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ABPN NDD practice questions

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Question 1General Neurology and Neuroscience Foundations

Which neurotransmitter is released at the neuromuscular junction to activate skeletal muscle?

  1. Acetylcholine Correct answer.
  2. GABA
  3. Dopamine
  4. Glutamate

Explanation

Motor neurons release acetylcholine, which binds nicotinic receptors on the motor end plate to trigger muscle depolarization. The other listed transmitters act primarily within the central nervous system.

Question 2General Neurology and Neuroscience Foundations

Damage to the left optic nerve before the chiasm produces which visual field defect?

  1. Bitemporal hemianopia
  2. Left homonymous hemianopia
  3. Right homonymous hemianopia
  4. Left monocular blindness Correct answer.

Explanation

A lesion of one optic nerve interrupts all fibers from that eye, causing blindness in that eye. Chiasmal and retrochiasmal lesions instead produce characteristic bilateral field cuts.

Question 3General Neurology and Neuroscience Foundations

The cell bodies of upper motor neurons that form the corticospinal tract are located primarily in which region?

  1. Cerebellar cortex
  2. Primary motor cortex Correct answer.
  3. Thalamus
  4. Basal ganglia

Explanation

Corticospinal fibers arise mainly from pyramidal neurons in the primary motor cortex and premotor areas. They descend through the internal capsule and brainstem to the spinal cord.

Question 4General Neurology and Neuroscience Foundations

A patient has a lesion of the right cranial nerve III. Which finding is expected?

  1. Right eye deviated down and out with ptosis Correct answer.
  2. Left eye deviated down and out
  3. Bilateral loss of the pupillary light reflex only
  4. Right eye deviated up and in with ptosis

Explanation

The oculomotor nerve innervates most extraocular muscles plus the levator palpebrae; loss leaves the eye pulled by the intact lateral rectus and superior oblique, producing a down-and-out position with ptosis.

Question 5General Neurology and Neuroscience Foundations

Which structure is the primary relay station for sensory pathways en route to the cerebral cortex?

  1. Amygdala
  2. Thalamus Correct answer.
  3. Hypothalamus
  4. Hippocampus

Explanation

Most ascending sensory pathways synapse in the thalamus before projecting to cortical areas. The hypothalamus, amygdala, and hippocampus serve autonomic, emotional, and memory functions.

Question 6General Neurology and Neuroscience Foundations

Cerebrospinal fluid is primarily produced by which structure?

  1. Ependymal cells of the spinal canal
  2. Choroid plexus Correct answer.
  3. Pia mater
  4. Arachnoid granulations

Explanation

The choroid plexus within the ventricles secretes most CSF. Arachnoid granulations resorb CSF rather than produce it.

Question 7General Neurology and Neuroscience Foundations

A lesion of the right cerebellar hemisphere most typically causes which finding?

  1. Loss of position sense in the feet
  2. Bilateral resting tremor
  3. Left-sided limb ataxia
  4. Right-sided limb ataxia Correct answer.

Explanation

The cerebellar hemispheres control the ipsilateral limbs, so a right hemisphere lesion produces right-sided ataxia. Resting tremor and sensory loss reflect other systems.

Question 8General Neurology and Neuroscience Foundations

Which artery is most commonly involved in a lesion causing contralateral hemiparesis and hemisensory loss with aphasia?

  1. Anterior spinal artery
  2. Posterior cerebral artery
  3. Middle cerebral artery Correct answer.
  4. Posterior inferior cerebellar artery

Explanation

Middle cerebral artery territory includes the lateral cortex, motor and sensory strips, and language areas, so occlusion can cause contralateral deficits with aphasia. The posterior circulation arteries supply different territories.

Question 9General Neurology and Neuroscience Foundations

The blood-brain barrier is formed primarily by which feature of cerebral capillaries?

  1. Fenestrations in endothelial cells
  2. Absence of pericytes
  3. Tight junctions between endothelial cells Correct answer.
  4. Open gaps between astrocytes

Explanation

Cerebral endothelial cells are joined by tight junctions, supported by pericytes and astrocyte end-feet, creating a restrictive barrier. Fenestrated endothelium is characteristic of circumventricular organs and peripheral capillaries.

Question 10General Neurology and Neuroscience Foundations

A patient has a lesion of the left dorsal columns at the cervical cord. Which finding is expected?

  1. Loss of vibration and position sense in the left arm and leg Correct answer.
  2. Loss of pain and temperature sense in the right arm and leg
  3. Loss of pain and temperature sense in the left arm and leg
  4. Spastic weakness of the left leg alone

Explanation

Dorsal columns carry ipsilateral vibration and proprioception and ascend without crossing until the medulla, so a left cervical lesion impairs these modalities on the left. Pain and temperature travel in the spinothalamic tract.

Question 11General Neurology and Neuroscience Foundations

Which cranial nerve carries taste from the anterior two-thirds of the tongue?

  1. Facial nerve Correct answer.
  2. Trigeminal nerve
  3. Glossopharyngeal nerve
  4. Hypoglossal nerve

Explanation

The facial nerve carries taste from the anterior two-thirds of the tongue via the chorda tympani. The glossopharyngeal nerve carries taste from the posterior third.

Question 12General Neurology and Neuroscience Foundations

A patient develops sudden severe headache with a dilated unreactive pupil and a painful third nerve palsy. Which mechanism best explains the pupil involvement?

  1. Lesion of the optic nerve
  2. Ischemia of the nerve core
  3. Loss of sympathetic tone to the iris
  4. Compression of parasympathetic fibers on the nerve surface Correct answer.

Explanation

Pupillary parasympathetic fibers travel on the superficial aspect of the oculomotor nerve, so compressive lesions such as a posterior communicating artery aneurysm affect them early. Ischemic diabetic palsies tend to spare the pupil.

Question 13General Neurology and Neuroscience Foundations

A lesion of the medial longitudinal fasciculus produces which examination finding?

  1. Impaired adduction of the eye on the side of the lesion with nystagmus of the abducting eye Correct answer.
  2. Downward deviation of one eye at rest
  3. Bilateral ptosis with normal eye movements
  4. Impaired abduction of the eye on the side of the lesion

Explanation

The MLF coordinates conjugate horizontal gaze; a lesion causes internuclear ophthalmoplegia, with impaired adduction ipsilaterally and abducting nystagmus. This reflects disruption of the pathway between abducens and oculomotor nuclei.

Question 14General Neurology and Neuroscience Foundations

Which basal ganglia pathway change is thought to underlie the hypokinesia of Parkinson disease?

  1. Decreased activity of the indirect pathway
  2. Increased dopaminergic input to the striatum
  3. Increased activity of the indirect pathway Correct answer.
  4. Loss of cortical motor neurons

Explanation

Loss of nigrostriatal dopamine reduces inhibition of the indirect pathway, increasing its activity and suppressing movement. This yields the hypokinesia and rigidity of Parkinson disease.

Question 15General Neurology and Neuroscience Foundations

Which finding localizes a lesion to the peripheral rather than central nervous system?

  1. Hyperreflexia with an extensor plantar response
  2. Unilateral cortical sensory loss
  3. Fasciculations with reduced reflexes and muscle atrophy Correct answer.
  4. Homonymous hemianopia

Explanation

Lower motor neuron lesions cause fasciculations, atrophy, and hyporeflexia, indicating peripheral involvement. Upper motor neuron and cortical findings point to central lesions.

Question 16General Neurology and Neuroscience Foundations

A patient has a lesion of the right hypoglossal nerve. Which finding is expected?

  1. Loss of taste on the right side of the tongue
  2. Tongue deviates to the left on protrusion
  3. Right-sided facial weakness
  4. Tongue deviates to the right on protrusion Correct answer.

Explanation

The genioglossus muscle pushes the tongue forward; weakness on the right lets the intact left genioglossus deviate the tongue to the right. Taste and facial movement are served by other nerves.

Question 17General Neurology and Neuroscience Foundations

Which type of nerve fiber is most susceptible to compression and ischemia?

  1. Small unmyelinated fibers
  2. Central axons within the spinal cord
  3. Preganglionic autonomic fibers only
  4. Large myelinated fibers Correct answer.

Explanation

Large myelinated fibers have high metabolic demand and are preferentially affected by compressive or ischemic injury, which is why vibration and proprioception are lost early. Small fibers are relatively spared in such lesions.

Question 18General Neurology and Neuroscience Foundations

Which structure connects the third ventricle to the fourth ventricle?

  1. Foramen of Monro
  2. Foramen of Luschka
  3. Cerebral aqueduct Correct answer.
  4. Central canal

Explanation

The cerebral aqueduct passes through the midbrain to link the third and fourth ventricles. Obstruction here causes noncommunicating hydrocephalus.

Question 19General Neurology and Neuroscience Foundations

A lesion of the right optic tract produces which visual field defect?

  1. Left monocular blindness
  2. Right homonymous hemianopia
  3. Left homonymous hemianopia Correct answer.
  4. Bitemporal hemianopia

Explanation

The optic tract carries fibers from the temporal retina of the ipsilateral eye and the nasal retina of the contralateral eye, representing the contralateral visual field. A right tract lesion therefore causes a left homonymous hemianopia.

Question 20General Neurology and Neuroscience Foundations

Which receptor type mediates fast excitatory synaptic transmission in the CNS?

  1. Muscarinic acetylcholine receptors
  2. Ionotropic glutamate receptors Correct answer.
  3. Adrenergic alpha-2 receptors
  4. Metabotropic GABA-B receptors

Explanation

Ionotropic glutamate receptors such as AMPA and NMDA open cation channels directly, producing rapid excitation. Metabotropic receptors act through second messengers and are slower.

Question 21General Neurology and Neuroscience Foundations

A patient with a right-sided lateral medullary infarction is expected to have which combination?

  1. Bilateral visual loss with a dilated pupil
  2. Ipsilateral facial sensory loss with contralateral body sensory loss Correct answer.
  3. Ipsilateral hemiparesis with contralateral facial weakness
  4. Contralateral tongue deviation with ipsilateral hemiparesis

Explanation

Lateral medullary syndrome affects the spinal trigeminal nucleus ipsilaterally and the spinothalamic tract contralaterally, producing crossed sensory findings. Other structures such as the pyramidal tract are typically spared.

Question 22General Neurology and Neuroscience Foundations

The primary somatosensory cortex is located in which gyrus?

  1. Postcentral gyrus Correct answer.
  2. Cingulate gyrus
  3. Precentral gyrus
  4. Superior temporal gyrus

Explanation

The postcentral gyrus of the parietal lobe houses the primary somatosensory cortex. The precentral gyrus contains the primary motor cortex.

Question 23General Neurology and Neuroscience Foundations

Which change in ion conductance produces the upstroke of the neuronal action potential?

  1. Rapid potassium influx
  2. Calcium efflux
  3. Chloride influx
  4. Rapid sodium influx Correct answer.

Explanation

Voltage-gated sodium channels open at threshold, allowing rapid sodium entry that depolarizes the membrane. Repolarization follows from potassium efflux.

Question 24General Neurology and Neuroscience Foundations

A patient has weakness of the right face and right arm with spared right leg strength, plus a right homonymous hemianopia and left hemineglect. Which lesion location best fits?

  1. Right posterior cerebral artery territory
  2. Right middle cerebral artery territory Correct answer.
  3. Left internal capsule lacune
  4. Left anterior cerebral artery territory

Explanation

Right MCA territory involves the lateral frontal and parietal cortex, giving contralateral face and arm weakness, a contralateral visual field cut, and neglect. Sparing of the leg reflects the medial ACA territory.

Question 25General Neurology and Neuroscience Foundations

Which neurotransmitter is deficient in the nigrostriatal pathway in Parkinson disease?

  1. Acetylcholine
  2. Serotonin
  3. Dopamine Correct answer.
  4. Norepinephrine

Explanation

Degeneration of substantia nigra pars compacta neurons reduces dopamine in the striatum, producing the motor features of Parkinson disease. Other transmitter systems are not the primary deficit.

Question 26General Neurology and Neuroscience Foundations

Which cells produce myelin in the central nervous system?

  1. Oligodendrocytes Correct answer.
  2. Schwann cells
  3. Ependymal cells
  4. Astrocytes

Explanation

Oligodendrocytes extend multiple processes that wrap axons in the CNS, forming myelin. Schwann cells perform the analogous role in the peripheral nervous system, while astrocytes and ependymal cells serve other support functions.

Question 27General Neurology and Neuroscience Foundations

The primary motor cortex is located in which gyrus?

  1. Superior temporal gyrus
  2. Cingulate gyrus
  3. Precentral gyrus Correct answer.
  4. Postcentral gyrus

Explanation

The primary motor cortex occupies the precentral gyrus of the frontal lobe, just anterior to the central sulcus. The postcentral gyrus houses primary somatosensory cortex, and the other listed gyri serve auditory or limbic functions.

Question 28General Neurology and Neuroscience Foundations

Which cranial nerve mediates the gag reflex afferent limb from the posterior pharynx?

  1. Trigeminal nerve
  2. Hypoglossal nerve
  3. Glossopharyngeal nerve Correct answer.
  4. Facial nerve

Explanation

Sensory fibers of the glossopharyngeal nerve (CN IX) carry afferent input from the posterior pharynx that triggers the gag reflex, with the vagus nerve supplying the motor efferent limb. The other listed nerves serve taste, facial sensation, or tongue movement.

Question 29General Neurology and Neuroscience Foundations

Which spinal cord tract carries fine touch and vibration sensation from the legs?

  1. Spinothalamic tract
  2. Dorsal spinocerebellar tract
  3. Lateral corticospinal tract
  4. Fasciculus gracilis Correct answer.

Explanation

The fasciculus gracilis is the medial portion of the dorsal columns and carries fine touch and vibration from the lower limbs. The corticospinal tract is motor, the spinothalamic tract carries pain and temperature, and the spinocerebellar tract is unconscious proprioceptive.

Question 30General Neurology and Neuroscience Foundations

Which structure produces the hormone melatonin?

  1. Pineal gland Correct answer.
  2. Adrenal medulla
  3. Pituitary gland
  4. Thyroid gland

Explanation

The pineal gland synthesizes melatonin, which helps regulate circadian rhythms. The pituitary, adrenal medulla, and thyroid produce different hormones.

Question 31General Neurology and Neuroscience Foundations

Which cells form the myelin sheath around peripheral axons?

  1. Microglia
  2. Astrocytes
  3. Schwann cells Correct answer.
  4. Oligodendrocytes

Explanation

Each Schwann cell wraps a single segment of a peripheral axon to form myelin. Oligodendrocytes myelinate central axons, and microglia and astrocytes are nonmyelinating glia.

Question 32General Neurology and Neuroscience Foundations

Which lobe of the brain contains the primary visual cortex?

  1. Parietal lobe
  2. Frontal lobe
  3. Temporal lobe
  4. Occipital lobe Correct answer.

Explanation

Primary visual cortex lies along the calcarine fissure in the occipital lobe. The parietal, temporal, and frontal lobes subserve somatosensory, auditory, and motor functions respectively.

Question 33General Neurology and Neuroscience Foundations

A patient has a lesion of the left optic nerve just anterior to the chiasm. Which visual field deficit is expected?

  1. Bitemporal hemianopia
  2. Left homonymous hemianopia
  3. Complete loss of vision in the left eye Correct answer.
  4. Right homonymous hemianopia

Explanation

A lesion of one optic nerve before the chiasm destroys all axons from that eye, producing monocular blindness on the affected side. Chiasmal lesions produce bitemporal defects, and postchiasmal lesions produce homonymous defects.

Question 34General Neurology and Neuroscience Foundations

A patient has a lesion destroying the left side of the cerebellum. Which finding is most expected?

  1. Bilateral loss of pain and temperature
  2. Global aphasia
  3. Right-sided spastic hemiparesis
  4. Left-sided limb ataxia and intention tremor Correct answer.

Explanation

The cerebellar hemispheres control the ipsilateral limbs, so a left cerebellar lesion causes left-sided incoordination and intention tremor. Spastic hemiparesis and sensory loss reflect corticospinal or spinothalamic lesions, and aphasia is cortical.

Question 35General Neurology and Neuroscience Foundations

A patient develops sudden hearing loss and right facial weakness with loss of taste on the anterior tongue and hyperacusis. Which cranial nerve is affected?

  1. Trigeminal nerve
  2. Vestibulocochlear nerve
  3. Facial nerve Correct answer.
  4. Glossopharyngeal nerve

Explanation

The facial nerve carries motor fibers to facial muscles, taste from the anterior tongue via the chorda tympani, and motor fibers to the stapedius muscle; stapedius paralysis causes hyperacusis. The other nerves serve facial sensation, hearing/balance, or posterior tongue sensation.

Question 36General Neurology and Neuroscience Foundations

A lesion of the left internal capsule posterior limb most likely produces which pattern of weakness?

  1. Contralateral face and limb weakness Correct answer.
  2. Isolated ipsilateral hand weakness
  3. Ipsilateral face and limb weakness
  4. Bilateral lower limb weakness only

Explanation

Corticospinal and corticobulbar fibers descend through the posterior limb of the internal capsule, so a left-sided lesion causes right-sided (contralateral) face and limb weakness. Ipsilateral or isolated distal weakness does not fit this compact tract lesion.

Question 37General Neurology and Neuroscience Foundations

A patient has a lesion of the right oculomotor nerve. Which finding is expected on examination?

  1. Bilateral failure of abduction
  2. Right eye deviated down and out with ptosis and a dilated pupil Correct answer.
  3. Right eye deviated inward with a constricted pupil
  4. Failure of upward gaze with normal pupil size

Explanation

The oculomotor nerve supplies most extraocular muscles and the levator palpebrae, and its parasympathetic fibers constrict the pupil. Loss produces a down-and-out eye, ptosis, and a dilated unreactive pupil.

Question 38General Neurology and Neuroscience Foundations

Which brainstem structure contains the cell bodies of the descending corticospinal fibers before they cross?

  1. Inferior cerebellar peduncle
  2. Superior colliculus
  3. Mammillary body
  4. Medullary pyramids Correct answer.

Explanation

Corticospinal fibers form the medullary pyramids and cross at the pyramidal decussation just below. The peduncle, colliculus, and mammillary body serve cerebellar, visual, and limbic functions.

Question 39General Neurology and Neuroscience Foundations

A patient has weakness of the left leg with preserved arm strength and a Babinski sign on the left. Which location best explains this finding?

  1. Left brachial plexus
  2. Right cerebellar hemisphere
  3. Right medial frontal cortex near the longitudinal fissure Correct answer.
  4. Left lateral frontal cortex

Explanation

Leg motor fibers are represented medially on the motor homunculus near the longitudinal fissure, so a right medial frontal lesion causes contralateral leg-predominant weakness. Lateral frontal lesions affect the face and arm, and the other options do not produce an upper motor neuron leg pattern.

Question 40General Neurology and Neuroscience Foundations

Which finding best distinguishes an upper motor neuron lesion from a lower motor neuron lesion?

  1. Reduced tone with hyporeflexia
  2. Fibrillations on needle electromyography
  3. Hyperreflexia with spasticity Correct answer.
  4. Fasciculations with muscle atrophy

Explanation

Upper motor neuron lesions increase tone and reflexes and produce spasticity, whereas lower motor neuron lesions cause flaccidity, hyporeflexia, atrophy, and spontaneous muscle fiber activity. Fasciculations, fibrillations, and reduced tone all point to lower motor neuron disease.

Question 41General Neurology and Neuroscience Foundations

A patient has loss of pain and temperature sensation on the right side of the body beginning two segments below a spinal cord lesion, with preserved vibration sense. Which tract is affected?

  1. Right spinothalamic tract Correct answer.
  2. Right dorsal columns
  3. Right dorsal spinocerebellar tract
  4. Left corticospinal tract

Explanation

The spinothalamic tract carries pain and temperature and crosses within the cord, so a lesion interrupts fibers that have already crossed and produces contralateral loss beginning a couple of segments below the lesion. The dorsal columns carry vibration, the corticospinal tract carries motor fibers, and the spinocerebellar tract is proprioceptive.

Question 42General Neurology and Neuroscience Foundations

Which structure is the main relay for sensory pathways traveling to the cerebral cortex?

  1. Basal ganglia
  2. Hippocampus
  3. Thalamus Correct answer.
  4. Hypothalamus

Explanation

Nearly all sensory pathways except olfaction synapse in the thalamus before reaching cortex. The hypothalamus regulates homeostasis, the basal ganglia modulate movement, and the hippocampus is critical for memory.

Question 43General Neurology and Neuroscience Foundations

Which change in ion conductance generates the downstroke (repolarization) of the neuronal action potential?

  1. Increased sodium conductance
  2. Increased potassium conductance with inactivation of sodium channels Correct answer.
  3. Increased calcium conductance
  4. Increased chloride conductance

Explanation

Repolarization results from voltage-gated sodium channel inactivation combined with opening of voltage-gated potassium channels, allowing potassium efflux. Sodium influx drives the upstroke, while chloride and calcium currents do not generate the repolarization phase.

Question 44General Neurology and Neuroscience Foundations

A patient has a lesion of the left frontal eye field. Which finding is expected?

  1. Inability to voluntarily look to the left
  2. Complete bilateral ptosis
  3. Loss of the pupillary light reflex
  4. Inability to voluntarily look to the right Correct answer.

Explanation

The frontal eye field drives conjugate gaze to the contralateral side, so a left frontal lesion impairs voluntary gaze to the right. Ptosis and pupillary reflexes are mediated by the oculomotor nerve and brainstem circuits, not the frontal eye field.

Question 45General Neurology and Neuroscience Foundations

A patient has a lesion of the left medial longitudinal fasciculus. Which finding is expected on attempted rightward gaze?

  1. The left eye abducts but the right eye fails to adduct
  2. Both eyes fail to abduct
  3. The right eye abducts but the left eye fails to adduct Correct answer.
  4. Both eyes fail to adduct on convergence

Explanation

The medial longitudinal fasciculus carries signals from the abducens nucleus to the contralateral medial rectus subnucleus for conjugate gaze. A left MLF lesion impairs adduction of the left eye on rightward gaze while the right eye abducts with nystagmus.

Question 46General Neurology and Neuroscience Foundations

A lesion at the level of the left cerebral peduncle involving the corticospinal tract and oculomotor nerve produces which syndrome?

  1. Ipsilateral third nerve palsy with ipsilateral hemiparesis
  2. Ipsilateral third nerve palsy with contralateral hemiparesis Correct answer.
  3. Contralateral third nerve palsy with ipsilateral hemiparesis
  4. Bilateral third nerve palsy with quadriparesis

Explanation

Weber syndrome results from a midbrain peduncular lesion affecting the oculomotor nerve fascicles and corticospinal fibers. The cranial nerve deficit is ipsilateral while the corticospinal deficit is contralateral because those fibers cross lower in the neuraxis.

Question 47General Neurology and Neuroscience Foundations

A patient has a lesion of the left parietal cortex and demonstrates neglect of the right side of space and the right side of the body. Which function is most impaired?

  1. Primary somatosensory detection threshold
  2. Spatial attention to the contralateral hemifield Correct answer.
  3. Primary visual acuity
  4. Voluntary movement of the left limbs

Explanation

The right hemisphere, particularly the parietal lobe, is dominant for spatial attention, and lesions cause contralateral hemispatial neglect. Visual acuity, primary sensation, and left-sided motor strength are not the functions most directly impaired by this attentional deficit.

Question 48General Neurology and Neuroscience Foundations

A patient has a lesion of the left hippocampus. Which deficit is most likely?

  1. Impaired recognition of familiar faces
  2. Impaired formation of new declarative memories Correct answer.
  3. Impaired production of fluent speech
  4. Impaired performance of learned motor skills

Explanation

The hippocampus is essential for consolidating new declarative (episodic and semantic) memories, so bilateral or dominant-side damage impairs anterograde declarative memory. Fluent speech, face recognition, and motor skill learning depend on other cortical and subcortical regions.

Question 49General Neurology and Neuroscience Foundations

A patient has a lesion of the left optic radiation in the temporal lobe. Which visual field defect is expected?

  1. Left superior quadrantanopia
  2. Right superior quadrantanopia Correct answer.
  3. Bitemporal hemianopia
  4. Right inferior quadrantanopia

Explanation

Meyer loop fibers pass through the temporal lobe and carry information from the inferior retina, which corresponds to the superior visual field. A left temporal radiation lesion therefore produces a right superior quadrantanopia.

Question 50General Neurology and Neuroscience Foundations

A 62-year-old man has a resting tremor, cogwheel rigidity, and bradykinesia. Degeneration of which neuronal population is most directly responsible for these motor findings?

  1. Cholinergic neurons of the pedunculopontine nucleus
  2. Glutamatergic neurons of the subthalamic nucleus
  3. GABAergic neurons of the globus pallidus externa
  4. Dopaminergic neurons of the substantia nigra pars compacta Correct answer.

Explanation

The cardinal motor features of Parkinson disease arise from loss of dopaminergic input from the substantia nigra pars compacta to the striatum, which releases the indirect pathway from inhibition and increases thalamic inhibition. This produces the hypokinetic-rigid syndrome of tremor, rigidity, and bradykinesia.