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How do the grey matter tracts of the spinal cord transition in the medulla?
- Substantia gelatinosa, posteromarginal nucleus, & nucleus proprius: Blend with spinal nucleus of trigeminal nerve to receive pain and temperature from face
- Lissauer's tract: This descending tract limits the incoming pain sensation to the spinal cord; in the medulla, it becomes the spinal trigeminal tract, made up of Aδ and C fibers from trigeminal ganglion
- Spinal accessory nucleus: These ventral horn motor neurons become the dorsal somatic motor column while the special visceral efferent (branchiomotor) cell column takes its position
How do the white matter tracts of the spinal cord transition in the medulla?
- Corticospinal tract: These motor fibers move from their lateral position to ventral via the pyramidal decussation
- Dorsal column: This sensory tract terminates in the caudal medulla and becomes the medial lemniscus
- Spinothalamic tract: Continues in a lateral position, but is forced dorsally by the olive
- Dorsal spinocerebellar tract: Carrying proprioception, this tract becomes inferior peduncle
- This tract is formed by Aδ and C fibers (pain, temp, and simple touch from the face) from trigeminal ganglion descending in the spinal tract to the spinal nucleus. At this level in the caudal medulla, the second neuron decussates and ascends in the ventral trigeminothalamic tract (near spinothalamic in medulla). These fibers go to the ventral posterior medial (VPM) nucleus of the thalamus and limbic areas, midbrain, and reticular areas. The VPM then projects to the primary sensory cortex.
Dorsal column system
- The cuneate and gracile fasciculi synapse in the corresponding nuclei. The second neuron axons decussate as internal arcuate fibers in the sensory decussation to form the medial lemniscus in the ventral midline. Fibers in the cuneate fasciculus send branches to the accessory cuneate nucleus which go to the cerebellum via the cuneocerebellar tract.
Carrying pain and temperature, this tract gets pushed dorsally by the olive but is still located laterally. Some fibers send branches to the medullary reticular formation, where pain is transmitted to midbrain and limbic structures.
Forms the bulk of the midmedulla. It sends its olivocerebellar fibers (derived from the inferior olivary complex) to the opposite cerebellum through the inferior cerebellar peduncle. Tells the cerebellum about planned movements and if they are being executed properly.
What efferent nuclei are found in the medulla?
- Hypoglossal nucleus: Motor innervation to intrinsic and extrinsic tongue muscles by CNXII emerging from preolivary sulcus (in rostral medulla, found just medial to dorsal motor nucleus of vagus)
- Nucleus ambiguus: Motor innervation to muscles of pharynx, larynx, soft palate (levator palati), and upper esophagus via CN IX and X; visceromotor control of cardiac ganglia in the heart wall and SA/AV nodes (conservative effects such as slowing heart rate and decreasing ventricular contraction force)
- Dorsal motor nucleus of the vagus: Viceromotor control through parasympathetics of swallowing, altering GI peristalsis, and enzyme secretion in GI tract; found next to the floor of the fourth ventricle
Nucleus of the solitary tract (NTS) and solitary tract
- Afferents (first neuron) from CN VII, IX, and X enter the brainstem and travel in the solitary tract to the NTS
- They carry viseral sensory information from the tongue, tonsils, pharynx and larynx, tympanic cavity, auditory tube, thoracic and abdominal visera, aortic arch and carotid sinus, and carotid body
- Taste from the tongue (CN VII and IX) and epiglottis (CN X) terminate in the NTS also
- Information about baroreceptor activity is also relayed upwards to the SSN (??)
Autonomic control of cardiovascular function in medulla *Remember, autonomics include afferents*
- Visceral sensory afferents from the carotid sinus (carotid sinus nerve to CN IX), aortic arch (CN X) and carotid body travel to the nucleus of the solitary tract (NTS), where the second neurons then run to the external nucleus ambiguus (influence parasympathetics) and to the ventrolateral medulla (carry info up to hypothalamus and down to influence sympathetics). These descending neurons from the rostral ventrolateral medulla then reach preganglionic sympathetic neurons in the spinal cord and synapse, going to the renal artery, peripheral vascular beds, and the heart.
- Baroreceptor activation is detected and carried to the NTS by CN X. From the NTS, parasympathetics are excited and travel along CN X to slow the heart rate.
- Also from the NTS, axons travel to the caudal ventrolateral medulla and then to the rostral ventrolateral medulla. Axons from the rostral ventrolateral medulla then join the sympathetics in the spinal cord in the intermediolateral horn.
- Glutamate: An excitatory NT, used at both the NTS an the caudal ventrolteral medulla
- GABA: An inhibitory NT, released in the rostral ventrolateral medulla; activation of the baroreceptors thus results in inhibition of sympathetic nerve activity to reduce vasculomotor tone
- It is also believed that axons from the NTS ascend to influence cerebral blood flow
- This reflex is activated with each pulsation of the heart--higher blood pressure, lower sympathetic nerve activity
- One neuron, one neurotransmitter
- This does not hold true; a given neuron may synthesize multiple different neurotransmitters in both PNS and CNS
Is blood pressure higher or lower when doing light activity when compared to eating or drinking?
Lower, feeding results in greater blood pressures
Von Bezold-Jarisch Reflex
- Triggered by serotonin or atrial stretch (chemo or mechanoreceptors) as a cardioprotective reflex
- Results in low blood pressure and low heart rate
- Triggered by low oxygen
- Results in increased arterial pressure in order to perfuse vital organs better
Contraction band necrosis
- Seen in sudden cardiac death, these calcified bands form when heart cells die in contraction and become very hard
- This is caused by increasses stress or high excitement and subsequent activation of SNA
- Free radicals accumulate, calcium enters the cells, and cells die
NTS lesion cardiac effects
- Attenuated baroreflex function
- Variable arterial pressures
- Necrosis of cardiac myocytes
- Abnormal cardiac rhythyms (ectopic ventricular beats, lack of systole, irritable beats)
When the brain is experiencing ischemia, activation of parasympathetics or sympathetics results in protection?
Parasympathetic activation can help the brain protect itself during ischemia
Control of respiration in medulla
- Baroreceptor (carotid sinus/aortic arch), chemosensory (carotid body), and lung mechanosensory inputs are sent to the dorsal respiratory group of the NTS to support inspiration
- The reticular formation contains the ventral respiratory group, which is responsible for expiration
- Efferents are carried by the phrenic nerve
- [Inspiration is ended by inhibition of the dorsal respiratory group by the pneumotaxic center of the pons. The apneustic center of the pons may also help drive inspiration]
Reflexes mediated by medulla
- Baroreceptor, chemoreceptor, and respiratory drive
- Swallowing: The swallowing center in the reticular formation is found between the NTS and nucleus ambiguus
- Vomiting: Vomiting center is found dorsally
- Therefore, dysphagia, nausea, and vomiting almost always accompany medullary lesions
- Gag: Afferent from oropharynx by CN IX to NTS and spinal nucleus of V (somehow); efferent by CN X to levator palati, pharyngeal constrictors, and larynx; lateral medullary syndrome results in diminished or absent gag reflex
Vestibular and cochlear nuclei location and lesions
- Location: Near the cerebellar peduncles in the pons and medulla ("straddling")
- Lesion: Signs similar to damage of the semicircular canals/labyrinth (vertigo, trunk ataxia, and nausea)
Central tegmental tract
- The diffuse white matter found in the center of the medulla. Contains the hypothalamospinal tract, which gives inputs to the preganglionic sympathetics. Lesions can lead to Horner's syndrome
- Cause: Lesions to the hypothalamospinal pathway, which carries preganglionic sympathetics; this pathway is found in the central tegmental tract of the medulla or upper spinal cord
- Signs: Miosis (constricted pupil), loss of reflex dilation, ptosis (drooping eyelid), anhidrosis (lack of sweating), enopthalmos (sunken eye), and flushing (from capillary dilation)
Medial medullary syndrome
- Occlusion of the anterior spinal artery
- Hypoglossal nucleus/nerve: Tongue points to affected side when protruded (ipsilateral)
- Pyramid: Loss of UMN to contralateral body
- Medial lemniscus: Loss of contralateral 2-point discrimination, vibration, and kinesthesia to body
Lateral medullary syndrome (Wallenberg's)
- Occlusion of vertebral artery or PICA
- Nucleus ambiguus: Dysphagia (difficulty swallowing), displaced uvula, and flaccid vocal cord
- Spinal nucleus of V: Loss of contralateral pain and temperature over face
- Spinothalamic tract: Loss of contralateral body pain, temperature, and crude touch
- Hypothalamospinal: Horner's syndrome
- Inferior cerebellar peduncle: Ipsilateral ataxia
- Vestibular nuclei: Vomiting and nausea
- Reticular formation: Hiccups
Name all these parts of the medulla!