Brain Stem

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Brain Stem Definition It is the stalk of the brain below the cerebral hemispheres. It is the major route for communication between the forebrain, the spinal cord, and peripheral nerves. It controls autonomic functions:- respiration heart rhythms perceptual functions such as sound localization

Brain stem consists of The midbrain The pons varolii Medulla oblongata Reticular formation

Midbrain Situated around the cerebral aqueduct between the cerebrum and pons varolii Consists of nerve cells and nerve fibres connecting the cerebrum with lower parts of the brain and with the spinal cord The nerve cells serve as relay stations for the ascending and descending fibres

Pons Varolii Situated in front of the cerebellum Below the midbrain Above the medulla oblongata Consists of the 1. Fibres connecting the two hemispheres of the cerebellum 2. Fibres connecting the higher centres of the brain and the spinal cord 3. Nuclei of the 5th 6th 7th and 8th cranial nerves 4. Nuclei which act as relay stations

Medulla Oblongata Extends from the pons varolii above and is continuous with the spinal cord below 2.5 cm Situated within the cranium above the foramen magnum Central fissures anteriorly and posteriorly Outer white matter Inner grey matter – passing from brain to the spinal cord. Some cells to relay sensations to the cerebrum from the spinal sensory fibres

Vital centres Medulla oblongata contains vital centres Cardiac centre Respiratory centre Vasomotor centre Reflex centres of vomiting, coughing, sneezing and swallowing

Special features Decussation of the pyramids – motor fibres from motor area in the pyramidal tracts cross Sensory decussation – some of the sensory nerves ascending from the spinal cord cross – others cross in the spinal cord itself Cardiac centre – controls the rate and force of cardiac contraction sympathetic and parasympathetic nerve fibres originating in the medulla pass to the heart. Sympathetic stimulation ↑ the rate and force of heart beat – parasympathetic does the opposite

Respiratory centre : controls the rate and depth of respiration Respiratory centre : controls the rate and depth of respiration. Impulses pass to the phrenic and intercostal nerves. Gets stimulated by excess of CO2 and to a lesser extent, by deficiency of O2 by nerve impulses from the chemoreceptors from the carotid bodies Vasomotor centre : controls the diameter of the blood vessels : small arteries and arterioles through the autonomic nervous system Reflex centres : irritating substances in the stomach or respiratory tract – stimulate reflex centres – reflex actions of vomiting coughing and sneezing.

Reticular Formation A collection of neurones in the core of the brain stem constanttly receives “information” being transmitted in ascending and descending tracts Surrounded by neural pathways which pass nerve impulses between the brain and the spinal cord It has synaptic links with other parts of the brain

Functions of reticular formation Coordination of skeletal muscle activity associated with voluntary motor movement and the maintenance of balance Coordination of activity controlled by the autonomic nervous system, e.g. cardiovascular, respiratory and gastrointestinal activity Selective awareness that functions through the reticular activating system which selectively blocks or passes sensory information to the certebral cortex

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Reticular formation Go to 11

Reticular activating system Go to 11

Cell Bodies Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system Go to 6