The Spinal Cord

Types of Reflexes

- Four groups (Superficial, deep tendon, Visceral, pathological) -Segmental spinal reflex involves the afferent neuron and its axon within a peripheral nerve and dorsal root and a motor unit at the same level -For a reflex to be present, the reflex arc must be intact -Spinal reflexes can provide information that is highly useful in the localization of lesions

Cross Section Make-up

- Gray matter makes up the "H" in the middle; mostly cell bodies -White matter makes up the outside of the cord; mostly axons and transmits information -Central canal in the middle made up of ependymal cells; filled with CSF

Pathway

- Entire neuronal circuit responsible for a specific function, including all nuclei and tracts (e.g. spinothalamic pathway) -Based on function -Term to describe bundles of axons in white matter

Laminae I to IV

- Exteroceptive sensation and comprise the dorsal horn - I (Lissauer's tract) and II (Substantia gelatinosa) - pain

Tract

- Fiber bundles with a common function (some can decussate or cross the midline) (e.g. Corticospinal tract - name tells you origin and destination) - Based on function -Term to describe bundles of axons in white matter

Gray Matter Laminae

- Laminae: Layers of nerve cells found in a cross section of the gray matter - New Way - "Rexed's Laminae" - Superficial laminae tend to be involved in pain signaling. Deep laminae in both pain and non-pain sensation

Dorsal Roots

- Largely sensory - Receives info from skin, skeletal muscles, and joints -Lateral division: pain/ temperature information will end in Rexed laminae I, II, and IV of the gray matter - Medial Division: Skin, skeletal, joint information

White Matter Descending Fiber Systems

- Lateral Corticospinal Tract - Anterior Corticospinal Tract - Vestibulospinal Tract - Rubrospinal Tract - Reticulospinal System - Descending Autonomic System - Tectospinal Tract - Medial Longitudinal Fasciculus

Lower Motor Neuron Lesions (LMNs)

- Lesions of the LMNs may be in the cell body (ventral gray column of the spinal cord/brain stem) or in their axons (in ventral roots of spinal or cranial nerves) -Can result in flaccid paralysis, muscle atrophy,diminished deep tendon reflexes and absence of pathologic reflexes

Laminae V and VI

- Proprioceptive sensations

Funiculus aka column

- location in the spinal cord (e.g. Dorsal Funiculus aka Dorsal Column) - Based on location -Term to describe bundles of axons in white matter

Gamma motor neurons

Small anterior horn neurons that, via the ventral root, innervate the intrafusal muscle fibers, increasing tension on the muscle spindle, which increases its sensitivity to overall muscle stretch, setting the "gain" on the muscle spindle

Simple Reflex Arc

- A receptor (special sense organ/cutaneous end-organ/muscle spindle whose stimulation initiates an impulse) - An afferent neuron (transmits the impulse through a peripheral nerve to the CNS where the nerve synapses with an LMN or an intercalated neuron) - Possibly 1 or more intercalated neuron aka "interneuron" (which for some reflexes relay the impulse to the efferent neuron) -An efferent neuron (usually an LMN - passes outward in the nerve and delivers the impulse to the effector) - An effector (the muscle or gland that produces the response) - Any interruption in the simple reflex arc abolishes the response

Anterior Corticospinal Tract

- About 10% fibers do not decussate in medulla, but descend uncrossed in the anterior white columns (anterior corticospinal tract) - Many may decussate after descending within the spinal cord via anterior white commissure - It controls gross and postural motor function (proximal and axial musculature)

Lateral Corticospinal Tract

- About 90% fibers decussate at pyramidal decussation in the medulla downward into the lateral white columns (lateral corticospinal tract) - Many of these fibers terminate throughout the ventral gray column and at the base of the dorsal column - It controls fine motor function (distal musculature) and modulation of sensory functions

Rubrospinal tract

- Arises in the red nucleus - Descends in the lateral column - Ends at the ventral horn interneurons - Plays a role in motor function

Intermediate Column and Lateral Horn

- Autonomic neurons innervating visceral and pelvis organs - Leaves through ventral roots - Sympathetic and parasympathetic

Lesions of Spinal Cord

- Can occurs anywhere, and each one has unique signs and symptoms. Example of specific spinal disorder - Spinal Cord Compression (weakness/sensory loss in legs, babinski reflex, hyper reflexia, pain)

Gray Matter Columns

- Columns - Gray matter extending the entire length of the spinal cord - There are 3 main ones 1. Ventral Gray Column (Anterior Horn) 2. Intermediate Column and Lateral Horn 3. Dorsal horn

Spinal Cord landmarks

- Deep anterior (ventral) median fissure -shallow posterior(dorsal) median sulcus -Posterolateral sulcus: Dorsal never roots enter the spinal cord along a vertical groove called this. -Anterolateral sulcus: Ventral nerve roots exit in this

Tectospinal Tract

- Descending tract and goes in the contra lateral ventral white column - Arises from superior colliculus in the roof of midbrain - Causes reflex head turning in response to sudden visual or auditory stimuli

Ventral Roots

- Represent motor outflow tracts from the spinal cord - Contains axons of efferent fibers - Large diameter alpha motor neurons axons to extrafusal muscles fibers - Smaller gamma motor neuron axons supply intrafusal muscles of the muscle spindles - Combine with dorsal root fibers after dorsal root ganglion to form the spinal nerve

Dorsal Horn

- Sensory nuclei that receive and process incoming somatosensory info; send its axons up to the midbrain and diencephalon - Many laminae

Lamina X

- Surrounds the central canal and contains neuroglia.

Spinal Cord

-31 segments, defined by 31 pairs of nerves exiting the cord - Cord is shorter than vertebral column, lower cord segments are above the similar vertebral body -Each segment gives rise to four roots (2 dorsal, 2 ventral)

Vertebral Column

-33 Vertebrae joined by ligaments and cartilage -Upper 24 separate and mobile -Lower 9 fixed -Intervertebral disks help absorb stress and strain within the column - contains nucleus purposes, thick annulus fibrosus; attached to vertebral bodies by hyaline cartilage - We shrink due to water loss

Spinothalamic Tract

-Ascending pathway -Originate in the skin -After synapsing, fibers cross to the opposite side of the spinal cord and ascend within either the anterior or lateral spinothalamic tracts -Like the dorsal column system, the spinothalamic tracts show somatotopic organization (lower - lateral, higher -medial)

Visceral Efferent

-Autonomic motor fibers to the viscera -Sympathetic fibers from thoracic, L1, and L2 travel to viscera, glands, and smooth muscle -Parasympathetic fibers from middle three sacral travel to pelvis and abdominal viscera -Other parasympathetic go to CN 3, CN 7, CN 9, and CN 10

Cross Section Enlargements

-Cervical enlargements LMNs, which form the brachial plexus, travel to the upper extremities -Lumbosacral enlargement LMNs ,which form the lumbosacral plexus, travel to the lower extremities

Laminae VIII-IX

-Comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle

Ventral Spinocerebellar Tract

-Convey information from muscle spindles, Golgi tendon organs, and touch/pressure receptors - 2nd order neurons in laminae V, VI, and VII in lumbar and sacral segments of spinal cord, send axons ipsilateral AND contralateral that ascend through the superior cerebellar peduncle to the paleocerebellar cortex - Ascends in the lateral column - Involved with movement and position mechanisms

Dorsal Spinocerebellar Tract

-Convey information from muscle spindles, Golgi tendon organs, and touch/pressure receptors -Enter spinal cord via dorsal roots at levels T1 to L2 and synapse on second-order neurons of the nucleus dorsalis (Clarke's column). Lower lumbar and sacral afferent fibers ascend within spinal cord to reach the lower portion of the nucleus dorsalis (or accessory cuneate nucleus above C8). 2nd order neurons from the nucleus dorsalis ascend upward as the dorsal spinocerebellar tract. 2nd order neurons from the lateral cuneate nucleus form the cuneocerebellar tract. Both remain on the ipsilateral side of spinal cord, ascend in the inferior cerebellar peduncle to terminate in the paleocerebellar cortex - Ascends in the lateral column - Involved with movement and position mechanisms

Somatic Afferent

-Convey sensory information from the skin, joints, and muscles to the CNS -Cell bodies are unipolar cells in the spinal ganglia (in dorsal root ganglia) -Peripheral branches of these ganglionic cells are distributed to somatic structures - central branches convey sensory impulses through the dorsal roots to the dorsal gray column and ascending tracts of the spinal cord

Medial Longitudinal Fasciculus

-Descending pathway -Arises from vestibular nuclei in the brain stem - As it descends in the ventral white column, it runs close to, and intermingles with, the tectospinal tract - It coordinates head and eye movements - It interconnects the extraocular muscle nuclei to facilitate yoked eye movements - Lesions: Eye movement deficits, internuclear ophthalmoplegia, nystagmus, exotropia - Last two descending fiber systems descend only to the cervical segments of the spinal cord

White Matter Ascending Fiber Systems

-Dorsal Column Tracts - Spinothalamic Tracts - Anterior Spinothalamic Tract - Lateral Spinothalamic Tract - Spinoreticular Pathway - Spinocerebellar Tracts - Dorsal Spinocerebellar Tract - Ventral Spinocerebellar Tract

Dorsal Root and nerves

-Each dorsal root receives sensory axons from the body (these sensory/dorsal nerves have their cell bodies in the dorsal root ganglia) -Sometimes the portion of the spinal nerve outside the vertebral column is called a "peripheral nerve" -Each ventral root contains motor axons leaving to the body -Nerve roots exit the vertebral column through the intervertebral foramina

Lamina VII

-Equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum

Optometry Case- Herpes Zoster Ophthalmicus 85yr. WF -Chief Complaint: Pain, Scabbing, Eye pressure, tearing, eye redness, decreasing vision -Pain in the distribution of the trigeminal nerve may be severe -Reactivation of varicella-zoster virus (VZV) from its dormant status in the dorsal ganglion cells of the central nervous system. From there, it may travel along neurons to the sensory axons of the skin to form vesicular lesions

-Herpes Zoster Ophthalmicus (HZO), commonly known as shingles, is a viral disease characterized by a unilateral painful skin rash in one or more dermatome distributions of the fifth cranial nerve (trigeminal nerve), shared by the eye and ocular adnexa - Treatment: Treat skin rash, oral antivirals, pain management

Alpha motor neurons

-Large anterior horn neurons that, via the ventral root, innervate the motor end-plate of extrafusal muscle fibers producing rapid muscle contraction -Damage to alpha motor neuron fibers can make a muscle weak (acutely phase) or spastic

Corticospinal Tract

-Large bundle of myelinated axons descend through brainstem via medullary pyramid tract (1 million axons) - Mostly upper motor neurons that provide input to the anterior horn cells of the spinal cord -These anterior horn cells, project directly to muscle and control muscular contraction (called lower motor neurons)

Dentate (Denticulate) Ligament

-Long, white, pail tissue runs along lateral margin of SC between dorsal/ventral rootless -Aids in stabilizing SC from side to side

Dorsal Column Tract

-Make up a part of the medial lemniscal system -Ascending pathway -Originates in the skin, joints, and tendons -Ascends without crossing in dorsal white column -Senses fine touch, vibration, proprioception, two-point discrimination (Fasciculus Gracilis and Fasciculus Cuneatus) -One dorsal column contains fibers from all segments of the ipsilateral half of the body arranged in an orderly somatotopic fasion from medial to lateral

Fasciculus

-More specific location within a funiculus (e.g. Fasciculus Gracilis and Fasciculus Cuneatus are in the Dorsal Funiculus) - Based on location -Term to describe bundles of axons in white matter

Somatic Efferent

-Motor fibers that innervate skeletal muscle -Originate in anterior gray column of spinal cord - form ventral root of the spinal nerve

External Anatomy of SC

-Occupies the upper 2/3 of the spinal canal within the vertebral column (42-45cm long) - Connect at medulla - Distal end called conus medullaris -Ends at L1/L2 -Lowest nerve roots descend almost vertically below cord to form the cauda equina

Myotomes

-Skeletal musculature innervated by motor axons in a given spinal root -While most muscles are Innervated from motor axons from several spinal roots, lesions in a single root can cause weakness or atrophy

Spinal Reflex example

-Stretch Reflexes (aka tendon or deep tendon reflexes) -Provides feedback for maintaining muscle tone -Involves specialized sensory receptors (muscle spindles), afferent nerve fibers (primarily Ia fibers) from receptors via dorsal roots to spinal cord, two types of LMNs (alpha and gamma motor neurons) that project back to muscle, and specialized inhibitory interneurons (Renshaw cells) -The muscle spindle and afferent fibers provide information about both muscle length (static response) and the rate of change in muscle length (dynamic response) - Deep tendon reflexes are concerned with resisting inappropriate stretch on muscles and thus contribute to the maintenance of body posture. The Ia fibers from a muscle spindle end monosynaptically on and produce excitatory postsynaptic potentials in, motor neurons supplying extrafusal muscle fibers in the same muscle

Dermatome

-Well-defined segmental portion of the skin to which the sensor components of each spinal nerve is distributed -Territories of dermatomes tend to overlap- difficult to identify absence of single innervation

Radicular Arteries

Arise from intercostal arteries- largest branch "Artery of Adamkiewicz" supplies most of the arterial blood to the lower half of the spinal cord

Posterolateral Spinal Arteries

Arise from vertebral arteries and course downward to lower cervical and upper thoracic segments

Sulcus Arteries

Branches of radicular arteries that enter the intervertebral foramen and unite with posterior and anterior spinal arteries to form an irregular ring of arteries (arterial corona)

Anterior spinothalamic tract

Carries info about light touch

Lateral spinothalamic tract

Carries info about pain and temperature sensibility

Descending Fibers Systems

Carry motor impulses from the brain to the muscles

Ascending Fiber systems

Carry sensory information from the periphery of the body to the brain - Typically made up of 3 neurons 1st Order - Cell Body (soma) located in the dorsal root ganglion 2nd Order - Interneuron (when present) connects 1st and 3rd order neurons 3rd Order - Cell Body (soma) above the spinal cord (e.g. thalamus)

Ventral Gray Column (Anterior Horn)

Cell bodies/ origin of lower motor neurons (alpha dnd gamma); somatic motor

Anterior Medial Artery

Continuation of ASA below T4

Visceral Afferent

Convey sensory information from the viscera - their cell bodies are in the dorsal root ganglia

Superficial

Corneal - Afferent CN V - Center in the Pons - Efferent CN VII -Corneal Sensitivity Test For example diabetics

Upper Motor Neuron Lesions

Damage to the cerebral hemispheres or lateral white column of the spinal cord can produce signs of upper-motor-neuron lesions (spastic paralysis or paresis, little or no muscle atrophy,hyperactive deep tendon reflexes, diminished or absent superficial reflexes, and pathologic reflexes and signs - especially the extensor plantar reflex (Babinski's sign))

Matter in Spinal Cord

In the spinal cord, in GENERAL... -The gray matter mainly contains the cell bodies of neurons and glia -The white matter mainly contains myelinated and unmyelinated nerve fibers (axons) -Transmits information—->Receiving or Sending

Renshaw cells

Inhibitory interneuronsin the ventral horn that project to alpha motor neurons. They receive excitatory synaptic input via collaterals, which branch from alpha motor neurons (part of local feedback circuits that prevent overactivity in alpha motor neurons)

Fasciculus Gracilis

Input from lower half of body - from the lowest, most medial segments) - More medial in dorsal column

Fasciculus Cuneatus

Input from upper half of body - thoracic is more medial than cervical - More lateral in dorsal column

Veins in SC Circulation

Irregular external venous plexus in he epidural space, which communicates with other veins- eventually to the vena cavae

Visceral

Light- Afferent CN II - Center in the Midbrain - Efferent CN III - Pupil Testing with Light Accommodation - Afferent CN II -Center in Occipital Cortex -Efferent CN III - Pupil Testing with Accommodation

Anterior Spinal Artery

Midline union of paired branches of the vertebral arteries

Posterior Spinal Arteries

Much smaller than anterior artery - form posterolateral arterial plexus, supplying dorsal white columns and posterior portion of dorsal gray columns

Pathway overview

Remember that the corticospinal, spinothalamic, and dorsal column track show somatotopic organization

Golgi tendon organs

Stretch receptors in muscle tendons that, via afferent groupIb fibers running to dorsal roots and spinal gray matter, end on interneurons that inhibit alpha motor neurons bound for the agonist muscle, thus mediating the inversestretch reflex - a type of feedback to prevent overactivity of alpha motor neurons

Reflexes

Subconscious stimulus-response mechanisms. The reflexes are extremely important in the diagnosis and localization of neurologic lesions.

White Matter Columns

The spinal cord has white columns (funiculi) - dorsal, lateral, and ventral -Closer to the neck, the dorsal column is separated by the posterior intermediate septum into fasciculus gracilis (medial) and fasciculus cuneatus (lateral)

Meninges

Three membranes that surround the cord 1. Dura Mater: tough, fibrous 2. Arachnoid Mater: Thin, subarachnoid space contains CSF 3. Pia Mater: surrounds; sends septa into its substance; contributes to formation of filum terminale internum/ externum