Medulla Oblongata Notes

Medulla Oblongata

Overview

• The medulla oblongata, or simply medulla, is a stem-like structure that forms the lower part of the brainstem.
• It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions.
• The medulla contains:
  • Cardiovascular center
  • Respiratory center
  • Vomiting center
  • Vasomotor center
• It is responsible for:
  • Breathing
  • Heart rate
  • Blood pressure
  • Sleep-wake cycle

Functions

• Contains nuclei of the four inferior-most cranial nerves:
  • Glossopharyngeal nerve (CN IX)
  • Vagus nerve (CN X)
  • Accessory nerve (CN XI)
  • Hypoglossal nerve (CN XII)
• Serves as a conduit for ascending and descending nerve tracts between the brain and spinal cord.
• Houses centers for vital functions:
  • Heart rate
  • Blood pressure
  • Breathing

Surface Anatomy

• Located between the pons and the spinal cord.
• Its ventral (anterior) surface faces the basilar part of the occipital bone and the dens of the axis (C2), separated by meninges and ligaments.
• The dorsal (posterior) surface faces the fourth ventricle of the brain.

Posterior Surface

• Marked by the dorsal median sulcus in the midline, continuous with its spinal counterpart.
• Two vertical prominences run parallel to the sulcus:
  • Gracile fasciculus: Located immediately lateral to the sulcus, ascending from the spinal cord.
    • Contains the gracile nucleus, a relay nucleus that synapses with the fibers of the gracile fasciculus.
    • Forms a visible enlargement called the gracile tubercle.
  • Cuneate fasciculus: Lateral to the gracile fasciculus.
    • Contains the cuneate nucleus, a relay nucleus for the cuneate fasciculus.
    • Forms a large cranial enlargement called the cuneate tubercle.
  • Lateral to each cuneate nucleus is the trigeminal tubercle formed by the spinal nucleus of the trigeminal nerve (CN V).
  • Caudal to the trigeminal tubercle is the lateral funiculus of the medulla oblongata.

Central Canal

• The central canal of the spinal cord, a caudal continuation of the fourth ventricle, courses through the dorsal half of the medulla, carrying cerebrospinal fluid (CSF).
• The ventral and dorsal surfaces of the medulla oblongata are marked by unique protuberances and fissures formed by the presence of different nuclei and their associated tracts.

Anterior Surface

• Features the anterior median fissure along the midline.
  • This is a continuation of the anterior median fissure of the spinal cord.
• A vertical protuberance on either side of the fissure is the medullary pyramid, formed by the fibers of the corticospinal tract.
• Below the base of the pyramids, the median fissure is interrupted by the fascicles of the corticospinal tract that decussate and form the pyramidal decussation in the midline.
• Each half of the ventral medullary surface shows two sulci:
  • Anterolateral sulcus
  • Posterolateral sulcus
• Between the sulci, and just lateral and posterior to each pyramid, is another oval structure known as the olive.

Cranial Nerve Exit Points

• Hypoglossal nerve (CN XII) exits through the anterolateral sulcus, just medial to the olive.
• Glossopharyngeal (CN IX), vagus (CN X), and accessory nerves (CN XI) exit through the posterolateral sulcus, lateral to the olive, going from rostral to caudal.

Internal Structure

• Three parts, from ventral to dorsal:

  • Basis

  • Tegmentum

  • Tectum

  • The basis of the medulla contains the pyramidal decussation of the corticospinal tract.

  • The tegmentum of the medulla houses the inferior olivary nuclei, as well as the nuclei of the cranial nerves IX-XII.

  • The tectum of the medulla is composed of the inferior medullary velum, which is the posteroinferior part of the fourth ventricle.

Levels Of The Medulla

• Two levels:

  • Caudal half (''Closed'' medulla) – contains the central canal and two decussations:

    • Pyramidal decussation

    • Decussation of the medial lemniscus

  • Upper half (“Open” medulla) – contains the inferior olivary nucleus; one can see the dorsal surface of the medulla through the fourth ventricle.

Level Of The Pyramidal Decussation

• Organization is similar to the cervical spinal cord.
• Dorsal aspect contains the fasciculus gracillis and fasciculus cuneatus.
• Spinocerebellar and spinothalamic tracts are in the same general lateral position as in the spinal cord.
• Some anterior horn cells can be seen at this level as well.

Level Of The Decussation Of The Medial Lemniscus

• Slightly rostral to the pyramidal decussation, a second major decussation called the decussation of the medial lemniscus is present.
• The pyramidal decussation involves fibers from the sensorimotor cortex that are specially involved in motor function.

Nuclei and Tracts

• Nuclei (gray matter) are mainly within the dorsal part of the medulla.
• Tracts (white matter) pass mainly through the ventral part.
• A nucleus is a cluster of neuronal cell bodies within the central nervous system. For example, the dorsal nucleus of vagus nerve contains the neurons that control the visceral motor functions of the thoracic and abdominal viscera.
• A tract is a bundle of axons, usually myelinated, that connects the nuclei of different parts of the CNS.
• Nuclei and tracts are three-dimensional structures spanning multiple levels.
• The same tract can be found in cross-sections of several different, or even all, levels of the medulla oblongata.

Nuclei of the Medulla Oblongata

• Involved in many important bodily functions.
  • Cranial nerve nuclei (CN IX-XII):
    • Inferior salivatory nucleus
    • Spinal nucleus of trigeminal nerve
    • Solitary nucleus
    • Nucleus ambiguus
    • Dorsal nucleus of vagus nerve
    • Nucleus of accessory nerve
    • Nucleus of hypoglossal nerve
  • Relay nuclei (relaying stimuli from peripheral receptors to higher cortical centers):
    • Gracile nucleus
    • Cuneate nucleus
    • Arcuate nucleus
    • Olivary nuclei.
  • Reticular nuclei (controlling autonomic functions):
    • Raphe nuclei
    • Gigantocellular (magnocellular) nucleus
    • Perihypoglossal (Roller’s) nucleus
    • Lateral reticular nucleus
    • Other small clusters of neurons.

Specific Nuclei and Their Functions

• All nuclei are bilaterally distributed.

  • From medial to lateral, and dorsal to ventral:

    • Raphe nuclei:

      • Belong to the reticular formation.

      • Main source of serotonin.

      • Involved in mood, pain, wakefulness/arousal, and thermoregulation.

    • Perihypoglossal (Roller’s) nucleus:

      • Close to the hypoglossal nucleus.

      • Part of circuits involving eye movements (visual pursuit).

    • Hypoglossal nucleus:

      • Motor nucleus for the hypoglossal nerve.

      • Innervates intrinsic and extrinsic muscles of the tongue (except palatoglossus).

    • Dorsal nucleus of vagus nerve:

      • Visceral motor nucleus providing parasympathetic innervation to smooth muscles of the digestive tract, lungs, and viscera of the abdomen and thorax.

    • Medial vestibular nucleus (of Schwalbe):

      • Part of the vestibular system, spanning the caudal pons and rostral medulla.

      • Receives inputs from the semicircular canals of the inner ear.

      • Projects via the medial longitudinal fasciculus (MLF) to nuclei of the oculomotor, trochlear, and abducens nerves, mediating the vestibulo-ocular reflex.

    • Cuneate nucleus:

      • Receives sensory information about light touch, proprioception, and vibration from the ipsilateral upper limb, carried by the cuneate fasciculus.
      • Gives off fibers that decussate and join the medial lemniscus.

    • Spinal trigeminal nucleus: sensory nucleus getting touch, vibration, pain, and temperature from the face.

    • Nucleus ambiguus: motor nucleus contributing fibers to the glossopharyngeal, vagus, and accessory cranial nerves.

    • Lateral reticular nucleus: integrates impulses from various sources and conveys them to the ipsilateral cerebellum. Helps the cerebellum with motor planning and coordination.

    • Olivary nuclei: dorsal accessory, inferior, and medial accessory olivary nuclei. Connect with the cerebellum to aid motor coordination.

Blood Supply

• Supplied by branches of:
  • Vertebral and basilar arteries
  • Anterior and posterior spinal arteries
  • Posterior inferior cerebellar arteries (PICA)
• Branches enter along the roots of the glossopharyngeal, vagus, accessory, and hypoglossal nerves.

Ascending and Descending Tracts

• White matter is composed of:
  • Motor tract: Corticospinal (pyramidal) tract
  • Sensory tracts:
    • Cuneate fascicle
    • Gracile fascicle
    • Medial lemniscus
    • Spinal tract of the trigeminal nerve
    • Spinothalamic tract
    • Spinocerebellar tract
    • Inferior cerebellar peduncle
  • Medial longitudinal fascicle (MLF): connects nuclei of the oculomotor, trochlear, and abducens nerves, coordinating eye movements relative to visual stimuli (saccadic eye movements).

Specific Tracts

• Pyramidal tract:
  • Most ventral tract.
  • Projects into the pyramids on the ventral surface of the medulla.
  • Main voluntary motor tract from the cerebral cortex to the spinal cord.
  • Synapses with lower motor neurons that innervate skeletal muscles.
• Medial lemniscus:
  • Passes dorsal to the pyramidal tract.
  • Synapses with fibers from the cuneate and gracile nuclei, transmitting somatosensory information.
  • Passes through the thalamus and ends in the primary somatosensory cortex.
• Inferior cerebellar peduncle:
  • Passes lateral to the medial longitudinal fasciculus.
  • Contains tracts connecting the spinal cord with the brainstem and cerebellum.
  • Carries proprioceptive somatosensory fibers and outputs from the cerebellum, regulating posture and balance.
• Medial longitudinal fasciculus (MLF):
  • Courses dorsal to the medial lemniscus.
  • Connects nerves responsible for eye movements and integrates the movement of the eyes and head.
  • Plays a role in reflex eye movements and contains tracts innervating neck muscles and upper limbs.
• Spinal tract of the trigeminal nerve:
  • Courses ventral to the inferior cerebellar peduncle.
  • Carries sensory information from the face to the spinal nucleus of the trigeminal nerve.
• Spinothalamic tract:
  • Passes medial and ventral to the spinocerebellar tract.
  • Conveys information about pain and temperature from the contralateral side of the body to the thalamus.
  • Fibers continue towards the primary somatosensory cortex.
• Spinocerebellar tract:
  • Lies anterior to the lateral lenticular nucleus.
  • Carries proprioceptive information from skeletal muscle to the cerebellum.

Function Of The Medulla Oblongata

Cranial Nerves

• Controls bodily functions via:

  • Glossopharyngeal nerve (CN IX):

    • swallowing, salivation, and visceral, general and special (taste) sensation in the oral cavity.

  • Vagus nerve (CN X):

    • parasympathetic supply to the head, thorax and abdomen, gland secretion control, peristalsis, phonation, taste, visceral and general sensation of these regions.

  • Accessory nerve (CN XI):

    • phonation and movements of the head and shoulders.

  • Hypoglossal nerve (CN XII):

    • movements of the tongue, speech and swallowing.

Vasomotor Center

• The control system of the arterial blood pressure lies within the vasomotor center of the medulla oblongata.

  • Its function is to collect baroreceptor signals from the aortic body about the blood pressure, and to initiate an autonomic response according to those information in the following way:

    • To stimulate the parasympathetic nervous system through the vagus nerve and influence the heart rate.

    • To initiate the vasoconstricting sympathetic nervous system response through the spinal cord, all in order to control the diameter of all the arteries, arterioles, veins and venules of the body.

• Composed of reticular nuclei, divided into three groups:

  • Vasoconstrictor area:
    • In the anterolateral part of the rostral medulla.
    • Connects with spinal cord neurons that contribute to the peripheral sympathetic nerves, causing systemic vasoconstriction.
    • Most potent in the kidneys, skin, spleen, and intestines; less potent in the brain and skeletal muscles.
  • Vasodilator area:
    • In the anterolateral part of the caudal medulla.
    • Connects with the vasoconstrictor area and inhibits its function, causing systemic vasodilation.
    • Stimulates the vagus nerve, decreasing heart rate.
  • Sensory area:
    • Within the nucleus of the solitary tract.
    • Receives baroreceptor stimuli from the aortic body via the glossopharyngeal nerve.
    • Signals sent to both vasoconstrictor and vasodilator areas to control their activity.

Respiratory Center

• Complex group of nuclei in the pons and medulla oblongata.
• Consists of three parts:
  • Dorsal respiratory group
  • Ventral respiratory group
  • Pneumotaxic center
• The former two are found within the dorsal and ventral medulla, respectively, while the latter lies within the rostral pons.
• Dorsal respiratory group:
  • In charge of inspiration (inhaling air).
  • Most neurons are within the nucleus of the solitary tract.
  • Receives information from peripheral chemoreceptors about blood oxygen saturation.
  • Stimulates the phrenic nerve to contract the diaphragm and the thoracic spinal nerves to contract the intercostal muscles, resulting in inspiration.
• Ventral respiratory group:
  • Consists of the rostral part of the nucleus ambiguus and the nucleus retroambiguus.
  • Inactive during normal, non-forced breathing.
  • Stimulated by the dorsal respiratory group when increased pulmonary ventilation is needed.
  • Stimulates the accessory respiratory muscles.

Clinical Conditions

• Lateral Medullary Syndrome (Wallenberg’s syndrome):
  • Results from a vascular lesion of the vertebral and posterior inferior cerebellar arteries (PICA).
  • Involves loss of pain and temperature sensation on the opposite side of the body and ipsilateral face, loss of coordination, loss of the gag reflex, hoarseness, and difficulty with speech and swallowing.
  • Damage to descending autonomic fibers can cause Horner’s syndrome.
  • Affected structures include the spinothalamic tracts, spinal nucleus and tract of the trigeminal nerve, nucleus ambiguus and its axons, descending autonomic fibers, and vestibular nuclei.
• Medial Medullary Syndrome (De’jerime’s syndrome):
  • Results from a vascular lesion of the anterior spinal or paramedian branches of the vertebral arteries.
  • Involves the pyramidal tract, medial lemniscus, and root fibers of the hypoglossal nerve.
  • Three typical neurologic signs include:
    • Loss of conscious proprioception, touch, and pressure from the contralateral side of the body.
    • Contralateral upper motor neuron paralysis.
    • Paralysis of the ipsilateral aspect of the tongue and deviation of the tongue upon protrusion to the side ipsilateral to the lesion.