Week 5 Neuro

Major Motor Pathways in the Central Nervous System

• The major motor (descending) pathways include the corticospinal tract and extrapyramidal pathways.

Corticospinal Tract (CST)

• Starts in the cortex and travels through the corona radiata, then passes through the internal capsule (mainly the posterior limb).
• Continues to the midbrain (crus cerebri), and extends to the spinomedullary junction to connect to skeletal muscle.
• Includes the lateral and anterior/ventral tracts.
• Purpose is for voluntary movement.
• Originates about 30% from the primary motor cortex, but other areas of the cerebral hemisphere also participate.
• Most fibers (about 90%) cross the midline, but 10-20% remain ipsilateral.

Anterior/Ventral CST

• Includes the cortex, corona radiata, internal capsule, and midbrain.
• Does NOT decussate at the spinomedullary junction, unlike the lateral CST.
• Impacts skeletal muscles of the cervical and thoracic regions.

Extrapyramidal Pathways

• Support movement and locomotion.
• Primary origin is in the brainstem.
• Don't get funneled through the pyramid-like structure that the corticospinal tract goes through.
• Function mostly on an unconscious level without voluntary control.
• Four specific extrapyramidal tracts include the reticulospinal, tectospinal, vestibulospinal, and rubrospinal tracts.
• The reticulospinal tract originates in the reticular formation and impacts locomotion and postural control.
• The tectospinal tract originates in the tectum of the midbrain and orients the head in response to auditory stimuli.
• The vestibulospinal tract originates in the vestibular nuclei of the pons and adjusts the head position with respect to gravity.
• The rubrospinal tract originates in the red nucleus of the midbrain and controls large muscle movements, mainly flexors.
• The rubrospinal tract is the only extrapyramidal tract that crosses the midline.

Corticobulbar Tract

• Includes the same fibers as the corticospinal tract, but they don't descend into the spinal cord -- terminate within motor nuclei of the cranial nerves.
• Provides voluntary drive to the brainstem, reticular formation, and some cranial nerves.
• Provides bilateral input except for cranial nerves 7 & 12.

Motor Tracts Function

• Can be functionally divided into two major groups.
• Pyramidal tracts originate in the cerebral cortex and carry motor fibres to the spinal cord and brain stem.
• Responsible for voluntary control of the musculature of the body and face.
• Extrapyramidal tracts originate in the brain stem and carry motor fibres to the spinal cord.
• Responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion
• No synapses within the descending pathways.
• At the termination of the descending tracts, the neurones synapse with a lower motor neurone.
• All the neurones within the descending motor system are classed as upper motor neurones.
• Their cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining within the CNS.

Pyramidal Tracts

• Derive name from the medullary pyramids of the medulla oblongata, which they pass through.
• Responsible for the voluntary control of the musculature of the body and face.
• Functionally, these tracts can be subdivided into two:
• Corticospinal tracts supply the musculature of the body.
• Corticobulbar tracts supply the musculature of the head and neck.

Corticospinal Tracts Beginnings

• Begin in the cerebral cortex, from which they receive a range of inputs:
• Primary motor cortex
• Premotor cortex
• Supplementary motor area
• Also receive nerve fibres from the somatosensory area, which play a role in regulating the activity of the ascending tracts.
• After originating from the cortex, the neurones converge, and descend through the internal capsule (a white matter pathway, located between the thalamus and the basal ganglia).
• The internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as a capsular strokeʻ.
• After the internal capsule, the neurones pass through the crus cerebri of the midbrain, the pons and into the medulla.
• Fibers within the lateral corticospinal tract decussate (cross over to the other side of the CNS).
• Descend into the spinal cord, terminating in the ventral horn (at all segmental levels).
• Lower motor neurones go on to supply the muscles of the body.
• The anterior corticospinal tract remains ipsilateral, descending into the spinal cord.
• Then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.

Corticobulbar Tracts Sources

• Arise from the lateral aspect of the primary motor cortex.
• Receive the same inputs as the corticospinal tracts.
• The fibres converge and pass through the internal capsule to the brainstem.
• Neurones terminate on the motor nuclei of the cranial nerves and synapse with lower motor neurones, which carry the motor signals to the muscles of the face and neck.
• Many fibres innervate the motor neurones bilaterally.
• Fibers from the left primary motor cortex act as upper motor neurones for the right and left trochlear nerves
• A few exceptions to this rule: upper motor neurones for the facial nerve (CN VII) have a contralateral innervation and affects muscles below the eyes only; upper motor neurons for the hypoglossal (CN XII) nerve only provide contralateral innervation.

Extrapyramidal Tract Origins

• Originate in the brainstem, carrying motor fibres to the spinal cord.
• Responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion.
• The vestibulospinal and reticulospinal tracts do not decussate, providing ipsilateral innervation.
• The rubrospinal and tectospinal tracts do decussate, and therefore provide contralateral innervation.

Vestibulospinal Tracts

• There are two pathways, medial and lateral -- arise from the vestibular nuclei, which receive input from the organs of balance.
• Convey this balance information to the spinal cord, where it remains ipsilateral.
• Fibres in this pathway control balance and posture by innervating the ‘anti-gravity' muscles (flexors of the arm, and extensors of the leg), via lower motor neurones.

Reticulospinal Tracts

• Have differing functions:
• Medial reticulospinal tract arises from the pons, facilitates voluntary movements, and increases muscle tone.
• Lateral reticulospinal tract arises from the medulla, inhibits voluntary movements, and reduces muscle tone.

Rubrospinal Tracts

• Originate from the red nucleus, a midbrain structure.
• Fibres emerge, they decussate (cross over to the other side of the CNS), and descend into the spinal cord -- have a contralateral innervation.
• Exact function is unclear, but it plays a role in the fine control of hand movements

Tectospinal Tracts

• This pathway begins at the superior colliculus of the midbrain, which is a structure that receives input from the optic nerves.
• Neurones then quickly decussate and enter the spinal cord.
• Terminate at the cervical levels of the spinal cord.
• Coordinates movements of the head in relation to vision stimuli.

Upper Motor Neurone Lesion

• Also known as supranuclear lesions.

Damage to the Corticospinal Tracts Clinical Relevance

• Pyramidal tracts are susceptible to damage, because they extend almost the whole length of the central nervous system -- particularly vulnerable as they pass through the internal capsule (a common site of cerebrovascular accidents (CVA)).
• A unilateral lesion of the left or right corticospinal tract, symptoms will appear on the contralateral side of the body.
• The cardinal signs of an upper motor neurone lesion are:
  • Hypertonia (an increased muscle tone)
  • Hyperreflexia (increased muscle reflexes)
  • Clonus (involuntary, rhythmic muscle contractions)
  • Babinski sign (extension of the hallux in response to blunt stimulation of the sole of the foot)
  • Muscle weakness

Damage to the Corticobulbar Tracts

• Unilateral lesion usually results in mild muscle weakness.

Exceptions to Unilateral Lesions

• Hypoglossal nerve causes spastic paralysis of the contralateral genioglossus, resulting in deviation of the tongue to the contralateral side.
  • Notethe tongue deviates towards the damaged side in a lower motor neurone lesion.
• Facial nerve causes spastic paralysis of the muscles in the contralateral lower quadrant of the face.

Damage to the Extrapyramidal Tracts

• Extrapyramidal tract lesions are commonly seen in degenerative diseases, encephalitis and tumours.
• Result in various types of dyskinesias or disorders of involuntary movement.

Clinical Impairments along the Motor Pathways

• A lesion can result in impairments, depending on the location and extent of the damage.
• Clinically, these pathways are often compromised due to vascular issues like stroke.

Corticospinal Tract (CST) Lesion

• Can lead to contralateral deficits in muscle function.
• While a small percentage of the CST fibers remain ipsilateral, the impact of their damage is not clinically relevant.

Middle Cerebral Artery (MCA) Stroke

• Impacts the lateral aspects of the frontal, parietal, and temporal lobes, affecting the corticospinal and corticobulbar tracts.
• Results in weakness in the face, hand, and arm due to the involvement of the motor homunculus.

Anterior Cerebral Artery (ACA)

• Leads to more pronounced weakness in the trunk and leg.
• In both ACA and MCA strokes, weakness can occur in both the upper and lower limbs, but one is more severely affected based on the motor homunculus and the specific vessel compromised.

Spasticity damage relevance

• Damage to the corticospinal tract can cause spasticity, which is involuntary muscle resistance to a quick stretch -- the individual loses voluntary control over motor neurons, while extrapyramidal pathways continue to send excitatory input to the muscles.

Complete Spinal Cord Injury Relevance

• Muscle weakness would occur bilaterally below the level of injury -- loss of sensation below the level of the injury and muscle tone would be characterized by hypertonicity and hyperreflexia.
• In where no signals can pass through the lesion,

Corticobulbar Tract Lesion

• A lesion involving both the corticospinal and corticobulbar tracts on one side (e.g., the left) would result in right-sided hemiparesis in both the upper and lower extremities, weakness of the right side of the lower half of the face, and deviation of the tongue to the right with tongue protrusion.
• Unilateral Lesions: Typically do not produce significant clinical effects due to the bilateral influence to specific cranial nerve nuclei, with a couple exceptions.

Exception to Unilateral Lesions

• Facial Nucleus causes a stroke on the left motor cortex resulting in difficulty of weakness in the muscles on the lower right half of the face, while the individual would still be able to wrinkle their forehead.
• The Neurons innervating the lower half of the face receive mainly unilateral influence from the opposite motor cortex.*
• Hypoglossal Nerve causes a left-sided stroke in the motor cortex to produce right-sided tongue weakness, leading to deviation of the tongue to the right upon protrusion. The Innervation to specific muscles within the tongue receives only unilateral input

Peripheral Nerve Lesion

With a lesion impacting a peripheral nerve, anything distal to the lesion will start to degrade and atrophy, including the nerve and muscle. If the lesion remains, the muscle itself will likely not get stronger, and rehab efforts will focus on compensatory strategies.

CNS Influence on Peripheral Muscle Contraction

• The central nervous system (CNS) exerts significant influence on peripheral muscle contraction through descending motor pathways, including both pyramidal and extrapyramidal tracts -- enable the brain to control and modulate muscle activity.

Contraction Influence on the Corticospinal Tract

• Crucial for voluntary movement.
• Originating in the motor cortex, its fibers descend through various brain structures, including the corona radiata, internal capsule, midbrain, and medulla, before reaching the spinal cord.
• Most CST fibers cross (decussate) at the medulla and then synapse on the ventral horn of the spinal cord -- message is transmitted to the muscle, resulting in contraction.
• These fibers are excitatory -- meant to impact positive movement.
• Damage to the CST can disrupt the brain's ability to initiate and control voluntary movements, potentially leading to muscle weakness or paralysis.

Extrapyramidal Origin

• The Tracts originating in the brainstem and extrapyramidal tracts including the reticulospinal, tectospinal, vestibulospinal, and rubrospinal tracts primarily support movement and locomotion.
• Those that operate largely at an unconscious level.
• They influence muscle tone, balance, posture, and reflexive responses to sensory stimuli.
• These tracts may still provide excitatory input to the muscles -- even if the corticospinal tract is compromised.

Voluntary control.

• To the muscles of the head and neck through cranial nerves through the corticobulbar tract.
• These tracts control muscles via cranial nerves 5, 7, 9, 10, and 12.
• The corticobulbar tracts provide bilateral influence to cranial nerve nuclei, with a few exceptions, including cranial nerve 7 (facial nucleus) and 12 (hypoglossal nerve).

Central Vs Peripheral lesions.

• Impact of lesion is based on if its central/ peripheral and/ or spinal levels remain intact.
• In the case of a central lesion, the muscle can still be strengthened via alternative pathways or methods such as electrical stimulation. If the lesion is not removed a peripheral lesion directly affects the peripheral nerve, disrupting the transmission of signals to the muscle: Muscle will Atrophy.

CNS Muscle Physiology

• A central lesion will alter: Muscles may undergo physiological changes, such as a decrease in type II muscle fibers, atrophy, and increased intramuscular fat, contributing to muscle weakness. CNS also influences muscle fatigue: CNS may need to recruit additional resources to maintain muscle function, leading to central fatigue.
• First, Cerebral cortex travel down through fibers called the: Cortico spinal tract. As it travels through, it gets closer to the Corona raita which is the key white portal on the train before meeting again at the: Internal Capsule or where the Cerebral corex and Subcortical structures get combined.
• The message then follows the: Brain stem and the Crus Cerebi and exits at the ventral portion of the spinal cord at: respective Muscle.

What are the Major Functions of the each extrapyramidal tract?

• Reticulospinal Tract: Originates in the reticular formation of the brainstem. It is involved in locomotion and postural control.
• Tectospinal Tract: Originates in the tectum of the midbrain. It controls head orientation in response to auditory stimuli.
• Vestibulospinal Tract: Originates in the vestibular nuclei of the brainstem. It adjusts head position with respect to gravity.
• Rubrospinal Tract: Originates in the red nucleus of the midbrain. It controls large muscle movements, mainly flexors. The rubrospinal tract is also the only extrapyramidal tract that crosses the midline.
• These extrapyramidal tracts generally support movement and locomotion and operate at a very unconscious level, without voluntary control. They play a crucial role in balance, posture, and reflexive responses to sensory stimuli.*