GROSS ANATOMY OF THE BRAIN Dr. G.R. Leichnetz

The cerebrum has five lobes: frontal, parietal, temporal, occipital, and insular. The central sulcus separates the frontal and parietal lobes (note the two vertical gyri, pre- and post-central). The lateral sulcus separates the frontal & parietal lobes from the temporal lobe. A line drawn from the preoccipital notch to the parieto-occipital sulcus delineates the occipital lobe from the parietal and temporal lobes. The insular lobe is hidden in the lateral sulcus. Parietal Lobe Frontal Lobe Occipital Lobe Temporal Lobe Insular Lobe

The central sulcus delineates the frontal lobe from the parietal lobe separating two vertical gyri, the pre- and post- central gyri. The lateral sulcus delineates the frontal & parietal lobes from the temporal lobe. A line drawn from the preoccipital notch to the parietooccipital sulcus on the dorsal margin of the hemisphere delineates the parietal & temporal lobes from the occipital lobe.

Cerebral Hemisphere Lateral Aspect Central sulcus Parietal Lobe Frontal Lobe Occipital Lobe Temporal Lobe Lateral sulcus Pre-occipital notch

Opening the lateral sulcus reveals the insular lobe.

Frontal Lobe The precentral gyrus, the primary motor cortex, runs vertically in the posterior frontal lobe (anterior to the central sulcus). The remainder of the frontal cortex is divided into the superior, middle, and inferior frontal gyri that run in parallel, perpendicular to the frontal pole.

Lateral Aspect: Frontal Lobe Precentral gyrus Central sulcus Superior frontal gyrus Middle frontal gyrus Pars opercularis, inferior frontal gyrus Pars triangularis, inferior frontal gyrus Inferior frontal gyrus Broca’s motor speech area is in the pars opercularis and triangularis in the inferior frontal gyrus of the left cerebral hemisphere. Pars orbitalis, inferior frontal gyrus

The frontal lobe can also be divided into functional subdivisions based upon unique cytoarchitectural differences. The precentral gyrus is the primary motor cortex where movements are initiated. The premotor cortex is concerned with planning movements. The prefrontal cortex is involved in “executive function” (cognition, decision-making), working memory, and the orchestration of complex behavioral responses.

Parietal Lobe The postcentral gyrus, the primary somatosensory cortex, is posterior to the central sulcus. The remainder of the parietal lobe is divided by the horizontal intraparietal sulcus into superior and inferior parietal lobules. The inferior parietal lobule is further subdivided into the supramarginal and angular gyri (U-shaped gyri at the caudal terminus of the lateral and superior temporal sulci respectively).

Parietal Lobe Inferior parietal lobule Postcentral gyrus Central sulcus Superior parietal lobule Intraparietal sulcus Inferior parietal lobule Angular gyrus Supramarginal gyrus

Temporal Lobe The lateral aspect of the temporal lobe is subdivided into three parallel superior, middle, and inferior temporal gyri that run perpendicular to the temporal pole.

Lateral Aspect: Temporal Lobe Superior temporal sulcus Lateral sulcus Superior temporal gyrus Middle temporal gyrus Inferior temporal gyrus

Temporal Lobe: Primary Auditory Cortex Opening the lateral sulcus reveals the superior transverse temporal gyri of Heschl on the superior aspect of the temporal lobe, the primary auditory cortex. Superior transverse temporal gyri Insular cortex Superior temporal gyrus

Inferior Aspect of the Brain Frontal lobe Diencephalon (hypothalamus) Temporal lobe Midbrain Pons Medulla The brainstem consists of the midbrain, pons, and medulla. Cerebellum

Inferior Aspect of Frontal and Temporal lobes The inferior aspect of the frontal lobe contains the orbitofrontal gyri. The inferior aspect of the temporal lobe contains the inferior temporal gyrus, occipitotemporal (fusiform) gyrus, and the parahippocampal gyrus. Brainstem & cerebellum removed

Inferior Aspect, Frontal Lobe Olfactory bulb Orbitofrontal gyri Olfactory tract

The inferior aspect of the temporal lobe contains three parallel gyri running perpendicular to the temporal pole. The inferior temporal gyrus lies most laterally on the lateral margin of the temporal lobe, separated by the inferior temporal sulcus from the occipitotemporal (or fusiform) gyrus. The occipito- temporal gyrus is separated from the parahippocampal gyrus by the collateral sulcus. The hippocampal formation is rolled within the parahippocampal gyrus.

Medial Aspect/ Cerebral Hemisphere (Mid-Sagittal Section) Central sulcus Parietooccipital sulcus Parietal Frontal Occipital Cerebellum Brainstem The central sulcus and parietooccipital sulcus are used to delineate lobes on the medial aspect of the hemisphere.

Medial Aspect: Frontal & Parietal Lobes Central sulcus Paracentral lobule Precuneus Superior frontal gyrus Cingulate gyrus Corpus callosum Parietooccipital sulcus The cingulate gyrus (above the corpus callosum) is part of the “limbic lobe.” The paracentral lobule is a continuation of the pre-and post-central gyri onto the medial aspect of the hemisphere, and contains the representation of the leg.

Medial Aspect: Occipital Lobe Parietooccipital sulcus Cingulate gyrus Precuneus The cuneus and lingual gyri are the primary visual cortex. Splenium, corpus callosum Cuneus gyrus Lingual gyrus Calcarine fissure Cerebellum

Corpus Callosum- the largest commissure of the brain, is a large bridge between the specialized hemispheres. A commissure interconnects homologous cortical areas in opposite hemispheres. Genu Splenium

Diencephalon: Thalamus & Hypothalamus

Mid-Sagittal: Diencephalon Pineal gland Septum pellucidum Fornix Thalamus Anterior commissure Superior colliculus Inferior colliculus Hypothalamus Midbrain Optic chiasm Mammillary body Pons Pituitary stalk (infundibulum)

Thalamus The two thalami lie in the walls of the dorsal part of the third ventricle. It is the principal relay station for major sensory and motor pathways to the cortex.

Diencephalon: Inferior Aspect Optic nerve (C.N. II) Optic chiasm Pituitary stalk Tuber cinereum (hypothalamus) Mammillary bodies

Mid-Sagittal: Ventricular System Massa intermedia Pineal gland Lateral ventricle Fornix Cerebral aqueduct Interventricular foramen of Monro Anterior commissure Midbrain Fourth ventricle Lamina terminalis Lateral Recess & Foramen of Luschka Pons Third ventricle The interventricular foramina of Monro connect the lateral ventricles to the third ventricle. The cerebral aqueduct of the midbrain connects the third ventricle of the diencephalon to the fourth ventricle which lies above the pons and medulla below the cerebellum.

Hypothalamus Hypothalamus The hypothalamus coordinates higher-level complex autonomic (visceroendocrine) functions (eg. appetite, water & electrolyte balance, reproductive function) .

Brainstem

Brainstem: Midsagittal View Midbrain Cerebellum IVth vent Pons Medulla The brainstem has three portions: midbrain, pons, medulla

The brainstem consists of the midbrain, pons, and medulla. Netter

Brainstem: Dorsal (Posterior) View Midbrain Pons Medulla

Midbrain: Dorsal Aspect The roof of the midbrain (midbrain tectum) contains four elevations (“corpora quadrigemina”), the two superior colliculi and inferior colliculi. The trochlear nerves (C.N. IV) exit through openings inferior to the inferior colliculi.

Midsagittal: Midbrain/ Pons The midbrain tectum is the roof of the midbrain above the cerebral aqueduct that contains the superior and inferior colliculi. In a midsagittal section, the midbrain tegmentum extends from the cerebral aqueduct to the interpeduncular fossa. It is contiguous inferiorly with the pontine tegmentum.

Midbrain: Ventral Aspect Optic Nerve Optic Chiasm Optic Tract Oculomotor Nerve (CN III) Tuber cinereum and pituitary stalk (hypothalamus) Trochlear Nerve (CN IV) Mammillary bodies Cerebral peduncle Pons On the ventral aspect of the midbrain the oculomotor nerves (C.N. III) exit from the interpeduncular fossa (between the two cerebral peduncles. The trochlear nerves (C.N. IV) exit from the dorsal midbrain and curve ventrally around the cerebral peduncle.

Brainstem: Ventral (Anterior) View

Ventral Aspect: Pons & Medulla Trigeminal Nerve (CN V) Middle cerebellar peduncle Pons Abducens Nerve (CN VI) Medulla Vestibulocochlear Nerve (CN VIII) Glossopharyngeal (IX) and Vagus (X) Nerves Facial Nerve (CN VII) Hypoglossal Nerves (CN XII) Spinal Accessory Nerve (CN XI) Olive Pyramid Pyramidal Decussation

Cerebellar Lobes The cerebellum has three lobes with a vermal portion and hemispheric portion. Anterior Lobe Posterior Lobe Flocculonodular Lobe The cerebellum is a sensorimotor integrator. It receives spinal and vestibular input and modifies the size and velocity of ongoing movements.

Cerebellum: Anterior (Superior) Aspect Lateral hemisphere Posterior Lobe Vermis Primary Fissure Anterior Lobe Midbrain

Cerebellum, Posterior (Inferior) Aspect Vermis Pyramis Posterior Lobe Hemisphere Uvula Tonsils of cerebellum Medulla

Mid-sagittal: Cerebellar Vermis Primary fissure Anterior lobe vernis Flocculus- hemispheric portion of the flocculonodular lobe of the cerebellum Fourth ventricle Posterior lobe vermis Nodule (vermal portion of the F-N lobe) Prenodular fissure Tonsil

Cerebellar Peduncles Large bundles containing tracts that connect the cerebellum to the brainstem, form the walls of the fourth ventricle. Netter

Cerebellar Peduncles The cerebellar peduncles connect the cerebellum to the three divisions of the brainstem: SCP to midbrain MCP to pons ICP to medulla. They carry major tracts into and out of the cerebellum. Anterior lobe hemisphere removed to reveal peduncles Superior cerebellar peduncle Midbrain Middle cerebellar peduncle Pons

Dorsal Aspect of the Brainstem (with cerebral cortex and cerebellum removed) The cerebellar peduncles form the walls of the fourth ventricle. The floor of the fourth ventricle is the rhomboid fossa. Thalamus Midbrain Midbrain Pons Rhomboid fossa Medulla

The floor of the fourth ventricle is the rhomboid fossa. The “rhomboid” can be divided into two triangles. The rostral triangle is over the pons. The caudal triangle is over the medulla. The fossa is divided vertically in half by the dorsal median sulcus. The sulcus limitans separates medial motor areas from lateral sensory areas. Superior colliculus Inferior colliculus Anterior medullary velum Superior cerebellar peduncle Pons Middle cerebellar peduncle FC Medulla Obex

Dorsal Aspect of the Medulla The tracts in the dorsal columns of the spinal cord (fasciculus gracilis and cuneatus) ascend to terminate in the dorsal medulla in the nucleus gracilis and nucleus cuneatus respectively. Obex Gracile tubercle Cuneate tubercle Fasciculus graciclis Tuberculum cinereum Fasciculus cuneatus Dorsal intermediate sulcus Dorsal median sulcus

Ventral Aspect of the Medulla Pontomedullary junction Glossopharyngeal (IX) and vagus (X) nerves exiting postolivary sulcus Olive Hypoglossal nerve (C.N. XII) exiting preolivary sulcus Pyramidal tract Ventral median fissure Pyramidal decussation

Brainstem: Ventral Aspect Cranial Nerves Optic (II) Oculomotor (III) Trigeminal (V) Facial (VII) and Vestibulocochlear (VIII) Glossopharyngeal (IX) & Vagus (X) Nerves