Brain Stem - Brain Stem • The brainstem consists of the...

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Unformatted text preview: Brain Stem • The brainstem consists of the medulla, the pons and the midbrain. • The activities of the brainstem may be divided into 3 general types: – Conduit functions; – Cranial nerve functions; – integrative functions. Conduit Functions • The conduit functions are apparent, since the only way for ascending tracts to reach the thalamus or cerebellum (or for descending tracts to reach the spinal cord) is through the brainstem. • Many of these tracts, however, are not straight-through affairs, and relay nuclei in the brainstem are frequently involved. Cranial Nerve Functions • The cranial nerves are the head's equivalent of spinal nerve fibers; – with the addition of special fibers involved in the special senses of olfaction, sight, hearing, equilibrium, and taste. • A wide assortment of sensory and motor nuclei related to cranial nerve function can be found at various brainstem levels. Integrative Functions • A number of integrative functions are organized at the level of the brainstem. • These include complex motor patterns, aspects of respiratory and cardiovascular activity, and even some regulation of the level of consciousness. • Much of this is accomplished by the reticular formation. Internal Structure of the Brainstem At any given brainstem level rostral to the obex, 3 general areas can be identified in cross section. • The area posterior to the ventricular space; • The area anterior to the ventricular space; • And large structures "appended" to the anterior surface of the brainstem. Colliculi tectum ventricular space tegmentum Posterior to the Ventricular Space • The midbrain is the only place where this part contains a substantial amount of neural tissue. • This region of the midbrain is called the tectum (L. roof). • It consists of the superior and inferior colliculi. • In the pons and rostral medulla, the 4th ventricle is covered posteriorly by the superior and inferior medullary vela. • Posterior to these, of course, is the cerebellum. Anterior Medullary Velum 4th Ventricle Anterior to the Ventricular Space • This part is called the tegmentum (L. covering). • The tegmentum contains most of the structures: – the reticular formation, – cranial nerve nuclei and tracts, – ascending pathways from the spinal cord, and some descending pathways. • Structures Appended to the Anterior Surface • large fiber bundles of the cerebral peduncles, the basal pons, and the pyramids of the medulla. • These structures contain fiber descending from the cerebral cortex to: – the spinal cord; – certain cranial nerve nuclei – the pontine nuclei, which in turn project to the cerebellum. • Midbrain • Tectum- rooflike region dorsal to the aqueduct consisting of the Corpora Quadrigemina • Corpora quadrigemina (Superior and inferior colliculi) visual reflexes & relay center for auditory information. – Two pairs of rounded knobs on the upper surface of the midbrain mark the location of four nuclei, which are called collectively the "corpora quadrigemina.“ – These masses contain the centers for certain visual reflexes, such as those responsible for moving the eyes to view something as the head is turned. They also contain the hearing reflex centers that operate when it is necessary to move the head so that sounds can be heard better. Anterior Medullary Velum 4th Ventricle 1. Superior medullary velum 2. Pons 3. Medulla oblongata 4. Pia mater 5. Ependyma 6. Choroid plexus Midbrain • Contains the rostral end of the reticular formation (which results in the loss of consciousness or coma if impaired). Midbrain • The ventral or anterior part has the cerebral peduncle, which is a huge bundle of axons traveling from the cerebral cortex into/ through the brainstem; fibers are important for voluntary motor function. • Red nucleus- so named because they have a pinkish color in fresh Brain specimens, because of an abundant Blood supply. The red nuclei aid in the unconscious regulation and coordination of motor activities. Midbrain Substantia nigra• A nuclear mass between the Tegmentum and Cerebral Peduncles, is a pigmented region of the midbrain with cytoplasmic Melanin Granules that give it a dark grayto-black color. – has interconnections with other Basal Ganglia Nuclei of the Cerebrum and is involved in Coordinating Movement and Muscle Tone. • Sends inhibitory signals to the thalamus and basal ganglia • Degeneration of the neurons leads to muscle tremors of Parkinson’s disease Midbrain • Medial lemniscus- a continuation of the gracile and cuneate tracts of the spinal cord and brainstem • Contains two cranial nerve nuclei that control eye movement: Cranial Nerves III (Oculomotor) and IV (Trochlear). Ventral View Pons • The pons (L. bridge) is dominated by the massive, transversely oriented structure on its ventral surface called the basal pons it and looks like a bridge interconnecting the two cerebellar hemispheres. It does not, however, interconnect them. • Many of the fibres descending in a cerebral peduncle synapse in scattered pontine nuclei which acts as relay station that connects the motor cortex with the cerebellum concerning the coordination of voluntary movements. • These connections are made via the middle cerebellar peduncles The Entry and Exit of Cranial Nerves around the Pons • The trigeminal nerve (CN V-innervates the face and chewing muscles) enters the brainstem at the midpons. • Three other cranial nerves enter (or leave) along the groove between the basal pons and the medulla; • The abducens nerve (CN VI- innervates an eye moving muscle) is the smallest and most medially located of these three, exiting where the pyramid emerges from the basal pons; • The facial nerve (CN VII- supplies muscles of facial expression) is farther lateral and consists of two parts: a larger and more medial motor root and a smaller sensory root (sometimes referred to as the intermediate nerve); 1.Olfactory Bulbs (I) 2. Dura Mater 3. Optic Nerve (II) 4.Optic Chiasm5.Pituitary Gland (Hypophysis)6a.Trigeminal Nerve (V) Opthalmic Branch (V)6b.Trigeminal Nerve (V) - Maxillary Branch (V)6c.Trigeminal Nerve (V) - Mandibular Branch (V) 7.Pons 8.Vestibulcochlear Nerve (VIII) 9.Abducens Nerve (VI)10.Facial Nerve (VII)11.Medulla Oblongata12.Spinal Accessory Nerve (XI)13.Hypoglossal Nerve (XII)14.Spinal Cord The Entry and Exit of Cranial Nerves around the Pons • The vestibulocochlear nerve (CN VIII) is slightly lateral to the facial nerve and also has two parts: a vestibular division and a more lateral cochlear division. • trochlear nerve (CN IV- innervate eye movement muscle) emerges from the dorsal surface of the brainstem between the junction of the pons and midbrain The Cerebellum ("little brain") • The cerebellum is involved in the coordination of movement • it compares what you thought you were going to do (according to motor cortex) with what is actually happening down in the limbs (according to proprioceptive feedback), and corrects the movement if there is a problem. • is also partly responsible for motor learning, such as riding a bicycle. • Unlike the cerebrum, which works entirely on a contralateral basis, the cerebellum works ipsilaterally. • The cerebellum operates in 3's: – there are 3 highways leading in and out of the cerebellum, – there are 3 main inputs, and – there are 3 main outputs from 3 deep nuclei. They are: CEREBELLAR PEDUNCLES – bundles of fibers connecting the cerebellum with the underlying brain stem • There are 3 pairs: – inferior- transmit info about ongoing movement from spinal cord – middle- sensory info from pons – superior-main output path • info to thalamus and brain stem • info to red nucleus of midbrain (red nucleus relays info from cerebellum to spinal cord) The 3 deep nuclei are the • fastigial- is primarily concerned with balance, and sends information mainly to vestibular and reticular nuclei. • interposed, and dentate nuclei- are concerned more with voluntary movement, and send axons mainly to thalamus and the red nucleus. • Cranial nerve nuclei VIIIXII • The core of the medulla contains much of the reticular formation some of which influence the autonomic (visceral motor) functions – cardiac center- adjust the force and rate of heartbeat – vasomotor center- regulates blood pressure by controlling blood vessel dilation and constriction – medullary respiratory centercontrols the basic rhythm and rate of breathing – additional centers regulate hiccuping, swallowing, coughing, and sneezing. ...
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This note was uploaded on 12/08/2011 for the course AMY 2A taught by Professor Jamesivey during the Spring '06 term at Riverside Community College.

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