Organization G.Brainstem contd. 3.Midbrain a.The midbrain extends from the superior pontine sulcus, its caudal boundary, to the hypothalamus and posterior commissure, its rostral boundary. The midbrain has a roof (tectum) composed of the superior and inferior colliculi (hills) that are sensory integration centers for vision and audition, respectively. Motor cranial nerves 3 & 4 are associated with the midbrain. The ventricular component here is a tubular structure, the cerebral aqueduct. The gray matter surrounding the aqueduct is the periaqueductal gray. The midbrain tegmentum lies ventral to the periaqueductal gray. The cerebral peduncles (containing axons from the cerebral cortex) lie on the ventral surface of the midbrain tegmentum. The tegmentum contains the cranial nerve nuclei for 3 & 4 as well as the red nucleus and substantia nigra, both associated with motor integration.

Organization G.Brainstem contd. 4.Diencephalon a.The diencephalon has two major subdivisions, the thalamus & hypothalamus. It contains the slab-shaped 3 rd ventricle, midline structure dividing the left and right sides of the diencephalon. The diencephalon is composed mainly of the thalamus and hypothalamus. The thalamus on each side is interconnected with the ipsilateral cerebral cortex as well as other sensory and motor centers. Cranial nerve 2 is connected to the thalamus. The hypothalamus, lying ventral to the thalamus, is a control center for autonomic and hormonal activity.

Organization G.Brainstem contd. 5.Cerebral hemispheres (Telencephalon) a.The two cerebral hemispheres are divided by the longitudinal fissure and are interconnected by a massive white matter bundle, the corpus callosum. The cortex is responsible for sensory awareness, information processing, memory and motor control. Cranial nerve 1 is connected with the telencephalon. The cortex is composed of lobes made up of gyri (ridges) and sulci (valleys). The superficial coating of gray matter is arranged in 6 layers. The lobes and their functions are : 1.Frontal – motor control and formulation 2.Parietal – somatosensory and polysensory analysis 3.Occipital – Visual sensation 4.Temporal – Auditory sensation and memory formation

Organization II.Flow of information The flow of information: Receptors to sensory pathway ( usually includes thalamus) to sensory cortex to association cortex to motor cortex to motor pathway to motor neurons to muscles A.Note that incoming sensory information usually crosses the midline to become represented in the contralateral cortex (e.g., sensation from the left side of the body is represented in the right cerebral cortex.) B.Motor impulses arising in the cortex also usually cross the midline to influence the contralateral musculature (e.g., motor impulses arising on the left motor cortex control muscles on the right side of the body.)