Spinal Cord And Medulla

Some aspects of spinal cord and medulla

Tract, fasciculus and lemniscus  Tract---

collection of nerve fibers (in the white matter) having the same origin, course and termination.  Fasciculi (bundles) and leminisci (ribbons) are the other names of the tracts.  Functionwise the tracts are sensory (ascending, afferent (signals coming to CNS) and motor (descending,efferent, signals going away from CNS).

Naming the tracts  Named

by the grey matter which they connect.  E.g. Corticospinal tract (from cerebral cortex to spinal cord (descending tract), spinothalamic tract (ascending tract).

nuclei and ganglia  Collections

of cell bodies of neurons in CNS are called the nuclei (not to be confused with the nucleus of a cell!!).  Collections of neurons in the peripheral nervous system are called “ganglia”.

PF LF

AF

The term FUNICULUS or COLUMN is used for white matter in the cord WHAT ARE THE COMMISSURES?

Major ascending tracts, their function and location in the cord  

 

Lateral spinothalamic tract—lateral white column--- pain and temperature from opposite side of the body. Spinotectal tract--- lateral white column--- visual responses ie; turning of head and eyes towards the source of stimulation. Spinocerebellar (dorsal and ventral)– lateral white column--- proprioception. Fasciculus gracilis(Gall) and F.cuneatus (Burdach)--posterior white column—joint sense,vibration sense, twopoint discrimination. stereognosis,conscious kinesthesia.

Some important descending tracts in the cord  Lateral

corticospinal tract (crossed pyramidal tract)--- lateral white column— skilled movements especially hands and feet.  Anterior corticospinal tract (uncrossed pyramidal)--- anterior white column—same as above  Vestibulospinal --- anterior white column– unconscious maintenance of balance.

Sensory neurons  Three

order of neurons  First order neuron– in the dorsal root ganglion.  Second order neuron– in the CNS.  Third order neuron– in the thalamus.

SYRINGOMYELIA  Destruction

of central canal and its surrounding areas--- results in injury to anterior white commissure which contains crossing anterior spinothalamic tracts.  This results in bilateral loss of pain and temperature sensations below the lesion but other sensations are preserved in the uncrossed tracts of posterior column. (dissociated sensory loss)

Tabes dorsalis  Tertiary

syphilitic degeneration of posterior white columns which only sensory tracts.  There is loss of tactile discrimination,vibration and position sense. (Rhomberg’s sign is positive).

Medulla oblongata

Medulla oblongata  Lower

part of brainstem.  Contains central canal in the lower part and its upper part contains IV ventricle.  Presents pyramids on either side of the midline anteriorly (formed by corticospinal tracts on their way to spinal cord) and olive produced by underlying inferior olivary nucleus.

On

the back of medulla are gracile and cuneate tubercles which contain nuclei of the same name(2nd order neurons) for the fasciculi of the same name.

Gross features

structure  The

characteristic appearance of grey matter in spinal cord is lost in medulla.  The grey matter contains the nuclei of last four cranial nerves and reticular formation.  For the arrangement of grey and white matters medulla is studied at 3 levels.

Medulla at the level of pyramidal decussation  



Important features are: 1.Spinal nucleus and tract of V in line with the posterior horn of spinal cord. 2.Pyramidal decussation(75%of fibers cross the midline and descend to the cord as lat.corticospinal tracts.

continued  3.corresponding

to the anterior horn is the spinal nucleus of accessory.  4.diffuse zone of nerve cells and fibers corresponding to the lateral white column called the reticular formation.

The reticular formation

SPECIAL FEATURES OF V nerve  Has

a large sensory nucleus (V1,V2 and V3) and a smaller motor nucleus.  Hence it extends throughout the brain stem (mesencephalic nucleus, principal nucleus and spinal tract and nucleus).  Perhaps the only cranial nerve nucleus to do so.

Medulla at the level of sensory decussation 



 

Fasciculi gracilis and cuneatus end in corresponding nuclei. Second order neurons begin as internal arcuate fibres,cross the midline and form the medial lemniscus. Olivary nucleus makes its appearance. Pyramids on either side of midline.

Sensory decussation

continued  Central

grey matter (close to the central canal) contains  1.hypoglossal nucleus (close to the midline)  2.dorsal nucleus of vagus (parasympathetic)  3.nucleus of tractus solitarius (taste)

Purely motor cranial nerves  3rd,4th,6th

and 12th cranial nerves lie in the same line and have their nuclei close to the midline of the brain stem.

continued  4.MLF

appears just posterior to medial lemniscus (this is connected to 3rd,4th,6th,8th and spinal nucleus of XI)  Spinocerebellar and lateral spinothalamic tracts move laterally to the lateral white column.  Lateral and anterior spinothalamic tracts lie close to each other and form “spinal lemniscus”.

Upper medulla (open part)  central

canal opens into IV ventricle.  along with XII, vestibular nuclei(2 of the 4) appear here.  nucleus ambiguus (common motor nucleus for 9th,10th and 11th cranial nerves) is present.

Medulla (clinical aspects)

Cranial nerves IX, X, XI and XII originate from the medulla. Functions of the medulla include: 1) control of somatic motor reflexes of the larynx, pharynx and tongue which are important in speech and swallowing. (nucleus ambiguus) 2) control of visceral motor reflexes, e.g., coughing, swallowing, salivating, vomiting, secretion, sneezing; the medulla also receives taste and other visceral sensations.

 3)

the MLF contains fibers for the regulation of the position of the head and neck and for coordinated eye movements;  4) the medulla is a relay for the cochlear and vestibular responses;  5) the reticular formation regulates a variety of functions including skeletal motor activity, consciousness, visceral functions, conduction of sensation, etc.

The features of medullary syndromes reflect its anatomy and physiology, i.e., the ascending sensory tracts, the descending motor systems, cranial nerves IX - XI exiting at this level, and the cranial nerves that are associated at or near this level, viz., CN VIII

Lateral medullary syndrome (PICA) of Wallenberg  Dorsolateral

aspect of medulla is supplied by PICA.  In this area spinothalamic tract, spinal nucleus and tract of V, nucleus ambiguus, inferior cerebellar peduncle, vestibular nuclei are situated. Effects

Inferior cerebellar peduncle

Ipsilateral ataxia; also vertigo from interruption of connections of vestibular nuclei

Spinal tract of trigeminal

Loss of pain and temperature sensation, same side of face

Spinothalamic tract

Loss of pain and temperature sensation, opposite side of the body

Nucleus ambiguus

Paralysis of vocal cord and pharyngeal muscles on the side of the lesion

Medial medullary syndrome (hypoglossal alternating hemiplegia)  Due

to thrombosis of vertebral artery branches supply paramedian regions of medulla.  Structures present here are: pyramid, hypoglossal nerve, medial lemniscus. Effects

pyramid

Contralateral hemiplegia

Hypoglossal Ipsilateral paralysis and nucleus atrophy of half of tongue (when protruded, the tip deviates to the same side of lesion) Medial lemniscus

Contralateral loss of position and vibration sense