Chapter 15-1 The Brain and Cranial Nerves

The Brain The average male adult brain weighs about 3.5 lbs (1590 gms). Composed of 3 divisions: –Cerebrum = Cerebral cortex and Basal nuclei –Cerebellum – posterior and inferior to cerebrum –Brainstem: Diencephalon (Thalamus, Hypothalamus and Epithalamus), Midbrain (superior and inferior colliculi), Pons and Medulla oblongata

Midsagittal section

Brain Meninges

Arachnoid villi Absorbs CSF back into venous sinuses and blood

Brain Ventricles Cavities within the cerebrum and brain stem where cerebrospinal fluid (CSF) is manufactured and circulated. Two lateral ventricles over thalamus and roll out into temporal lobes, medial third and fourth ventricles joined by cerebral aqueduct. Lateral ventricles look like a “Rocky Mountain” sheep Rams horns and communicate to third ventricle via interventricular foreamen. CSF is formed by the ependymal cells of choroid plexus in all four ventricles; is an ultrafiltrate of blood- no cells present; high in Na and K. CSF circulates through ventricular system as well as central canal of spinal cord and subarachnoid space around brain and spinal cord.

Brain Ventricles

CSF formation and circulation into and from blood vascular system. CSF provides protection and nourishment to the brain and spinal cord. Has higher levels of Na and K than blood. CSF reduces weight of brain (1400 gm to 50 gm).

Choroid plexus and Arachnoid granulations

Dural sinuses

Cerebrum Cerebrum composes the vast majority of the brain mass (80%). It is divided into 2 halves or hemispheres, a right and a left hemisphere. Longitudinal fissure- midsagittal groove dividing the two hemispheres into a right and left side. Consists of an outer layer of gray matter over white matter (like crust on white bread). Sulci and gyri = valleys and ridges on surface of cerebral hemispheres. Five lobes: Frontal, parietal, occipital, temporal and insula.

Brain blood flow and barrier Blood flow to the brain is provided by the internal carotid arteries and the vertebral arteries. The blood brain barrier (BBB) is thought to be due to specialized endothelial cells that are influenced by the glial astrocytes. In the choroid plexus there is also a CSF-BBB formed by the ependymal cells. The BBB is absent in some places of the 3 rd and 4 th ventricles at patches called circumventricular organs where some substances may pass into the brain tissue.

Cerebral hemispheres Superior view

Cerebral Cortex The cortex is the outer layer of the cerebrum is composed of gray matter and is where our “consciousness” is located. It allows us to be who we are, perceive sensations, comprehend and understand, as well as learn and, communicate with the ability to act and react at a conscious level.

Sulcus or Sulci –sulcus/sulci – the valleys or grooves found between the gyri. Some of these also have specific names. central sulcus – separates the frontal and parietal lobes.. lateral sulcus – separates the temporal lobe from the parietal/ frontal lobes. parieto-occipital sulcus – separates the parietal and occipital lobes.

Gyrus or Gyri –gyrus/gyri – the elevated ridges of tissue in the cortex 1. precentral gyrus- motor cortex area 4 2. post central gyrus- sensory cortex areas homunculus (little man)- portrays primary motor and sensory areas of body overlaying precentral and postcentral gyri of the cortex

Sulci and Gyri

Humunculus “little man”

Broadmann areas

Functional areas of the Cortex Three important areas of cortex: Motor areas control voluntary movement. Sensory areas provide consciousness and awareness of sensations. Association areas act mainly to integrate diverse information into decisive and meaningful actions. This is done via interneurons or association neurons.

Cortical motor, sensory and association areas

Motor Areas of Cerebral Cortex –1° (primary) Motor Cortex – located in the precentral gyrus.Also known as Broadmann’s area # 4. -Contains large Pyramidal neurons that allow us to perform precise and skilled movements with our skeletal muscles. -Motor innervation is contralateral i.e. the left side of the brain controls the right side of the body and vice versa. Broca’s Area – located superior to lateral sulcus and anterior to precentral gyrus. Associated with motor speech and is usually located in the left hemisphere Premotor cortex anterior to precentral gyrus (Broadmann 6) thought to be site of memory bank for fine motor skils.

Sensory Areas of Cerebral Cortex 1° (primary) Somatosensory Cortex – located in the postcentral gyrus of the parietal lobe. Areas 1-3 Visual Sensory Area – Located on the extreme posterior tip of the occipital lobe. Receives input from retina; is largest cortical sensory area. Auditory Area – medial aspects of the temporal lobes. Gustatory Area –parietal lobe deep to the temporal lobe at the tip of the tongue of humunculus.

Cerebral anatomy to know Longitudinal and transverse fissures Lateral, Central and Parieto-occipital sulci Precentral (motor #4) and Postcentral (sensory #1-3) gyri Lobes: Frontal, Parietal, Temporal, Occipital and Insula Basal nuclei: Caudate nucleus, Putamen, Globus pallidus Corpus callosum, septum pellucidum, fornix, and internal capsule

Cerebral anatomy to know White matter is myelinated neuronal tissue in brain and spinal cord and contains neuronal tracts. Gray matter is unmyelinated and contains mostly neuronal nuclear groups or nuclei and unmyelinated tracts.

Coronal section

Cerebral White Matter is composed of myelinated processes. Within the white matter are “roads” of ordered groups of neuron processes called tracts. There are three major types of tracts in the cerebral cortex: 1.Commissural fibers – connect the gray matter between the two hemispheres. e.g. corpus callosum 2.Association fibers – connect adjacent gyri in same hemisphere. e.g. visual and auditory association areas. 3.Projection fibers – these connect to regions outside of the cerebrum e.g. internal capsule; corona radiata

Gray and White Matter in CNS

Communication fibers Association fibers Commissural fibers Projection fibers

Basal Nuclei “Corpus striatum” Masses of cerebral gray matter buried deep in each hemisphere in the white matter and lateral to the thalamus. Caudate nucleus, globus pallidus and putamen –Communicate with cortex in controlling movement –Complete function is ??, but may regulate start, stop and intensity of voluntary body movements from cortical levels.

Basal Nuclei “Corpus striatum”

The Limbic System Concerned with sense of smell in association with long term memory and emotional responses (anger, fear, rage, sex and hunger). Includes cingulate gyrus, amygdala, hippocampus, septum, fornix, thalamus, hypothalamus and olfactory bulbs.

The Limbic System