(an interactive figures are placed in this area to assist visualization of the anatomical features that are described below. Click on the italicized red text below to see images; Toggle (on-off) visualization of images by re-clicking. For those of you with older( Netscape or IE pre version 4) browsers, a separate page of these images will be provided to assist you.
Originally derived from the myelencephalon, it is similar in appearance to the spinal cord....
Caudal: The foramen magnum or the first cervical nerve or the decussation of fibers of the cortical spinal tract (tracts and pathways will be discussed later)
Rostral: On the ventral surface, to the transverse fibers of the pons; on the dorsal surface, to the most dilated portion of the fourth ventricle (ventral to the cerebellum)
Dorsal--from medial to lateral
dorsal median sulcus---divides the medulla (and cord) into halves
fasciculus ("bundle") gracilis--one of two dorsal columns
fasciculus ("bundle") cuneates--the other of the two dorsal columns
dorsolateral sulcus (not visible in figures provided)--a natural demarcation of afferent vs efferent tracts
Ventral--from medial to lateral
ventral median fissure--divides medulla into halves
pyramids--the corticospinal tracts pass through the medulla
cranial nerves--part of cranial nerveV; cranial nerves VI through XII
(Anatomical illustration to follow...)
In addition to cranial nerve functions, important areas of the medulla control respiration, heart rate and gastrointestinal motility
The reticular formation, a loosely organized neuronetwork approximately central medulla to the pons, is paramount in the regulation of arousal and consciousness as well as motor neuron activity in the brain and the spinal cord. This portion of the brainstem facilitates (or inhibits) reflex activity and cortical motor movements
The vasomotor center, rostral aspect, profoundly regulated the heart rate and vascular tone.
The ventrally located nucleus ambiguus and portions of the reticular formation are components of the medullary respiratory center.
Cranial Nerves of the Medulla
CN VI (abducens) contains GSE cell bodies and efferent fibers to the retractor bulbi muscle (pulls eye into orbit) and the lateral rectus muscle (rotates globe in lateral direction.
CN VII (facial)
contains SVE fibers with a more rostral component having GVE fibers. Taste (SVA) neurons (for anterior 1/3 of tongue) are located (elsewhere) in the geniculate ganglion. The SVE fibers emerge from cell bodies of the facial nucleus at approximate level of caudal cerebellar attachment. Fibers pass dorsomedial to the middle of the 4th ventricle, then ventrolaterally in complex form through the internal acoustic meatus of the petrosal bone, dorsal to the vestibular-cochlear nerve, ultimately emerging from the stylomastoid foramen. This nerve is motor to muscles of facial expression...ear, eyelids, nose, cheek, lips and (interestingly) the caudal portion of the digastricus muscle.
CN VIII (vestibular-cochlear)
consists of two nerves: the Vestibular and the Cochlear.
The vestibular consists of four nuclei which lie adjacent and lateral to the 4th ventricle. These receive afferent fibers from the vestibular fibers and from the fastigeal nucleus of the cerebellum. Efferent fibers project caudally and also rostrally along the medial longitudinal fasciculus to affect activity of the motor nuclei of CNIII, CNIV and CN VI...ultimately resulting in adjustments in ocular movement and through the midbrain and contralateral thalamus and internal capsule to the temporal lobe, affecting conscious positioning.. The caudal projections will be discussed under the general subject of tracts. Together, the nerve functions to regulate equilibrium via postioning of eyeballs, trunk, limbs and head.
The cochlear (auditory) nerve nuclei lie as dorsal and ventral bulges from the lateral medulla. Fibers are intermingled with the those of the vestibular nerve. Audition neurophysiology is complex...and will not be further described here.
CN IX (Glossopharyngeal).. along with CN X (vagus) is motor to muscles of the larynx and pharynx (SVE), taste (SVA) over the caudal 1/3 of the tongue, and provides parasympathetic fibers (GVE) to parotid and zygomatic salivary glands; sensory (GSA) fibers to the oral mucosa (including the tongue) and (via GVA) to the coratid sinus (asthe afferent arm of the refex arc). GSA inputs ultimately terminate in the nucleus of CN V.
CN X (Vagus)...is autonomic [parasympathetic] motor (GVE) and sensory (GVA) to the abdominal and thoracic viscera; sensory (GSA) to the external ear (NOTE: since the vagus is sensory also to the stomach, irritation of the ear can result in vomition...if the brain fails to distinguish the true source of the irritation). GSA inputs ultimately terminate in the nucleus of CN V.
CN XI (Accessory).. is motor (SVE) to the trapezius muscle and the intrinsic muscles of the larynx (except the cricothyroideus); is motor (GSE) to the trapezius, brachiocephalicus, sternocephalicus and omotransversarius muscles.
CN XII (Hypoglossal)..supplies motor (GSE) to intrinsic and extrinsic tongue muscles and to the geniohyoid muscle and is also proprioceptive (GSA) to the muscles of the tongue.
(more to come....in 2005 or 2006)