Brainstem III

by

Internal Structures and Vascular Supply

Functional Brainstem Anatomy

(the author breaks down the brainstem region into 4 functional groupings)

Cranial Nerve Nuclei and Related Structures – Six Columns

(this is a review of material covered in Chap. 12, Table 12:3 and Figure 12:5)

Somatic Motor

  • General Somatic Efferent

Branchial Motor

  • Special Visceral Efferent

Parasympathetic

  • General Visceral Efferent

Visceral Sensory

  • Special Visceral Afferent and General Visceral Afferent

General Somatic Sensory

  • General Somatic Afferent

Special Somatic Sensory

  • Special Somatic Afferent

Long Tracts

 

(this is a review of the material covered in Chap. 6 & 7)

  • Lateral corticospinal tract (motor functions)
  • Posterior columns (sensory functions – vibration, joint position, fine touch)
  • Anterolateral pathways (sensory functions – pain, temperature, crude touch)

Cerebellar Circuitry

 

(this is a preview of the material covered in Chap. 15)

  • Lesions to the cerebellar circuitry result in ataxia, ipsilateral to side of lesion because the cerebellar circuits decussate twice before reaching lower motor neurons.
  • Cerebellum is attached to brainstem via three white matter pathways:
  • Superior cerebellar peduncle (contains mainly cerebellar outputs; the fibers of the superior cerebellar peduncle decussate at the inferior colliculi in the midbrain then continue up the brainstem to reach the red nucleus at the level of the superior colliculi. Other fibers continue rostrally to influence the motor cortex
  • Middle cerebellar peduncle (provides input to the cerebellum arising from the pontine nuclei which receive input from the corticopontine fibers
  • Inferior cerebellar peduncle (carries input to the cerebellum from the spinal cord)

Reticular Formation and Related Structures

 

  • The reticular formation is a central core of nuclei that runs through the entire length of the brainstem (p. 589 provides a nice illustration)
  • Two main components:
  • Rostral reticular formation (maintains alert conscious state in the brain)
  • Caudal reticular formation (maintains a variety of important motor, reflex, and autonomic functions)
  • Other structures in the brainstem tegentum:
  • Periaqueductal gray matter in the midbrain (involved in pain modulation)
  • Chemotactic trigger zone in the medulla (involved in causing nausea)

 

Widespread Projection Systems of Brainstem and Forebrain: Consciousness, Attention, and Other Functions

Brainstem Reticular Formation and Thalamus

 

  • The pontomesencephalic reticular formation (in the region of the rostral reticular formation) forms a circuit together with intralaminar nuclei of the thalamus that is critical to maintaining normal consciousness.
  • The Intralaminar nuclei in turn project to the cerebral cortex.
  • Ascending and descending influences are both critical for consciousness.
  • Coma is caused by dysfunction of he upper brainstem reticular formation or by dysfunction of extensive bilateral regions of the cerebral cortex. Bilateral lesions of the thalamus can also cause coma.

Pontomesencephalic Reticular Formation

 

  • Also projects to the hypothalamus and basal forebrain which then project to the cortex.

 

Other Regions of the CNS Project to the Reticular Formation

 

  • Projections from the limbic system to the reticular formation are responsible for increased alertness in stressful or emotional situations.
  • Projections from the anterolateral pathway of the spinal cord transmit information about pain to the reticular formation.
  • Projections from the association cortices transmit information about cognitive processes (e.g., these areas will stimulate increased arousal/alertness when one needs to engage in problem-solving activity).
  • Other circuits that may play a role in attentional mechanisms include the superior colliculi, cerebellum, and thalamic reticular nucleus.

 

Identified Neurotransmitter Systems

 

(this is not a complete list of neurotransmitters but apparently this section of the book focuses on neurotransmitters that have widespread projections and are involved in the maintenance of alertness, arousal, and attention)

Acetylcholine

  • Major efferent neurotransmitter of the peripheral nervous system (reminder: efferent = a pathway that carries signals away from a structure)
  • Only has a limited role in CNS function
  • Found in the pontomesencephalic region of the brain stem and in the basal forebrain where it plays a role in arousal
  • Main receptor type = muscarinic

Dopamine

  • Found mainly in neurons located in the ventral midbrain (substantia nigra pars compacta & ventral tegmental area)
  • Three projection systems:
  • Mesostriatal pathway to caudate and putamen (this is the one implicated in Parkinson’s disease)
  • Mesolimbic pathway to limbic structures (implicated in positive symptoms of Schizophrenia)
  • Mesocortical pathway to prefrontal cortex (implicated in working memory and other executive skills and cognitve deficits in Parkinson’s disease and negative symptoms of Schizophrenia)

Norepinephrine

(also called noradrenaline)

  • Primarily found in the locus ceruleus (located near fourth ventricle in the rostral pons)
  • Also found scattered throughout the lateral tegmental area of the pons and medulla
  • Projects to the entire forebrain through the thalamus
  • Functions of the ascending norepinephrine projection system include modulation of attention, sleep-wake states, and mood
  • Psychostimulant treastment of ADD enhances noradrenergic transmission
  • Noradrenergic transmission also seems to be important in mood disorders including depression and bipolar and anxiety disorders

Serotonin

  • Found in neurons of the raphe nuclei of the midbrain, pons, and medulla
  • Rostral raphe nuclei project to entire forebrain including cortex, thalamus, and basal ganglia
  • Plays a role in several psychiatric disorders including depression, anxiety, OCD, and aggressive behavior
  • Caudal raphe nuclei project to the cerebellum, medulla, and spinal cord and are involved in pain modulation

Histamine

  • Found mainly in the neurons of the posterior hypothalamus
  • Diffuse histaminergic projections from the posterior hypothalamus to the forebrain may be important to maintaining the alert state
  • Histaimine has excitatory effects on thalamic neurons, and both excitatory and inhibitory effects on cortical neurons
  • Antihistamine medications are thought to cause drowsiness by blocking CNS histamine receptors
  • Histaminergic neurons participate in a circuit with the hypothalamus that regulates sleep and arousal

 

Anatomy of the Sleep-Wake Cycle

  • There are four stages of progressively deeper non-REM sleep followed by REM sleep, and this cycle repeats several times through the night.
  • Sleep-producing regions are located in the medulla.
  • Lesions of the pons can produce excessive sleep (or coma) while lesions of the medulla can result in decreased sleep because of the sleep-producing functions in the medulla.
  • During non-REM sleep, GABA neurons in the preoptic area of the hypothalamus inhibit histaminergic neurons which removes histaminergic activtation from the forebrain and sections of the brainstem.
  • During REM sleep, cells in the pontine reticular formation interact with other brainstem circuits to activate cholinergic inputs to the thalamus, inhibit tonic muscle activity, activate phasic eye movements, and activate other phasic motor activity.

The suprachiasmatic nucleus of the hypothalamus receives retinal inputs and is crucial for setting circadian rhythms and synchronizing them with the light-dark cycle.