Past Questions - Pathways Flashcards
(15 cards)
Indirect pathway of motor control with neurochemical properties of basal ganglia
The indirect pathway of motor control suppresses unwanted movement by increasing thalamic inhibition. The cortex excites the striatum, which inhibits the globus pallidus externus (GPe), reducing its inhibition on the subthalamic nucleus (STN). The STN then excites the globus pallidus internus (GPi) and substantia nigra pars reticulata (SNr), which strongly inhibit the thalamus, decreasing motor output. Neurochemically, glutamate (excitatory) is used by the cortex and STN, GABA (inhibitory) by the striatum, GPe, GPi, and SNr, and dopamine (D2 receptors) from SNc inhibits this pathway, facilitating movement. Dysfunction leads to Parkinson’s (excess inhibition) or Huntington’s (reduced inhibition).
A patient presents a lesion of the anterior funiculus of the spinal cord. Name and describe the pathways that could be possibly damaged by the lesion.
A lesion in the anterior funiculus of the spinal cord can damage both motor and sensory pathways. The main affected pathways include the anterior corticospinal tract, which controls voluntary movements of axial and proximal muscles, and the medial and lateral reticulospinal tracts, which regulate posture and muscle tone. Additionally, the vestibulospinal tracts (medial and lateral) may be affected, leading to impaired balance and coordination. Sensory deficits can arise from damage to the anterior spinothalamic tract, responsible for crude touch and pressure sensation. Since many of these tracts are bilaterally innervated, deficits may be partial or compensated by the unaffected side.
Explain the indirect pathways of basal nuclei. A patient has a lesion in subthalamic nucleus explain the symptoms he will have.
The indirect pathway of the basal nuclei inhibits unwanted movements by increasing thalamic inhibition. The cortex excites the striatum, which inhibits the globus pallidus externus (GPe), reducing its inhibition on the subthalamic nucleus (STN). The STN then excites the globus pallidus internus (GPi) and substantia nigra pars reticulata (SNr), which strongly inhibit the thalamus, decreasing motor output.
A lesion in the STN disrupts this pathway, reducing excitation of the GPi/SNr, leading to less inhibition of the thalamus. This causes excessive movement, seen as hemiballismus, a condition characterized by violent, involuntary, flinging movements of the limbs, usually affecting one side of the body (contralateral to the lesion).
Describe esopyramidal pathway
The extrapyramidal pathway refers to a set of motor pathways outside the pyramidal (corticospinal) tract, primarily involved in involuntary movement control, posture, muscle tone, and coordination. It includes several major tracts: the reticulospinal tract (regulates posture and muscle tone), the vestibulospinal tract (maintains balance and head position), the rubrospinal tract (facilitates flexor muscle activity), and the tectospinal tract (coordinates head and eye movements in response to stimuli). These pathways originate in the brainstem nuclei and are modulated by the basal ganglia and cerebellum. Dysfunction of the extrapyramidal system, such as in Parkinson’s disease, leads to movement disorders like rigidity, tremors, and bradykinesia.
Describe ventral spinothalamic pathway
The ventral spinothalamic pathway is an ascending sensory tract responsible for transmitting crude touch and pressure sensations to the brain. It originates from sensory neurons in the dorsal root ganglia, which synapse in the dorsal horn of the spinal cord. Second-order neurons then decussate (cross) at the spinal level and ascend in the ventral funiculus to the thalamus. Here, they synapse with third-order neurons, which project to the somatosensory cortex for perception. This pathway is part of the anterolateral system and works alongside the lateral spinothalamic tract (which transmits pain and temperature). Damage to this tract leads to contralateral loss of crude touch and pressure sensation below the lesion.
Describe the dorsal column pathway, and describe where it passes within the brainstem
The dorsal column pathway (medial lemniscus system) is an ascending sensory tract responsible for fine touch, vibration, and proprioception. It consists of the gracile fasciculus (carrying lower limb sensations) and the cuneate fasciculus (carrying upper limb sensations). First-order neurons travel from the dorsal root ganglia to the medulla oblongata, where they synapse in the gracile and cuneate nuclei. Second-order neurons then decussate (cross) in the medulla, forming the medial lemniscus, which ascends through the pons and midbrain to the thalamus. Third-order neurons project from the thalamus to the somatosensory cortex. A lesion in this pathway causes ipsilateral sensory loss below the lesion in the spinal cord but contralateral loss above the medulla due to decussation.
Describe spinocerebellar pathways
The spinocerebellar pathways are ascending sensory tracts that transmit proprioceptive information from muscles, tendons, and joints to the cerebellum, aiding in balance and coordination. There are four main tracts: the dorsal (posterior) spinocerebellar and cuneocerebellar tracts (which carry ipsilateral proprioceptive input from the lower and upper limbs, respectively), and the ventral (anterior) and rostral spinocerebellar tracts (which relay information about spinal cord interneuron activity). Unlike most sensory pathways, these tracts primarily do not decussate or re-cross to maintain ipsilateral cerebellar control. Damage leads to ataxia (lack of coordination) without sensory deficits, often seen in cerebellar disorders.
Describe anterior horn of the spinal cord
The anterior horn of the spinal cord, located in the gray matter, contains motor neurons that control voluntary muscle movements. It contains α-motor neurons, which innervate skeletal muscles, and γ-motor neurons, which regulate muscle spindle sensitivity for reflex control. The anterior horn is largest in the cervical and lumbar enlargements, where motor control of the limbs is concentrated. It receives input from descending motor pathways (e.g., corticospinal and extrapyramidal tracts) and transmits signals via the ventral roots to peripheral muscles. Damage to the anterior horn, as seen in polio or amyotrophic lateral sclerosis (ALS), causes flaccid paralysis, muscle atrophy, and hyporeflexia.
Describe the pathways of the anterior funiculus of spinal cord
The anterior funiculus of the spinal cord contains both ascending sensory and descending motor pathways. Major motor tracts include the anterior corticospinal tract, which controls voluntary movements of axial muscles, and the reticulospinal, vestibulospinal, and tectospinal tracts, which regulate posture, balance, and reflexive head movements. Sensory tracts in this region include the anterior spinothalamic tract, which carries crude touch and pressure sensations to the brain. Many of these tracts partially decussate (cross) either at the spinal level or in the brainstem. A lesion in the anterior funiculus can cause motor deficits, postural instability, and sensory impairments, often contralateral to the lesion.
A patient has a lesion at the level of the genu of the internal capsule, describe the pathway involved and the structures of the internal capsule
A lesion at the level of the genu of the internal capsule affects the corticobulbar tract, which carries motor signals from the motor cortex to the cranial nerve nuclei in the brainstem. The internal capsule is a white matter structure located between the thalamus and the basal ganglia, containing major motor and sensory tracts. The genu specifically contains the corticobulbar fibers that control cranial muscles, particularly those involved in facial movements, tongue, and swallowing. A lesion here would result in contralateral weakness or paralysis of the lower face, tongue, and muscles of mastication (due to involvement of the corticobulbar tract), with preserved upper face function (because of bilateral innervation of the upper face). Additionally, the posterior limb of the internal capsule carries the corticospinal tract (controlling body movements), so damage here could also result in contralateral motor deficits in the body.
Describe the perforant pathway of the hippocampus.
The perforant pathway is a key afferent pathway to the hippocampus, primarily involved in memory processing. It originates from the entorhinal cortex and projects through the perforant path to the dentate gyrus of the hippocampus. This pathway plays a critical role in sensory integration and spatial memory, as it carries cortical information to the hippocampus for further processing. The dentate gyrus then sends signals to the CA3 region of the hippocampus, which eventually relays information to the CA1 region and outputs it through the fornix to other brain structures. Disruption of the perforant pathway is implicated in memory disorders, such as Alzheimer’s disease.
Circuit in molecular layer of cerebellum and efferent pathways of cerebellum
The molecular layer of the cerebellum contains parallel fibers (axons of granule cells) that synapse with Purkinje cells, which are the principal inhibitory neurons of the cerebellar cortex. The climbing fibers (arising from the inferior olivary nucleus) also synapse on Purkinje cells, providing strong excitatory input that modulates their activity. This interaction between parallel fibers and climbing fibers is crucial for motor learning and coordination. The efferent pathways of the cerebellum transmit processed motor information via the deep cerebellar nuclei (dentate, interposed, and fastigial nuclei) to various brain regions. The dentate nucleus sends information to the thalamus (via the thalamocortical tract) to modulate voluntary motor control, while the fastigial and interposed nuclei influence the brainstem and spinal cord through vestibulospinal and reticulospinal tracts, playing a key role in maintaining balance and posture.
Describe the microscopic anatomy of the cerebral cortex and its cytoarchitecture
The cerebral cortex is the outermost layer of the brain, composed of gray matter that is involved in sensory processing, motor control, and higher cognitive functions. It has a characteristic six-layer structure, each with distinct types of neurons and functions. The layers are numbered from superficial to deep:
- Layer I (molecular layer), which contains few neurons and is rich in synaptic connections.
- Layer II (external granular layer), involved in local communication.
- Layer III (external pyramidal layer), where pyramidal neurons project to other cortical areas.
- Layer IV (internal granular layer), primarily receiving sensory input
- Layer V (internal pyramidal layer), with large pyramidal cells sending outputs to subcortical structures.
- Layer VI (multiform layer), which sends output to the thalamus.
The arrangement of these layers varies across different regions, giving rise to different cytoarchitectural areas, such as Broca’s area for speech and the primary motor cortex for voluntary movement. This layered organization enables the complex processing and integration of information.
Describe the cerebral cortex layers and the main zonal differences
The cerebral cortex is the outermost layer of the brain, composed of gray matter that is involved in sensory processing, motor control, and higher cognitive functions. It has a characteristic six-layer structure, each with distinct types of neurons and functions. The layers are numbered from superficial to deep:
- Layer I (molecular layer), which contains few neurons and is rich in synaptic connections.
- Layer II (external granular layer), involved in local communication.
- Layer III (external pyramidal layer), where pyramidal neurons project to other cortical areas.
- Layer IV (internal granular layer), primarily receiving sensory input
- Layer V (internal pyramidal layer), with large pyramidal cells sending outputs to subcortical structures.
- Layer VI (multiform layer), which sends output to the thalamus.
The arrangement of these layers varies across different regions, giving rise to different cytoarchitectural areas, such as Broca’s area for speech and the primary motor cortex for voluntary movement. This layered organization enables the complex processing and integration of information.
Pathways and their function passing in the lateral funiculus of spinal cord
- The lateral funiculus of the spinal cord contains both ascending sensory and descending motor pathways.
- The lateral corticospinal tract is the major descending motor pathway, controlling voluntary movement of limbs, with fibers crossing at the medullary pyramids to influence contralateral muscles.
- The spinothalamic tract carries ascending sensory information, including pain and temperature sensation, with fibers crossing at the spinal cord level.
- The dorsal and ventral spinocerebellar tracts also pass through the lateral funiculus, conveying proprioceptive information to the cerebellum for coordination and balance. These pathways are crucial for motor control, sensory perception, and coordination of limb movements.
