Motor Neurons and Descending Tracts Quiz

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Questions and Answers

Which tract specifically innervates skeletal muscle directly?

Corticobulbar tract (correct)

Dorsal column tracts

Anterior corticospinal tract

Lateral corticospinal tract

What type of paralysis results from upper motor neuron lesions?

Respiratory paralysis

Flaccid paralysis

Muscle atrophy

Spastic paralysis (correct)

What is a characteristic of lower motor neuron lesions?

Hyperreflexia

Decreased reflexes (correct)

Spastic paralysis

Increased muscle tone

Where do the main lateral corticospinal tracts decussate?

In the medulla (pyramids) Signup and view all the answers

What do motor signals from the brain primarily control?

Muscle contraction Signup and view all the answers

Which column primarily contains the corticospinal tracts?

Anterior (ventral) Columns Signup and view all the answers

What happens when there is damage to lower motor neurons at the lesion level?

Loss of voluntary movement Signup and view all the answers

Which part of the central nervous system contains lower motor neurons?

Anterior horn of the spinal cord Signup and view all the answers

What is the primary function of the corticospinal tract?

To provide motor innervation for voluntary skilled movements Signup and view all the answers

At what level do the majority of corticospinal fibers decussate?

At the pyramidal decussation in the medulla Signup and view all the answers

Which cranial nerves receive motor fiber innervation from the corticonuclear tract?

III, IV, V, VII, IX, XI, XII Signup and view all the answers

What role do the anterior corticospinal fibers play?

They remain ipsilateral and provide innervation to postural muscles. Signup and view all the answers

Which type of sensations does the spinothalamic tract carry to the brain?

Sensory information about pain, temperature, and touch Signup and view all the answers

Which descriptions best fit the axons of the corticospinal tract?

They primarily supply motor innervation to distal extremities. Signup and view all the answers

What happens to motor fibers of cranial nerves at their nuclei?

They remain ipsilateral and do not decussate. Signup and view all the answers

The corticospinal tract primarily carries which type of impulses?

Motor signals from the brain to the spinal cord Signup and view all the answers

What type of nerve roots carry motor information in the spinal cord?

Anterior (ventral) roots Signup and view all the answers

Which structure of the spinal cord is primarily responsible for sensory message transmission?

Posterior (dorsal) horn Signup and view all the answers

Which type of anaesthetic is injected directly into the fluid sac of the spinal cord?

Spinal anaesthetic Signup and view all the answers

What is the composition of the white matter in the spinal cord?

Ascending and descending nerve fibers Signup and view all the answers

Which structure contains the cell bodies of motor neurons to skeletal muscle?

Ventral horn Signup and view all the answers

In the spinal cord, how are the white matter structures arranged?

In columns (funiculus) Signup and view all the answers

What is the function of ascending tracts in the spinal cord?

Transmit sensory pathways to the brain Signup and view all the answers

Which lamina is considered most important for motor neuron cell bodies in the spinal cord?

Lamina IX Signup and view all the answers

What type of information is conveyed by the spinocerebellar tract?

Proprioceptive information Signup and view all the answers

Which pathway decussates at the medulla?

Dorsal column tracts Signup and view all the answers

What does the lateral pathway of the spinothalamic tract primarily transmit?

Temperature and pain sensation Signup and view all the answers

Which neurons are involved in the transmission of proprioceptive information from the body?

Three neurons with two synapses Signup and view all the answers

The artery of Adamkiewicz is crucial for supplying which part of the spinal cord?

Lower spinal cord Signup and view all the answers

What is the function of the internal vertebral venous plexus?

To drain venous blood from the spinal cord Signup and view all the answers

Which of the following options describes a characteristic of the spinal veins?

They are numerous, tortuous, and valveless Signup and view all the answers

What type of touch sensation is processed by the anterior pathway of the spinothalamic tract?

Crude touch and pressure Signup and view all the answers

What is one of the main functions of the spinal cord?

Carrying signals between the brain and the rest of the body Signup and view all the answers

What structural feature identifies the cervical enlargement of the spinal cord?

Increased volume of motor cells in the ventral horns Signup and view all the answers

Which white matter tract is primarily responsible for carrying sensory information to the brain?

Posterior (dorsal) columns Signup and view all the answers

What landmark identifies where the spinal cord ends in adults?

L2 Signup and view all the answers

What structure is found at the end of the spinal cord, contributing to the formation of spinal nerves?

Conus medullaris Signup and view all the answers

Which meningeal layer directly covers the spinal cord?

Pia mater Signup and view all the answers

What is the total number of spinal nerves in the human body?

31 pairs Signup and view all the answers

What type of information does the spinothalamic tract primarily convey?

Pain and temperature sensations Signup and view all the answers

Study Notes

Descending Tracts

Motor signals are sent from the brain to lower motor neurons via descending tracts.
These tracts are found in the anterior, lateral, and posterior columns of the spinal cord.

Motor Neurons

Upper Motor Neurons (UMNs):
- Located in the cerebral cortex.
- Control movement and inhibit muscle tone.
- Send signals to lower motor neurons via the corticospinal tract.
Lower Motor Neurons (LMNs):
- Located in the anterior horn of the spinal cord and cranial nerve nuclei in the brainstem.
- Directly innervate skeletal muscle to control movement.

Motor Neuron Lesions

UMN Lesions: lead to spastic paralysis (increased muscle tone, hyperreflexia, no muscle wasting)
LMN Lesions: lead to flaccid paralysis (decreased muscle tone and reflexes, long-term muscle wasting)

Spinal Cord & Brainstem Lesions

Damage LMNs at the level of the lesion.
Damage UMNs of all levels below the lesion.
Sensory input is lost at and below the level of the lesion.

Motor Decussations

The main corticospinal tracts decussate (cross over) at the medulla (pyramids).
Motor fibers of cranial nerves decussate at the level of their nuclei.

Corticospinal Tract

Responsible for voluntary skilled movements.
Originates in the motor cortex (pre-central gyrus).
Forms the pyramids in the medulla (brainstem).
Lateral Corticospinal Tract:
- 75-95% of corticospinal fibers.
- Decussates in the medulla.
- Supplies motor innervation to the distal parts of the extremities.
Anterior Corticospinal Tract:
- Decussates at the spinal cord segment (not in the pyramids).
- Supplies motor innervation to the back and proximal portions of the extremities.

Corticobulbar Tract

Conducts impulses from the brain to the cranial nerves.
Axons do not enter the spinal cord.
Synapse onto the nuclei of CN III, IV, V, VI, VII, IX, X, XI, XII.
Contains motor fibers for muscles of the eyes, face, mastication, and some neck muscles.

Spinothalamic Tract

Carries information about pain, temperature, itch, and light touch sensations to the brain.

Spinal Cord

Carries signals between the brain and the rest of the body.
Contains motor, sensory, and autonomic pathways.
Acts as a reflex center for basic reactions.

Spinal Cord Structure

Located in the back and neck regions.
Elongated, cylindrical shape.
Found within the vertebral canal.
Approximately 45cm long.
Extends from the brainstem (medulla oblongata).
Ends at L2 in adults (L3 in newborns).

Spinal Cord Enlargements

Cervical enlargement: due to increased volume of motor cells in the ventral horns (controls upper limbs)
Lumbosacral enlargement: due to increased volume of motor cells in the ventral horns (controls lower limbs)

Spinal Cord Endings

Conus medullaris: the tapered end of the spinal cord (ends at L2)
Cauda equina: collection of nerve roots extending from the conus medullaris (appears like a horse's tail)
Filum terminale: a thin thread of fibrous tissue extending from the conus medullaris (ends at Co1)

Spinal Cord Meninges

Dura mater: outermost layer
Arachnoid mater: middle layer
Pia mater: innermost layer

Spinal Nerves

31 pairs of spinal nerves.
Anterior (ventral) & Posterior (dorsal) rootlets form anterior and posterior roots.
Anterior roots are motor (efferent).
Posterior roots are sensory (afferent).
Anterior and posterior roots combine to form a mixed spinal nerve in the intervertebral foramen.
Spinal nerves branch into ventral and dorsal primary rami.

Dorsal Root Ganglia

Collection of cell bodies responsible for transmitting sensory messages from receptors to the CNS.
The sensory information processed includes:
- Thermoreceptors (temperature)
- Nociceptors (pain)
- Proprioceptors (position and movement)
- Chemoreceptors (chemical stimuli)

Cauda Equina

Lower nerve roots are progressively elongated within the dural sac (to S2).
Relevant for:
- Lumbar puncture: used to diagnose serious infections and multiple sclerosis (MS).
- Epidural anesthesia: pain relief.
- Spinal anesthetic: injection of numbing medicine into the fluid sac.

Spinal Cord Internal Structure

Composed of:
- Grey matter: centrally located, butterfly (H) shape.
- White matter: surrounds the grey matter.
A small central canal runs through the center.
Major fissures:
- Anterior median fissure: groove on the front of the spinal cord.
- Posterior median sulcus: groove on the back of the spinal cord.
- Posterolateral sulcus: lateral groove on the back of the spinal cord.

Grey Matter

Subdivided into laminae, known as Rexed's laminae (I-X).
Lamina IX is crucial as it contains the cell bodies of motor neurons to skeletal muscle.

White Matter

Highly organized, contains ascending and descending nerve fibers connecting the spinal cord and brain.
Divided into columns (funiculi).
Columns contain tracts (fasciculi) which contain axons with similar functions.

Spinal Cord Columns

Anterior (ventral) column: contains the anterior corticospinal tract.
Lateral column: contains the lateral corticospinal tract and the lateral spinothalamic tract.
Posterior (dorsal) column: contains the gracile and cuneate tracts.

Ascending Tracts

Sensory pathways that begin in the spinal cord and extend to the cerebral cortex.
Spinothalamic tract: carries pain, temperature, crude touch, and pressure information.
Spinocerebellar tract: carries proprioceptive information from the spinal cord to the cerebellum.
Gracile and Cuneate tracts: carry fine touch, pressure, vibration, and proprioceptive information from the ipsilateral side of the body.

Sensory Decussations

Spinothalamic tract: decussates a few levels above the spinal cord entry (pain).
Dorsal column tracts: decussate at the medulla.
Unilateral spinal cord lesions can cause different sensory effects on both sides of the body.

Arterial Supply

Anterior and Posterior spinal arteries: supply the spinal cord.
Lumbar arteries: arise from the abdominal aorta, contributing to the supply.
Artery of Adamkiewicz: supplies the lower spinal cord, reinforcing the anterior spinal artery.

Adamkiewicz Artery

This artery is crucial for blood supply to the lower spinal cord.
Damage to the artery can result in a significant blood supply loss to the lower limbs.

Venous Drainage

Numerous, tortuous veins with valveless vessels.
Internal vertebral venous plexus drains the spine and spinal cord.
This plexus runs alongside the nerve roots in the anterior and posterior regions.

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Description

Test your understanding of motor neuron functions, including upper and lower motor neuron roles, and the implications of different types of motor neuron lesions. This quiz highlights key aspects of descending tracts and their relation to movement control and paralysis. Perfect for students studying neuroscience and human physiology.