Midbrain

In general the midbrain acts as a relay station between forebrain and hindbrain

1. Cerebral peduncles - on anterolateral surface, consist of tracts
   1. Tracts (corticospinal) carrying motor signals from cerebral cortex to pons, medulla oblongata, and spinal cord
   2. Tracts carrying sensory signals from medulla oblongata to the thalamus
2. Nuclei
   1. Substantia nigra - dark pigmented nuclei
      1. Releases dopamine which inhibits over-activity of the cerebral nuclei, modifies muscle tone and motor activity
      2. Contains interval clock that measures time from seconds to minutes
   2. Red nucleus - highly vascularized
      1. Receives information from cerebrum and cerebellum, involuntary motor output for muscle tone, posture and limb position
   3. Nucleus of origin of cranial nerves 3 & 4 - control eye muscles
3. Medial lemniscus - major sensory tract from medulla oblongata nuclei to thalamus for touch, proprioception, pressure and vibrations
4. Corpora quadrigemina - posterior surface, control startle responses to bright light or sudden noise
   1. Superior colliculi - controls reflex eye and trunk movements in response to visual stimuli from thalamus
   2. Inferior colliculi - controls reflex head and trunk movements in response to auditory stimuli from nuclei in pons and medulla oblongata

Reticular activating system

• Diffuse system of sensory and motor neurons
• Extends through medulla oblongata, pons, midbrain, thalamus, to cerebral cortex
• System controls overall degree of CNS activity including sleep and wakefulness
• Almost any sensory system in the body can activate the RAS

1. Mesencephalic pathway - stimulation here causes generalized activation of entire brain, responsible for normal wakefulness and varying degrees of attentiveness
2. Thalamic pathway - stimulation of specific thalamic areas causes selective activation of specific cortical areas producing intense attention to one specific aspect of environment or memories stored in that area