14.3 The Midbrain

midbrain

brain region that develops from embryonic mesencephalon; short segment of brain stem that connects hindbrain to forebrain

cerebral aqueduct of midbrain

surrounded by central (periaqueductal) gray substance involved in pain awareness

cranial nerves of midbrain

contains motor nuclei for CN III and IV

tectum of midbrain

roof-like part of midbrain, posterior to cerebral aqueduct; contains four bulges

two superior colliculi of tectum

visual attention, tracking moving objects, and some reflexes

two inferior colliculi

relays signals from inner ear to thalamus and other parts of brain

cerebral peduncles of midbrain

two anterior midbrain stalks that anchor the cerebrum to the brainstem; each brainstem has three parts, the tegmentum, substantia nigra, and cerebral crus

tegmentum

within cerebral peduncle and dominated by red nucleus; pink color due to high density of blood vessels; connections go to and from the cerebellum for motor control

substantia nigra

nucleus within peduncle; dark nucleus pigmented with melanin; motor center that relays inhibitory signals to thalamus and basal nuclei, suppressing unwanted body movement

cerebral crus

bundle of nerve fibers that connect the cerebrum to pons; carries corticospinal tracts

reticular formation

lose web of gray matter that runs vertically through all levels of the brain stem and into the upper spinal cord; occupies space between white fiber tracts and brainstem nuclei

somatic motor control of reticular formation

adjust muscle tension to maintain tone, balance, and posture; relay signals from eyes and ears to cerebellum; integrate visual, auditory, balance and motion stimuli into motor coordination

gaze centers of reticular formation

allows eyes to track and fixate on objects

central pattern generators of reticular formation

neural pools that produce rhythmic signals to the muscles of breathing and swallowing

cardiovascular control of reticular formation

cardiac and vasomotor centers of medulla oblongata

pain modulation of reticular formation

some pain signals ascend through reticular formation; some descending analgesic pathways begin in the reticular formation, end in the spinal cord where they block transmission from pain signals

sleep and consciousness of reticular formation

plays a central role in consciousness, alertness, and sleep; injury here can esult in irreversible coma

habituation of reticular formation

reticular activating system modulates activity in the cerebral cortex so that it ignores repetitive, inconsequential stimuli

cerebellum

largest part of the hindbrain; second-largest part of the brain as a whole; contains more than half of all brain neurons

granule cells

found in cerebellum; most abundant type of neuron in entire brain

purkinje cells

large cerebellar neurons; axons project to deep nuclei to synapse with neurons that lead to the brainstem

right/left cerebellar hemispheres

connected by a bridge called the vermis

folia of cerebellum

superficial cortex of gray matter with folds

arbor vitae of cerebellum

branching white matter

cerebellar peduncles

three pairs of stalks that connect brainstem and cerebellum; their fibers carry signals to and from the cerebellum

inferior peduncles of the cerebellar peduncles

connected to the medulla oblongata; most spinal input enters the cerebellum through these

middle peduncles of the cerebellar peduncles

connected to pons; most input from the rest of the brain enters through these

superior peduncles of the cerebellar peduncles

connected to midbrain; carries cerebellar outputs

functions of the cerebellum

motor coordination and locomotor ability; sensory, linguistic, emotional, and other nonmotor functions