BRAIN STRUCTURE THE BRAIN STEM Nervous System CNS

BRAIN STRUCTURE. THE BRAIN STEM

Nervous System CNS (central nervous system) Brain Spinal Cord PNS (peripheral nervous system) Peripheral Nerves Ganglia

Gross Divisions of the Brain Cerebrum (cerebral hemisphere) Diencephalon Cerebellum Brainstem

Development

Brainstem

Brain Stem • Located btwn cerebrum and spinal cord – Provides a pathway for tracts running btwn higher and lower neural centers. • Consists of the midbrain, , pons, and medulla oblongata. – Each region about 2. 5 cm in length. • Deep gray matter surrounded by white matter fiber tracts. • Produces automatic behaviors necessary for survival.

Corpus callosum Posterior commissure Fornix Occipital Lobe Thalamus Anterior commissure Quadrigeminal cistern Hypothalamus vermis Optic nerve 4 th ventricle Mammillary body pyramid

Anterior view of brainstem basal ganglia internal capsule optic chiasm optic nerve hypothalamus optic tract mammillary body cerebral peduncle interpeduncular fossa pons flocculus inferior olivary nuclear complex cerebellum pyramid cerebellar tonsil pyramidal decussation

Lateral view of brainstem optic tract cerebral peduncle trigeminal nerve optic nerve middle cerebellar peduncle optic chiasm vestibulocochlear nerve flocculus hypothalamus cuneate tubercle pons inferior olivary nuclear complex anterior median fissure pyramid

Posterior view of brainstem Superior colliculus Cerebral peduncle Superior cerebellar peduncle Middle cerebellar peduncle Inferior colliculus 4 th ventricle Inferior cerebellar peduncle Medulla

Basic Strucrure of Brain Stem 1. Roof plate (tectum) 2. Tegmentum 3. Basal portion

Basilar (Basal) Portion Descending Fibers from Cerebral Cortex 1. Corticospinal Tract 2. Corticobulbar Tract Pyramidal Tract 3. Corticopontocerebellar Fiber Corticopontine Fiber Pons Proper (Ventral Pons) Pontocerebellar Fiber

Roof Plate Midbrain Superior Colliculus: vision Inferior Colliculus: hearing Closed Medulla Nucleus Gracilis et Cuneatus: discriminative touch sensation cf. Cerebellum: equilibrium

Tegmentum Cranial nerve nuclei Precerebellar nuclei Reticular formation Tracts Ascending cortical pathways Cerebellar circuits Descending tracts Composite bundles

Cranial Nerves Columns of Brain Stem Motor Column 1. GSE 2. SVE 3. GVE Sensory Column 4. VA (GVA & SVA) 5. GSA 6. SSA

Components of Cranial Nerves

Ventricles in brainstem • Mesencephalon cerebral aqueduct • Metencephalon 4 th ventricle • Mylencephalon 4 th ventricle

Components of the brainstem • Sensory ascending pathways (dorsal): – Relay nuclei, tracts • Motor descending pathways (ventral) – Tracts, motor nuclei brainstem • Cerebellar pathways – Tracts, cerebellar afferent and efferent nuclei • Cranial nerve sensory and motor tracts – Cranial nerve nuclei, nerve entry and exit points • Autonomic centers: rhythmic chewing, respiration, cardiovascular regulation & gain adjustments for reflexes • Modulatory systems: locus coeruleus, raphe & substantia nigra – Chemically coded nuclei

Brainstem: 3 major divisions • Midbrain • Pons • Medulla

Medulla Oblongata • Most inferior region of the brain stem. • Becomes the spinal cord at the level of the foramen magnum. • Two ridges (medullary pyramids) are visible ventrally. – These are formed by the large motor corticospinal tracts. – Right above the medulla-SC junction, most of these fibers cross over (decussate).

External Features of the Medulla Oblongata A. Anterolateral Surface: 1. The anterior median fissure. 2. The pyramid: formed by the pyramidal (corticospinal) tract. 3. Pyramidal decussation. 4. The olive: formed by the inferior olivary nucleus. 5. The anterolateral sulci: gives exit to the hypoglossal nerves. 6. The posterolateral sulci: gives exit to the glossopharyngeal, vagus and cranial accessory nerves. 7. The inferior cerebellar peduncle.

B. The posterior surface of the medulla oblongata: 1. The posterior surface of the upper half (open medulla): from medial to lateral: a. Posterior median fissure. b. Inferior fovea. c. Hypoglossal trigone (triangle). d. Vagal trigone. e. Vestibular trigone. 2. The posterior surface of the lower half (closed medulla): from medial to lateral: a. Posterior median fissure. b. Gracile tract. c. Cuneate tract.

Medulla Oblongata • Nuclei in the medulla are autonomic nuclei, cranial nerves nuclei, and relay (motor/sensory) nuclei. • Autonomic nuclei: – Cardiovascular centers • Alter the rate and force of cardiac contractions • Alter the tone of vascular smooth muscle – Respiratory rhythmicity centers • Receive input from the pons – Additional Centers • Emesis, deglutition, coughing, hiccupping, and sneezing

Medulla Oblongata • Sensory & motor nuclei of 5 cranial nerves: – • Auditory/Vestibular (8), Glossopharyngeal (9), Vagus (10), Accessory (11), and Hypoglossal (12) Relay nuclei – – Nucleus gracilis and nucleus cuneatus convey somatic sensory information to the thalamus Olivary nuclei relay info from the spinal cord, cerebral cortex, and the brainstem to the cerebellar cortex.

Medulla summary Crossing point of motor pathways Autonomic centers Cardiac center Respiratory center Vasomotor center Centers of cough, vomiting, swallow, hiccup Cranial nerves (nuclei): CN IX - Glossopharyneal CN X - Vagus CN XI - Accessory CN XII – Hypoglossals

Pons

Pons • Btwn the midbrain & medulla and in front of cerebellum and IV ventricle. • Contains: – Sensory and motor nuclei of 4 cranial nerves • Trigeminal (5), Abducens (6), Facial (7), and Auditory/Vestibular (8) – Respiratory nuclei: • Apneustic & pneumotaxic centers work w/ the medulla to maintain respiratory rhythm – Nuclei & tracts that process and relay info to/from the cerebellum – Ascending, descending, and transverse tracts that interconnect other portions of the CNS

Corpus trapezoideum 1. Vestibular nerve 2. Cochlear nerve 4. Tuberculum 7. Superior olivary nucleus. 8. Trapezoid body. 9. Trapezoid nucleus. 10. Central acoustic tract (lateral lemniscus).

External Features of the Pons A. The anterior surface of the pons presents with: 1. The basilar groove: for basilar artery. 3. The middle cerebellar peduncle. 4. The trigeminal nerve. 5. The abducent nerve. 6. The facial nerve. 7. The vestibule-cochlear nerve.

B. The posterior surface of the pons: presents with: 1. The median longitudinal sulcus: the middle line. 2. The medial eminence: for facial nucleus. 3. The facial colliculus: it produced by the facial nucleus. 4. The medullary stria: transverse nerve fibers which separate posterior surface of pons from that of medulla oblongata. 5. Superior fovea. 6. Vestibular trigon.

Internal structures of the pons 1. Pontine nuclei: part of the cortico-ponto-cerebellar pathway. 2. Transverse fibers: pontocerebellar fibers. 3. Longitudinal fibers: include pyramidal and corticopontine fibers. 4. Nuclei of the trigeminal nerve: a. Motor nucleus. b. Sensory nuclei. 5. Nucleus of the abducent nerve. 6. Nuclei of the facial nerve: a. Motor nucleus. b. Superior salivary nucleus: parasympathetic function.

Locus coeruleus • Physiological reactions to stress and panic. • Described in ХVIII c. by F. Vicq-d’Azyr, (J. C. Reil). • Dark blue, due to melanin in the neurons. – nucleus pigmentosus pontis • Main site of norepinephrin synthesis in the brain. • Locus coeruleus-noradrenergic system or LC-NA system.

Connections Projections – far and wide reaching • Spinal cord, brain stem, cerebellum, hypothalamus, amygdala, cerebral cortex. Afferents from • Cerebellum, hypothalamus, amygdala, cingulate cortex, nuclei raphae pontis

Functions • • • Neuroplasticity Arousal-dream Attention and memory Emotions Behavior flexibility, behavior inhibition and stress • Balance

7. Nuclei of the vestibulocochlear nerve. 8. Lateral lemniscus: for auditory sensation. 9. Spinal lemniscus: a band of ascending fibers carrying pain, temperature and crude touch from the opposite side of the body below the head. 10. Trigeminal lemniscus: a band of ascending fibers carrying pain, temperature and touch and proprioception from the opposite side of the face and scalp. 11. Medial lemniscus: deep sensation and fine touch from the opposite side of the body below the head.

Pons summary • Respiratory center • Norepinephrine center • Modulation of pain and panic • Control of homeostasis • Cranial nerves: Trigeminal (V), Abducent (VI), Facial (VII), Vestibulocochlear (VII)

Midbrain

Midbrain Btwn diencephalon and pons. • 2 bulging cerebral peduncles on the ventral side. These contain: • Descending fibers to cerebellum via pons • Descending pyramidal tracts • Cerebral aqueduct runs thru the midbrain; connects the 3 rd and 4 th ventricles of the brain. • The roof (tectum) of the aqueduct contains the corpora quadrigemina • 2 superior colliculi control reflex movements of the eyes, head and neck in response to visual stimuli • 2 inferior colliculi control reflex movements of the head, neck, and trunk in response to auditory stimuli

External features of the midbrain A. The anterior surface of the midbrain presents: 1. Two cerebral peduncles. 2. Interpeduncular fossa/posterior perforated substance 2. Oculomotor nerve: emerging from the medial side of the cerebral peduncle.

B. The posterior surface of the midbrain: presents the following features: 1. Four colliculi which include: a. Two superior colliculi (Visual function). b. Two inferior colliculi (Auditory function). 2. Two superior cerebellar peduncles. 3. Superior medullary velum (membrane): lies between the two superior cerebellar peduncles. 4. The trochlear nerve. C. The lateral surface of the midbrain: presents the following features: 1. Brachium of superior colliculus. 2. Brachium of inferior colliculus.

Internal Structures of the Midbrain On transverse section of the midbrain, the cerebral (Sylvius) aqueduct divides it into two main parts: 1. Tectum: the smaller dorsal part behind the Sylvius aqueduct, and consists of: a. Two Superior Colliculi: reflex centers of vision. b. Two Inferior Colliculi: reflex centers of hearing.

2. Two cerebral peduncles: the larger ventral part in front of aqueduct, it consists of three parts: a. Crus cerebri: the most anterior part which consists of pyramidal and corticonuclear fibers. b. Substantia Nigra: a thick lamina of gray mater. An extrapyramidal center. c. Tegmentum: the posterior part of the cerebral peduncle. Contains ascending tracts, nuclei of III & IV cranial nerves, reticular formation of the midbrain.

• Cranial nerves 3&4 (oculomotor and trochlear) exit from the midbrain • Midbrain also contains the headquarters of the reticular activating system

Midbrain On each side contains red nucleus and substantia nigra • Red nucleus - rich in blood vessels; - info from cerebrum and cerebellum; - subconscious control of muscle tone & posture • Substantia nigra - ventral to the red nucleus; - melanin-containing; - secretes dopamine; - inhibits the excitatory neurons of the basal nuclei. – Damage to the substantia nigra would cause what?

Internal Structures of the Midbrain The main nuclei of the midbrain: 1. Nucleus of oculomotor nerve. 2. Nucleus of the trochlear nerve. 3. Red nucleus: important extrapyramidal centre. 4. Mesencephalon nucleus of the trigeminal nerve: for deep sensation. 5. Inferior colliculus: a centre for auditory reflexes. 6. Superior colliculus: a centre for visual reflexes. 7. Substantia nigra: an extrapyramidal centre. 8. Reticular formation.

Brain death tests 1. Body temperature below 35°C. 2. Fixed pupils, no reaction to light. 3. Corneal reflex absent. 4. Vestibulo-ocular reflex absent. 5. Cranial nerve reflexes absent. 6. Vomiting reflex absent. 7. Respiratory movements absent if ventilation is stopped for 10 min.