Spinal Cord Reflexes Part 3

Question 1

Descending (Motor) Pathways are split into two important but separate systems.

Answer 1

Pyramidal System and Extrapyramidal System

Question 2

-Part of the Pyramidal System
-Function: Fine, voluntary motor control of the limbs. Helps with voluntary body & trunk postural adjustments
-Not functional until 18 months of age
-Heavily myelinated → fast speed

Answer 2

Corticospinal Tracts (CST)

Question 3

Long, big, powerful pathways originating in the brain, traveling down through the brainstem/spinal cord, and collateralizing off to peripheral neurons.
-Give us skills like playing an instrument, writing a paper, etc.
-Divided into Lateral and Anterior
-Both originate in motor areas of the Cortex, run the entire length of the spinal cord (all the way to the coccygeal segment), and synapse with alpha motor neurons via internuncial neurons

Answer 3

Corticospinal tracts

Question 4

Precentral Gyrus → Internal Capsule → Cerebral Peduncles → Basis Pontis → Medulla, where 85% of fibers cross (pyramids). The 85% who cross are the Lateral Corticospinal tract. LCST descends the entire spinal cord → Internuncial Neurons → Alpha Motor Neurons on the contralateral side → innervate or inhibit a muscle

Answer 4

Lateral Corticospinal Tract (LCST)

Question 5

Precentral Gyrus → Internal Capsule → Cerebral Peduncles → Basis Pontis → Medulla, where 85% of fibers cross (pyramids) and 15% do not cross. The 15% who do NOT cross are the Anterior Corticospinal Tract (ACST). ACST continues on until cervical region. By C8/T1, they have crossed and project to Alpha Motor Neurons on the contralateral side to innervate or inhibit a muscle (typically muscles in the neck).

Answer 5

Anterior Corticospinal Tract (ACST)

Question 6

Lesion (Stroke/injury) results in symptoms that are contralateral to the source of the problem.
-Right sided brain problem = left sided symptoms

Answer 6

Corticospinal Tracts

Question 7

Some of the Corticospinal fibers become ___________, and will synapse with pontine nuclei upon entering the Basis Pontis. The pontine nuclei will originate ponto-cerebellar fibers that project to the contralateral cerebellum via the Middle Cerebellar Peduncle.

Answer 7

Corticobulbar Tracts

Question 8

Some of the Corticospinal fibers that diverge, but do not project to pontine nuclei, but rather synapse with motor nuclei of CN’s III, IV, V, VI, VII, IX, X, XI, and XII to provide supranuclear control of their motor functions.
-Most project to the contralateral motor nuclei, but some are mixed (Facial nerve is mixed)
-Motor nuclei controlling a CN receive _________ fibers that have crossed over from the pons and go on to innervated muscles, such as the muscles of mastication or the muscles of facial expression, or the Ocular Motor System (FEF in Cerebral Cortex)
-Supranuclear control is usually contralateral

Answer 8

Corticobulbar Fibers

Question 9

CN III, IV, & VI Supranuclear Control

Answer 9

Extraocular Movement

Question 10

CN V Supranuclear Control

Answer 10

Jaw Opening

Question 11

CN VII Supranuclear Control

Answer 11

Muscles of Facial Expression (platysma)
-Facial Nerve is mixed. some fibers cross over and some don’t.

Question 12

CN IX Supranuclear Control

Answer 12

Stylopharyngeus Muscle

Question 13

CN X Supranuclear Control

Answer 13

Larynx, Pharynx, and Palate Muscles

Question 14

CN XI Supranuclear Control

Answer 14

Trapezius and SCM muscles

Question 15

CN XII Supranuclear Control

Answer 15

Muscles of the tongue

Question 16

Lesion to this affects voluntary, fine digital movement.
-Function is to control limb movements, trunk control, and postural adjustments
-Lesion causes hypotonicity, limb flailing, etc.
-Runs the entire length of the spinal cord
-Crosses in the lower aspect of the medulla
-Affects the contralateral side

Answer 16

Lateral Corticospinal Tract (LCST)

Question 17

Function is to help with positioning of the head, and movement of the head/neck in response to stimuli. May also contribute a little to the Brachial Plexus.
-Lies on either side of the Anterior Median Fissure
-Only found in the cervical region; crosses over by C8/T1
-Affects the contralateral side (after crossing over in the cervical region)

Answer 17

Anterior Corticospinal Tract (ACST)

Question 18

A system made up of 4 tracts:
-Reticulospinal
-Vestibulospinal
-Rubrospinal
-Tectospinal

Answer 18

Extrapyramidal System

Question 19

Functions to regulate spinal cord Gamma Motor Neuron Bias, and regulates muscle tone.
Splits into:
-Medial (Pontine)
-Lateral (Medullary)
Can have ascending and descending fibers:
-Descending: takes info from the RF down to the spinal cord
-Ascending: fibers originate from the RF and project up to several structures, such as the Hypothalamus, Limbic structures, Cerebral Cortex, etc.

Answer 19

Reticulospinal Tracts

Question 20

Originates in the Medial Reticular Formation or the Lateral Reticular Formation → fibers CROSS after emerging from the RF → Medial Reticular Tract or Lateral Reticular Tract → Gamma Motor Neurons run throughout the entirety of the spinal cord → facilitate or inhibit muscle activity

Answer 20

Reticulospinal Tracts

Question 21

-Excitatory to Gamma Motor Neurons
-Lies proximally in the brainstem (runs from midbrain to medulla)
-Fibers exit the RF and CROSS. Motor fibers project to the contralateral side
-Neurons are mostly located in the Upper Pons
-Makes the Antigravity Muscles contract (Elbow Flexors and Knee Extendors)

Answer 21

Medial (Pontine) Reticulospinal Tract

Question 22

What are the antigravity muscles?

Answer 22

Elbow Flexors and Knee Extendors

Question 23

-Inhibitory to Gamma Motor Neurons
-Fibers exit the RF and CROSS. Motor fibers project to the contralateral side
-Neurons are mostly located in the Medulla
-Makes the Antigravity Muscles relax & lengthen (Elbow Flexors and Knee Extendors)

Answer 23

Lateral (Medullary) Reticulospinal Tract

Question 24

Lesions in these tracts will be contralateral to symptom presentation (issues with muscle tone)

Answer 24

Reticulospinal Tracts

Question 25

The Reticular formation also receives fibers from the Hypothalamus, which then projects to neurons in the spinal cord, giving rise to the sympathetic (Thoracolumbar) and parasympathetic outflow.
-Tracts provide a pathway for the Hypothalamus to control sympathetic outflow from the thoracolumbar spinal cord segments and parasympathetic outflow from the sacral spinal cord segments (S2-S4)

Answer 25

Reticulospinal Tracts

Question 26

The Reticular Formation projects to the Hypothalamus to give it control of Sympathetic/Parasympathetic outflow:
-Posterior hypothalamus: Sympathetic
-Anterior Hypothalamus: Parasympathetic

Answer 26

Reticulospinal Tracts Ascending Fibers

Question 27

Responsible for maintaining balance by adjusting trunk posture, head position, and limb position. Allows you to balance yourself against gravity to do functional skills.
-Has fibers that coordinate extraocular movements

Answer 27

Vestibulospinal tracts

Question 28

Balance Receptors in the Semicircular Canals & Vestibule (Inner Ear) → Vestibulocochlear Nerve (CN VIII) → brainstem (pons & Medulla) → 4 Vestibular Nuclei (Lateral, Medial, Superior, & Inferior) → Motor Pathways

Answer 28

Vestibulospinal Tracts Origin

Question 29

-Originates in the Lateral Vestibular Nucleus
-DOES NOT CROSS
-Descends the entire length of the spinal cord
-Fibers synapse with Alpha Motor Neurons to facilitate extension and inhibit flexion in the ipsilateral upper and lower limbs
-Pathway utilized when you think you’re about to fall → extension of the limbs to catch yourself

Answer 29

Lateral Vestibulospinal Tract

Question 30

-Originates in the Medial Vestibular Nucleus
-Pathway is mixed (some are contralateral and some are ipsilateral)
-Only descends to approx. T1, sending fibers to synapse with Alpha Motor Neurons from C1-T1
-Facilitates control of neck musculature, serving as a backup for the ACST.
-Works to “right the head” when we’re falling → pulls head back into a level position.
-Brachial Plexus is in the same region, so we also get some upper limb control (Extension of the Upper Limb) to maintain balance.

Answer 30

Medial Vestibulospinal Tract (Descending)

Question 31

-Also called the Medial Longitudinal Fasciculus (MLF)
-Originates in all 4 Vestibular Nuclei
-Fibers CROSS at the level of the Vestibular Nuclei, and then ascend on the Contralateral side to the upper brainstem to synapse with neurons in the motor nuclei of CN III, IV, and VI.
-Control extraocular movements, such as conjugate eye deviation (eyes should move together in the same direction and at the same speed)

Answer 31

Medial Vestibulospinal Tract (Ascending)

Question 32

Lesion can cause Internuclear Ophthalmoplegia. Injury to Right MLF → right eye doesn’t move (left eye is normal)
Lesion can also cause Nystagmus → eyes drift to one side and then snap back to the center (abnormal eye movement)

Answer 32

Lesion to the Medial Vestibulospinal Tract (Ascending); also called the Medial Longitudinal Fasciculus (MLF)

Question 33

Lesions are ipsilateral above the level of the Vestibular Nuclei, because the fibers cross between the Vestibular Nuclei in the brainstem

Answer 33

Medial Vestibulospinal Tract (Ascending); also called the Medial Longitudinal Fasciciulus (MLF)

Question 34

Red Nucleus (midbrain) → Fibers CROSS immediately in the Ventral Tegmental Decussation (still in the Midbrain) → fibers descend on the Contralateral side → brainstem → spinal cord → Alpha & Gamma Motor Neurons down to the lower Thoracic Segments (T7-T8).

Answer 34

Rubrospinal Tracts

Question 35

Tracts that affect the Upper Limbs & Trunk.
-Facilitates upper limb flexors
-Inhibits upper limb extensors

Answer 35

Rubrospinal Tracts

Question 36

Primitive pathways that, once we get CSTs working, drop off (around 18 months of age)
-Receive supranuclear control from the motor areas of the Cerebral Cortex.
-Corticorubral Tract exists that synapses in the Red Nucleus to facilitate Rubrospinal Tract function

Answer 36

Rubrospinal Tracts

Question 37

-Decorticate Rigidity
-RN connections are still intact, so some things might still work
-Ex: Lesion of the rostral brainstem is above the RN. Will see loss of supranuclear flexor inhibition, leading to upper limb flexor domination (Decorticate)

Answer 37

Lesion ABOVE Red Nucleus

Question 38

-Decerebrate Rigidity
-RN has been disconnected. RN provided inhibition, so loss of inhibition is occurring
-Ex: Lesion in the spinal cord leads to loss of Extensor inhibition, so Upper Limb Extensors dominate, leading to extension of elbows and pronation of wrists

Answer 38

Lesion BELOW Red Nucleus

Question 39

Superior/Inferior Colliculi (the Tectum) → Fibers CROSS → project to the Upper Cervical Spinal Cord → synapse with internuncial neurons → Alpha Motor Neurons → axons via cervical ventral roots → extrafusal fibers in the neck muscles

Answer 39

Tectospinal Tracts

Question 40

Responsible for reflex movements of the head in response to visual/auditory stimuli.
Ex: loud noise, we turn towards it and pull head back (startle reflex)

Answer 40

Tectospinal Tracts

Question 41

Originates a motor pathway in the CNS
-Lesion = knocked out a pathway that controls motor function of the CNS
-Ex: Medial RST/ Lateral RST

Answer 41

Upper Motor Neuron

Question 42

Lesion of this causes spasticity, hypertonic, inc DTRs, and muscle weakness.
-Usually due to lesion in Lateral RST

Answer 42

Lesion of Upper Motor Neuron

Question 43

Part of the Nervous system that originates a peripheral axon.

Answer 43

Lower Motor Neuron

Question 44

Lesion in this causes flaccidity, hypotonic, dec/absent DTRs, muscle weakness, and muscle atrophy.
-Ex: Polio

Answer 44

Lesion in Lower Motor Neuron