Brain Stem
Definition
It is the stalk of the brain below the cerebral hemispheres.
Major route for communication between the forebrain, the spinal cord, and peripheral nerves.
Controls various autonomic functions such as
respiration
heart rhythms
sound localization
The parts
The midbrain
The pons varolii
Medulla oblongata
Reticular formation
Midbrain
Situated around the cerebral aqueduct between the cerebrum and pons varolii
Consists of
1. nerve cells which serve as relay stations for the ascending and descending fibres
Pons Varolii
2. nerve fibres connecting the cerebrum with lower parts of the brain and with the spinal cord
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
Continues 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 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.
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 centre of the brain stem
Surrounded by neural pathways from brain to spinal cord
Has links to other parts of the brain
Receives information transmitted in ascending and descending tracts
Functions of reticular formation
Coordination
Selective awareness
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)
Sselective blocking or passing sensory information to the cerebreal cortex, (e.g. the slight sound made bya sick child moving in bed may arouse his mother but the noise of regularly passing trains may be suppressed)
