Neuro Unit 2 - Transcripts

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What is the basic job of an upper motor neuron (UMN)?

• It carries motor signals from the brain toward the spinal cord and helps initiate/control movement

• It does not directly innervate muscle

What is the basic job of a lower motor neuron (LMN)?

• It carries motor signals from the spinal cord or cranial nerve motor nuclei to the muscle or target organ

• It is the final common pathway for skeletal muscle movement

Why is the LMN called the final common pathway?

• Because all voluntary skeletal muscle activation ultimately depends on the LMN reaching the muscle

• The UMN cannot create movement without an intact LMN

Where do many UMNs originate?

• Primary motor cortex in the precentral gyrus, just anterior to the central sulcus

• Also premotor cortex and related cortical motor areas

Which Brodmann area is the primary motor cortex?

Brodmann area 4 = precentral motor cortex / primary motor cortex

Which Brodmann area is the premotor area?

Brodmann area 6 = premotor area

What is the overall goal of the UMN system?

Precise control of movement, especially predominantly contralateral control of movement

What is the UMN's direct neural target?

The UMN communicates with the alpha motor neuron / LMN, not directly with the target organ

Where do UMNs terminate in the spinal cord?

They terminate/synapse in the ventral horn of the spinal cord, where LMNs receive the signal

What structures are included in a motor neuron?

• Cell body/soma

• Axon

• Dendrites

• Neuromuscular junction/motor end plate

What are the major adjustment centers for descending UMN activity?

• The cerebellum and basal ganglia

• They help adjust movement by influencing excitation/inhibition of LMN output

How does sensory information influence motor output?

Ascending sensory data interacts with descending motor data and helps adjust LMN output to muscle

What are the two major UMN pathway categories?

1. Pyramidal systems

2. Extrapyramidal systems

What makes a tract part of the pyramidal system?

• It travels through the pyramids of the medulla

• Examples include corticospinal and corticobulbar tracts

What tracts are included in the pyramidal system?

• Corticospinal tract

  • Includes lateral and anterior divisions

• Corticobulbar tract

What is the main movement system within the pyramidal tracts?

The corticospinal tract, especially for voluntary skilled movement

• Corticospinal = movement

What does the corticobulbar tract control?

Motor control of the head and neck through cranial nerve motor nuclei

• Corticobulbar = head & neck via cranial nerves

What does extrapyramidal mean?

• Outside the medullary pyramids

• These tracts fine-tune movement and are more involved in unconscious/involuntary motor control

Name the main extrapyramidal descending tracts.

1. Reticulospinal: posture and muscle tone

2. Rubrospinal: limb flexor movement

3. Vestibulospinal: balance and posture

4. Tectospinal: reflex head/neck turning

What neurotransmitter is key at the neuromuscular junction?

Acetylcholine (ACh)

Which Brodmann areas are Broca's areas?

Brodmann areas 44 and 45

What is Broca's aphasia?

The patient generally understands language but has difficulty producing speech/language effectively

Which Brodmann area is Wernicke's area?

Brodmann area 22

What is Wernicke's aphasia?

Fluent speech that does not make sense, impaired language comprehension, and unreliable history if the patient is alone

Broca vs Wernicke aphasia: what is the quick difference?

• Broca: understands but cannot produce well

• Wernicke: fluent but nonsensical and poor comprehension

How are spinal tracts named?

• Usually by origin → termination

• Examples:

  • Spinothalamic = spinal cord to thalamus

  • Corticospinal = cortex to spinal cord

What are tracts?

Bundles of long axons traveling within CNS white matter

What should you know for each descending tract?

• Location in the spinal cord

• Whether it crosses/decussates

• Function

What does the lateral corticospinal tract control?

• Fine, skilled limb movement

• It crosses in the medulla at the medullary pyramids

What does the anterior corticospinal tract control?

More axial/upper body control: UE, head, neck, shoulders, and upper trunk depending on lecture framing

What does the tectospinal tract do?

Reflexive head turning in response to visual/auditory stimuli and support of postural control

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¼ extrapyramidal descending tracts

Where does the tectospinal tract cross?

It crosses in the midbrain

What are the main functions of the reticulospinal tracts?

Posture, balance, tone/postural preparation, and regulation of pain awareness

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¼ extrapyramidal descending tracts

Do the lateral and medial reticulospinal tracts cross?

They generally do not cross

What are the main functions of the vestibulospinal tracts?

Posture, balance, antigravity control, and maintaining head position when the base of support changes

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¼ extrapyramidal descending tracts

Which ascending tracts are part of the DCML system?

• Fasciculus gracilis

• Fasciculus cuneatus

What does fasciculus gracilis carry?

• Sensory input from below about T6/T7

• Lower limb/lower trunk position, deep touch, vibration, and proprioception

What does fasciculus cuneatus carry?

• Sensory input from above about T6/T7

• Upper limb/upper trunk position, deep touch, vibration, and proprioception

What is the Dorsal Column–Medial Lemniscus (DCML) system?

The DCML system is an ascending sensory tract that carries fine/discriminative touch, vibration, and conscious proprioception

Where does the DCML system cross?

In the medulla

What does the lateral spinothalamic tract carry?

Pain and temperature

What does the anterior spinothalamic tract carry?

Crude/gross touch, pressure, and poorly localized/indiscriminate touch

What does the spinoreticular tract carry?

Pain sensation related to tissue injury

What do the spinocerebellar tracts carry?

Unconscious proprioceptive information from muscle to the cerebellum

Which spinocerebellar tract remains ipsilateral?

Posterior/dorsal spinocerebellar tract

Which spinocerebellar tract crosses?

• Anterior spinocerebellar tract

• Later detail: it crosses twice, so it functionally ends up ipsilateral in the cerebellum.

What structures are included in the LMN system?

• Alpha motor neuron

• Gamma motor neuron

• Neuromuscular junction/motor end plate

• Muscle spindle

• Golgi tendon organ

• Free nerve endings

• Muscle

Where is the Golgi tendon organ located?

In the tendinous portion of muscle

Where is the muscle spindle located?

Within the central portion/belly of the muscle

Where do alpha and gamma motor neurons synapse?

In the ventral horn of the spinal cord

What is a motor unit?

One alpha motor neuron and all the muscle fibers it innervates

When an alpha motor neuron activates, what happens to its muscle fibers?

ACh is released at all of its NMJs, and all fibers innervated by that alpha motor neuron contract

Why do eye muscles have fine motor control?

They have very small motor units, sometimes about 2 muscle fibers per alpha motor neuron

Why are gastrocnemius-soleus motor units less precise than eye muscles?

They may have ~2,000 muscle fibers per alpha motor neuron, so one neuron controls many fibers

What is alpha-gamma coactivation?

Simultaneous activity of alpha and gamma motor neurons so extrafusal contraction occurs while the muscle spindle stays sensitive to stretch

What does the alpha motor neuron innervate?

• Extrafusal skeletal muscle fibers

• The muscle fibers responsible for force, bulk, power, and contraction

What does the gamma motor neuron innervate?

• Intrafusal fibers in the muscle spindle, helping maintain stretch sensitivity during contraction

What is the role of the gamma motor neuron?

• It adjusts muscle spindle sensitivity

• Helps regulate alpha motor neuron activity/tone

What are motor pools?

Clusters of alpha motor neurons in the ventral horn that innervate specific muscles or muscle groups

Anterior vs posterior motor pools: what do they mostly innervate?

• Anterior motor pools mostly innervate extensors

• Posterior motor pools mostly innervate flexors

What is a myotome?

A group of muscles innervated by a single spinal nerve/root level

How does the somatic nervous system connect to skeletal muscle?

One LMN projects directly to skeletal muscle

How does the autonomic nervous system differ from the somatic motor system?

• ANS uses a two-neuron chain

• Preganglionic neuron → ganglion → postganglionic neuron → target organ

In a UMN lesion, why is there minimal to no muscle atrophy?

Because the LMN and muscle remain connected, so there is not true denervation of the muscle

What does paresis mean?

Weakness

What does hemiparesis mean?

Weakness on one side of the body with the other side relatively maintained, commonly seen after stroke

Why do UMN lesions often cause hyperreflexia?

The spinal reflex arc is intact, but descending UMN regulation/inhibition is lost, so the reflex becomes exaggerated

What DTR grade suggests hyperreflexia?

3+ or 4+ instead of normal 2+

What is the normal adult Babinski response?

Hallux flexion, often with possible withdrawal

What is a positive Babinski sign?

• Hallux extension toe, often with toe fanning/splaying

• An UMN sign in adults

How do you test Babinski?

Use a cutaneous stimulus along the lateral plantar foot, then across the metatarsal heads toward the medial side

What is clonus?

Involuntary, repeating, rhythmic contraction of a muscle group in response to sustained stretch

How do you test gastrocnemius/ankle clonus?

Apply a light sustained upward pressure into dorsiflexion at the ball of the foot and watch/feel for rhythmic beating

What is sustained clonus?

• Rhythmic beats continue as the stretch is maintained

• Sustained clonus is pathologic

Why is clonus clinically useful?

It is a quick screen that raises concern for possible UMN involvement

What is spasticity?

Velocity-dependent resistance to quick passive stretch, strongly associated with UMN lesions

How do you test for biceps spasticity?

Move the elbow into extension at increasing speed and feel for velocity-dependent resistance from the biceps

How do you test for triceps spasticity?

Move the elbow into flexion at increasing speed and feel for velocity-dependent resistance from the triceps

What scale is commonly used to grade spasticity?

Modified Ashworth Scale

What is selective motor control?

The ability to isolate and coordinate intended muscle activation without unwanted co-contraction or abnormal synergy

What is co-contraction?

Agonist and antagonist muscles fire at the same time when they should normally take turns

Why do abnormal synergies occur after UMN lesions?

Loss of descending inhibition/fractionation disrupts normal alpha-gamma regulation, so muscles are coupled abnormally

What is a common upper extremity flexor synergy pattern after stroke?

Elbow flexion, wrist flexion, and upper limb held close to the chest

In LMN lesions, why does muscle atrophy occur?

There is disruption/denervation between the LMN/alpha motor neuron and muscle itself

What type of paralysis is associated with LMN lesions?

Flaccid paralysis

What happens to reflexes in LMN lesions?

Hyporeflexia or areflexia because the reflex arc/output pathway is disrupted

What is a fasciculation?

• Visible or felt involuntary spontaneous twitching caused by activation of part or all of a motor unit

• May be benign or abnormal

What is a fibrillation?

• Muscle fiber-level activity from an unstable/denervated muscle membrane

• Typically detected on EMG and associated with neuropathic/myopathic conditions

Fasciculation vs fibrillation: which is often visible?

• Fasciculation (twitch) is often visible/felt

• Fibrillation usually requires EMG detection

Why is EMG useful in weakness?

It helps determine:

• Motor unit death/dysfunction

• Whether the cause is neurogenic, myopathic, ALS, trauma, electrolyte imbalance, etc

What causes fibrillations physiologically?

Denervation/axonal death can create unstable muscle membranes and small ineffective depolarizations at the muscle fiber level

List classic UMN lesion signs

• Weakness/paresis

• Minimal atrophy

• Hyperreflexia

• Positive Babinski

• Clonus

• Spasticity

• Abnormal synergies/loss of selective motor control

List classic LMN lesion signs.

• Atrophy

• Flaccid paralysis/weakness

• Hyporeflexia or areflexia

• Fasciculations

• Fibrillations/denervation findings

Why is spinal cord injury usually considered an UMN lesion?

If the lesion is within the spinal cord above the anterior horn cell, descending UMN pathways are disrupted while LMN/reflex circuits below may remain intact

Which disorder is both UMN and LMN?

Amyotrophic lateral sclerosis (ALS)

Why are cranial nerves tricky when classifying UMN vs LMN?

Their nuclei are central in the brainstem, but their effector organs are outside the CNS, so cranial nerve motor output can be considered LMN

Give examples of UMN-related disorders from the lecture.

• Cerebral palsy

• stroke

• multiple sclerosis

• CNS tumors

• seizures/epilepsy

• dementia/Alzheimer's

• Parkinson's disease

• infection/meningitis

• Huntington's

• Friedreich's ataxia

• SCI

What is cerebral palsy?

Damage to the immature brain occurring in utero, during the perinatal period, or immediately after birth

What structure is involved in Parkinson's disease?

• Substantia nigra

• Clinical signs include rigidity

What movement disorder is especially associated with cerebellar lesions?

Ataxia