, fibre structure and a central integrating region to distinguish a transmission defect from destruction of the cerebellum or skeletal muscle.
207. Four interventions and observations are listed below.
| Record | Intervention | Observed or expected result |
|---|
| P | Sodium-potassium pumping stops for a prolonged period | Resting ion gradients become difficult to maintain |
| Q | Potassium efflux is blocked after depolarisation | Repolarisation becomes faster |
| R | Pre-synaptic vesicle fusion is prevented | Post-synaptic response is reduced |
| S | A medullary respiratory centre is damaged | Interhemispheric communication alone is lost |
Which records preserve the expected mechanism?
ⓐ. Q and S only
ⓑ. P and R only
ⓒ. P, Q and R only
ⓓ. P, R and S only
Correct Answer: P and R only
Explanation: Record P is consistent with the active role of the sodium-potassium pump. Moving \(3\) \(\mathrm{Na^+}\) outward for every \(2\) \(\mathrm{K^+}\) inward helps maintain the unequal ion distributions of a resting neuron. Prolonged pump failure weakens those gradients. Record R is also valid: without vesicle fusion, neurotransmitter is not released normally, so receptor activation and the post-synaptic potential decline. Record Q reverses the potassium effect. Outward \(\mathrm{K^+}\) movement supports repolarisation, so blocking it delays rather than accelerates recovery. Record S assigns a corpus-callosum function to the medulla. Medullary respiratory-centre damage disrupts breathing control, whereas interhemispheric communication depends on cerebral connecting fibres. Only P and R preserve the appropriate intervention-to-outcome pathway. Record Q predicts the opposite of the expected recovery effect, and Record S assigns a corpus-callosum function to a medullary centre. Only P and R preserve mechanism and outcome.
208. An identified neuron has one axon and several dendrites. Its axon is wrapped by a myelin sheath, and the fibre carries a command from the CNS towards skeletal muscle. The complete classification is:
ⓐ. bipolar neuron, non-myelinated fibre and autonomic efferent pathway
ⓑ. unipolar neuron, myelinated fibre and visceral afferent pathway
ⓒ. multipolar neuron, myelinated fibre and somatic efferent pathway
ⓓ. multipolar neuron, non-myelinated fibre and autonomic afferent pathway
Correct Answer: multipolar neuron, myelinated fibre and somatic efferent pathway
Explanation: One axon with several dendrites identifies a multipolar neuron. The presence of a myelin sheath establishes that the relevant axonal fibre is myelinated; this label concerns its covering rather than its direction. The impulse travels from the CNS towards skeletal muscle, making the pathway efferent in direction and somatic in target. These labels describe different levels of biological organisation and can apply simultaneously. Multipolar refers to the number of neuronal processes, myelinated refers to axonal covering, and somatic efferent refers to the direction and destination of the neural pathway. A neuron does not become bipolar merely through having a myelinated axon, and a skeletal-muscle target does not fit an autonomic classification. Combining the three evidence types produces the complete diagnosis.
209. Three interventions are applied separately to the same receptor-to-effector route.
| Intervention | Observation |
|---|
| P | Afferent fibre is cut | Peripheral stimulation produces no CNS recording, but direct CNS stimulation still activates the target |
| Q | Post-synaptic receptors in the CNS are blocked | Transmitter is released, but the next neuron shows no normal potential |
| R | Sodium permeability in the efferent axon cannot rise | A central command fails to propagate to the target |
The strongest conclusion is:
ⓐ. all three interventions damage the same pre-synaptic structure
ⓑ. each intervention blocks a different part of the same afferent pathway
ⓒ. all three observations identify failure of afferent conduction
ⓓ. P, Q and R isolate afferent, synaptic and efferent failures
Correct Answer: P, Q and R isolate afferent, synaptic and efferent failures
Explanation: Intervention P removes the pathway carrying peripheral information towards the CNS. The preserved response to direct CNS stimulation confirms that the outgoing route can still function, localising the defect to the afferent stage. Intervention Q permits transmitter release but prevents the receiving membrane from responding, isolating a post-synaptic chemical-transmission defect. Intervention R acts on the outward axon. Without the stimulus-linked rise in sodium permeability, depolarisation and action-potential propagation along the efferent pathway are impaired, so the command fails to reach the target. The three manipulations affect successive parts of one integrated route rather than one shared structure. Together they demonstrate how peripheral input, central synaptic transfer and regulatory output can be tested independently through appropriate controls and observations. Each observation also fixes the level of failure: input cannot enter, one central chemical transfer cannot proceed, or output cannot propagate. The three stages are mechanistically distinct.
210. Consider the following statements about synaptic transmission within an integrated neural pathway.
I. Electrical synapses allow direct current flow between closely placed membranes.
II. Chemical synapses use transmitter release and post-synaptic receptors.
III. Loss of axonal myelin automatically converts a chemical synapse into an electrical one.
IV. A chemical post-synaptic potential may be excitatory or inhibitory.
ⓐ. I, II and IV only
ⓑ. I and III only
ⓒ. II and III only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Electrical synapses are characterised by closely placed neuronal membranes and direct passage of electrical current, making Statement I valid. Chemical synapses require a sequence involving vesicle fusion, neurotransmitter release, receptor binding and post-synaptic ion-channel activation, supporting Statement II. Statement IV is also valid, as the resulting post-synaptic potential may promote or inhibit excitation. Statement III incorrectly merges axonal conduction with synapse classification. Myelin affects the manner and speed of impulse propagation along an axon, but its loss does not change the structural mechanism by which two cells communicate at an existing synapse. The valid statements distinguish conduction along one neuron from transmission between cells. These processes occur consecutively in a pathway but remain structurally and mechanistically different.
211. A probe passes inward through the cranium and three meninges before entering a folded region rich in neuronal cell bodies. Deeper tissue contains many myelinated fibre tracts. The complete outside-to-inside relation is:
ⓐ. cranium, pia mater, arachnoid, dura mater, white cortex, grey tracts
ⓑ. dura mater, cranium, arachnoid, pia mater, white cortex, grey tracts
ⓒ. cranium, dura mater, arachnoid, pia mater, grey cortex, white matter
ⓓ. cranium, arachnoid, dura mater, pia mater, white cortex, grey matter
Correct Answer: cranium, dura mater, arachnoid, pia mater, grey cortex, white matter
Explanation: The cranium forms the external bony protection. Immediately inside it lies the dura mater, followed by the arachnoid and then the pia mater, which directly contacts brain tissue. The probe next enters the folded cerebral cortex. Its concentration of neuronal cell bodies gives it a grey appearance. Deeper cerebral tissue contains tracts of myelinated nerve fibres and forms white matter. The relation combines external protection with internal cerebral organisation. The meningeal sequence cannot be reversed without moving the pia away from the neural surface, and the grey-white arrangement cannot be exchanged without placing myelinated tracts in the outer cortex. Following the physical path from skull to deep cerebrum fixes every position in one continuous anatomical sequence. The folded tissue is identified only after the protective layers and its outer cortical position are considered together.
212. Sodium-potassium pumping is gradually inhibited in hypothalamic neurosecretory neurons, while their structure initially remains intact. Over time, the most defensible prediction is:
ⓐ. progressive strengthening of resting polarity and neurosecretory output
ⓑ. weakening of ion gradients, neural activity and neurosecretion
ⓒ. loss of ion gradients affects myelin before neuronal excitability
ⓓ. new myelin and nodes compensate for weakened membrane gradients
Correct Answer: weakening of ion gradients, neural activity and neurosecretion
Explanation: The sodium-potassium pump contributes to maintaining high internal \(\mathrm{K^+}\), high external \(\mathrm{Na^+}\) and the polarised resting condition required for normal neuronal excitability. Gradual inhibition does not instantly erase all existing gradients, but it makes them increasingly difficult to preserve. As the ionic conditions deteriorate, normal resting potential, action-potential generation and communication by the affected hypothalamic neurons may become impaired. These cells are neurosecretory, so disturbed neural activity can also weaken their contribution to endocrine regulation. The predicted outcome links membrane physiology with forebrain function without claiming immediate anatomical transformation. Myelin and nodes concern axons and Schwann cells, not dendrites, and pump inhibition does not create new neural tissue types. The effect develops through loss of ionic maintenance rather than direct destruction of the cells. As the gradients weaken, repeated electrical signalling and hormone release become progressively less reliable even before the cells show gross structural damage.
213. A pathway contains an electrical synapse followed by a chemical synapse. A treatment prevents neurotransmitter release but does not alter direct current flow or axonal conduction. After stimulation, what pattern is expected?
ⓐ. transmission crosses the electrical synapse but fails at the chemical synapse
ⓑ. transmission fails first at the electrical synapse and then resumes chemically
ⓒ. both synapses transmit normally since current reaches the first junction
ⓓ. direct current at the first junction bypasses the later release block
Correct Answer: transmission crosses the electrical synapse but fails at the chemical synapse
Explanation: Direct electrical transmission does not depend on neurotransmitter release. Current can still pass across the first junction through closely placed membranes under the stated comparison. The second junction is chemical and requires release of neurotransmitter from pre-synaptic vesicles. Blocking that release prevents the messenger from entering the cleft, binding post-synaptic receptors and opening ion channels. The signal reaches the chemical junction but cannot produce its normal post-synaptic effect. The treatment does not automatically change one synapse type into another, since electrical and chemical junctions differ structurally as well as functionally. The outcome localises failure through mechanism: transmitter-independent transfer remains, while transmitter-dependent transfer is interrupted. This distinction is visible only when the two synapse types occur sequentially in the same tested route. Preservation of direct current transfer acts as an internal control, showing that the treatment selectively affects the transmitter-dependent junction rather than all neural communication.
214. Arrange the following events in a complete sensory-to-response pathway.
P. A peripheral stimulus initiates sodium-dependent depolarisation in an afferent neuron.
Q. The afferent action potential reaches a pre-synaptic terminal in the CNS.
R. Neurotransmitter binds receptors and changes the next neuron’s membrane potential.
S. Central processing organises regulatory output.
T. An efferent impulse reaches the appropriate peripheral target.
ⓐ. Q, P, R, T, S
ⓑ. P, R, Q, S, T
ⓒ. P, Q, R, S, T
ⓓ. P, Q, S, T, R
Correct Answer: P, Q, R, S, T
Explanation: The sequence begins when the peripheral stimulus produces sodium-dependent depolarisation in the afferent neuron at P. The resulting action potential travels towards the CNS and reaches the pre-synaptic terminal at Q. Chemical transmission then occurs, and neurotransmitter binding changes the membrane potential of the receiving neuron at R. This transfer contributes to central processing, after which an appropriate regulatory output is organised at S. The command finally travels through an efferent pathway and reaches its peripheral target at T. The order nests membrane events, synaptic transmission and system-level information flow within one route. Central output cannot be organised from the stated stimulus before the afferent signal and its synaptic transfer have entered the processing network, and the target response cannot precede the outward command. The order does not imply that one synapse alone performs all central processing; it identifies the first chemical transfer before organised output is produced.
215. The table presents four lesion diagnoses.
| Record | Damage | Predicted effect |
|---|
| P | Most dendrites of a neuron are destroyed | Reception and inward conduction towards the soma are reduced |
| Q | Myelin is removed from an axon | Saltatory conduction becomes faster |
| R | Post-synaptic receptors are blocked | Transmitter may be released, but the receiving response is reduced |
| S | Hypothalamic homeostatic centres are damaged | Temperature, hunger and thirst regulation may be disturbed |
Which records are consistent with the affected structure?
ⓐ. P and Q only
ⓑ. P, R and S only
ⓒ. P, Q and S only
ⓓ. Q and R only
Correct Answer: P, R and S only
Explanation: Record P correctly links dendritic damage with reduced reception and impaired conduction of incoming information towards the cell body. Record R also preserves the mechanism of chemical transmission. Neurotransmitter release may occur normally, yet blocked post-synaptic receptors prevent normal receptor-linked ion-channel activation and reduce the receiving potential. Record S correctly connects hypothalamic damage with disturbed regulation of temperature, hunger and thirst. Record Q reverses the role of myelin. Removing the sheath disrupts node-to-node saltatory conduction and makes propagation less efficient rather than faster. The valid set spans neuronal structure, synaptic transmission and brain-region function, but every row is evaluated through the same diagnostic operation: identify what the damaged structure normally contributes and predict the most direct loss. Record Q reverses the effect of demyelination. Removing myelin reduces efficient saltatory conduction rather than accelerating it, leaving P, R and S as the consistent lesion-effect relations.
216. During an exercise-related neural response, information from peripheral tissues reaches the CNS, action potentials travel along neurons, chemical synapses transfer signals and central output adjusts body functions. Consider the following statements.
I. Peripheral-to-central information travels through afferent fibres.
II. Depolarisation depends on \(\mathrm{Na^+}\) influx, while repolarisation involves \(\mathrm{K^+}\) efflux.
III. Chemical transmission proceeds from pre-synaptic release to post-synaptic receptor activation.
IV. Respiratory regulation is assigned primarily to the cerebellum rather than the medulla.
ⓐ. I and IV only
ⓑ. I, II, III and IV
ⓒ. II, III and IV only
ⓓ. I, II and III only
Correct Answer: I, II and III only
Explanation: Statement I correctly identifies the inward information route from peripheral tissues to central processing. Statement II preserves the linked ion movements underlying neuronal excitation and recovery: sodium enters during depolarisation, while potassium leaves during repolarisation. Statement III gives the functional direction of a chemical synapse, beginning with transmitter release at the pre-synaptic terminal and continuing through receptors on the post-synaptic membrane. Statement IV assigns a medullary function to the wrong hindbrain structure. The cerebellum contributes to coordinated balance and sensory integration, whereas the medulla contains centres involved in respiratory control. The valid statements form a connected route from input through membrane conduction and synaptic transfer. The rejected statement concerns the central regulatory site that would organise part of the physiological response.
217. Physiological records are taken before, during and after vigorous exercise.
| Stage | Muscular activity | Respiratory and cardiovascular activity | Renal activity |
|---|
| P | Resting | Resting pattern | Resting pattern |
| Q | High | Adjusted to increased demand | Altered from resting pattern |
| R | Exercise stopped | Moving gradually towards baseline | Moving gradually towards baseline |
The complete record supports the inference that:
ⓐ. muscular work changes alone, while renal activity remains physiologically independent
ⓑ. supporting organs return instantly to baseline as soon as muscular work stops
ⓒ. respiratory, cardiovascular and renal activities adjust with demand and recover gradually
ⓓ. renal adjustment begins only after breathing and circulation have fully recovered
Correct Answer: respiratory, cardiovascular and renal activities adjust with demand and recover gradually
Explanation: Stage P supplies the resting reference. At Stage Q, muscular work is high, respiratory and cardiovascular activities have adjusted to the increased demand, and renal activity has also changed from its resting pattern. The response extends beyond the muscles, lungs and heart. Stage R adds the recovery evidence: exercise has stopped, yet the supporting organ systems move gradually rather than instantaneously towards their resting activities. This pattern illustrates biological coordination as complementary adjustment among several organs followed by organised restoration when the temporary demand ends. The table does not specify that every participating organ changes in the same direction or by the same amount. It shows that muscular, respiratory, cardiovascular and renal activities participate in one synchronised physiological response. The three stages establish both recruitment and recovery. A single exercise row would show association, but the resting and post-exercise rows reveal coordinated change over time.
218. A target muscle receives a rapid signal through a specified nerve, while a hormone released during the same physiological response contributes to a broader adjustment in several target tissues. The observations indicate that:
ⓐ. neural and endocrine mechanisms provide complementary components of one coordinated response
ⓑ. both signals are neural mechanisms acting through separate point-to-point pathways
ⓒ. both signals are endocrine mechanisms acting through circulating chemical messengers
ⓓ. endocrine signalling produces the rapid local command and neural signalling the distributed adjustment
Correct Answer: neural and endocrine mechanisms provide complementary components of one coordinated response
Explanation: The rapid signal travelling through a specified nerve has the defining point-to-point character of neural coordination. The second signal is explicitly a hormone and contributes to integration across several target tissues, identifying an endocrine mechanism. Their participation in the same physiological response does not make their routes identical. Instead, the two systems contribute complementary forms of control: a neural pathway can deliver a rapid command to a particular target, while hormonal communication can support broader integration through chemical messengers. The evidence also prevents an either-or interpretation in which only one system is capable of coordination. Shared purpose must be separated from communication mode. Neural and endocrine mechanisms can cooperate in maintaining organised body function while retaining their distinct routes and signalling agents. Removal of either route would therefore reduce a different component of the response rather than proving that the remaining mechanism is the sole coordinator.
219. Select the classification that preserves the hierarchy of the human neural system.
ⓐ. Sympathetic and parasympathetic are subdivisions of somatic output to skeletal muscle.
ⓑ. Autonomic and somatic are subdivisions of the sympathetic neural system.
ⓒ. Visceral and spinal are the two subdivisions of all autonomic output.
ⓓ. Sympathetic and parasympathetic are subdivisions of the autonomic neural system.
Correct Answer: Sympathetic and parasympathetic are subdivisions of the autonomic neural system.
Explanation: The autonomic neural system carries regulatory impulses from the CNS towards involuntary organs and smooth muscles. Within this division, sympathetic and parasympathetic neural systems form the two stated subdivisions. Somatic output belongs at a different classification level and is associated with skeletal muscles. Visceral communication describes the wider network of nerves, fibres, ganglia and plexuses carrying impulses between the CNS and viscera in both directions; it is not one of the two autonomic subdivisions named here. The hierarchy must preserve equal-level categories. Sympathetic and parasympathetic belong together beneath autonomic, whereas somatic and autonomic distinguish target-based forms of CNS output. No specific organ effects need to be imported to establish this classification.
220. Investigators record impulses travelling from the CNS to smooth muscle in an involuntary organ. The available observations do not show the detailed effect produced by the pathway. The strongest justified classification is:
ⓐ. It is sympathetic efferent, with smooth muscle proving the sympathetic subdivision.
ⓑ. It is autonomic efferent, with the subdivision unresolved by the observations.
ⓒ. It is parasympathetic efferent, with CNS origin proving the parasympathetic subdivision.
ⓓ. It is visceral afferent, with the involuntary organ proving direction towards the CNS.
Correct Answer: It is autonomic efferent, with the subdivision unresolved by the observations.
Explanation: The recorded impulse travels from the CNS towards a peripheral organ, establishing the efferent direction. Its target is smooth muscle in an involuntary organ, which places the pathway in the autonomic neural system. These observations support both parts of the classification autonomic efferent. They do not, however, distinguish between the sympathetic and parasympathetic subdivisions. Making that additional decision would require relevant evidence not supplied in the experiment. The pathway cannot be called afferent since its direction is away from the CNS. The result demonstrates an important limit of inference: direction and target identify the broad pathway, but they do not automatically reveal every lower subdivision. Accurate classification stops where the stated evidence stops. Further classification as sympathetic or parasympathetic would require additional pathway evidence not contained in the recorded direction and target.
