Cerebral cortex:Frontal, temporal, parietal, & occipital lobes

FRONTAL LOBE

The **frontal lobe** is one of the four major divisions of the cerebrum. It is the largest of the four lobes. It lies on the orbital plate of the frontal bone and is bound by its vertical and horizontal plates. It is the area of the cerebrum posterior to the frontal pole, anterior to the central (Rolandic) sulcus and superomedially to the Sylvian fissure (lateral sulcus) and temporal lobe.

Function: motor cortex, premotor cortex and prefrontal area

* **precentral gyrus**
	+ lies anterior to the central sulcus
	+ contains the primary motor cortex
	+ precentral sulcus, premotor cortex, and supplementary motor area lie anteriorly
* **superior frontal gyrus**
	+ Separated from the middle frontal gyrus by the superior frontal sulcus.
	+ Part of premotor cortex; performs initiation & planning motor control and supplementary motor area
	+ Medial part is the paracentral lobule
	+ Frontal eye field is located at junction of precentral sulcus & caudal-most part of superior frontal sulcus
* **middle frontal gyrus**
	+ separated from the inferior frontal gyrus by the inferior frontal sulcus
	+ executive functioning, working memory, attention
* **inferior frontal gyrus**
	+ pars opercularis in posterior aspect (contains Broca's area)
	+ pars triangularis
		- V-shaped area anteriorly used a landmark to find the pars opercularis and hence Broca's area

Medially, important areas include:

* **anterior cingulate gyrus:** processing of salience, pain, reward, emotion, and impulse control
	+ superior to corpus callosum
* **gyrus rectus (straight gyrus)**
* **olfactory sulcus**
	+ Contains olfactory bulbs, which transduce & relay odorant information centrally
* **orbital gyrus:** processes response inhibition and representations of reward, error, emotion, & valuation
* **orbital sulcus:** H-shaped sulcus separating medial, anterior, lateral, & posterior orbital gyri

Relations

* anterior - frontal bone
* posterior - central sulcus and parietal lobe
* inferolaterally - lateral sulcus and temporal lobes
* inferior - orbital plate frontal bone

Blood supply

* middle cerebral artery (MCA) - lateral frontal lobe
* anterior cerebral artery (ACA) - medial frontal lobe

TEMPORAL LOBE

The **temporal lobe** largely occupies the middle cranial fossa.

The lobe extends superiorly to the Sylvian fissure, and posteriorly to an imaginary line; the lateral parietotemporal line, which separates the temporal lobe from the inferior parietal lobule superiorly and the occipital lobe inferiorly. The middle cranial fossa forms its anterior and inferior boundaries.

Function: auditory cortex and temporal association cortex

The temporal lobe can be divided into two main sections:

* neocortex (sometimes referred to simply as temporal lobe)
	+ lateral and inferolateral surfaces
	+ comprised of standard cerebral cortex
* mesial temporal lobe (sometimes referred to as the limbic lobe): these are significant role in formation of memory
	+ including the hippocampus, amygdala, parahippocampal gyrus, dentate gyrus, uncus

Sulci and gyri

The temporal lobe is divided into 6 gyri by 5 sulci which are oriented largely parallel to the Sylvian fissure. Unfortunately not all gyri and sulci extend the full length of the lobe and as such not all are present at each angled coronal section. Furthermore nomenclature is variable.

The general order from superolateral to inferomedial is:

* **superior temporal gyrus:** Wernicke area, auditory area on the surface receives auditory stimuli.
* superior temporal sulcus
* **middle temporal gyrus:** wernicke area. Connects with auditory, somatosensory, visual association pathways.
* inferior temporal sulcus
* **inferior temporal gyrus:** higher visual association area
* lateral occipitotemporal sulcus
* **lateral occipitotemporal gyrus**
* collateral sulcus
	+ Most mesial temporal sulcus
	+ Lateral limit of parahippocampal gyrus
* **medial occipitotemporal gyrus** (aka lingual gyrus)
* anterior calcarine sulcus
* **parahippocampal gyrus:** on medial surface, merges into uncus. Separated from temporal lobe apex by incisura temporalis, part of primary olfactory area, contains amygdala

PARIETAL LOBE:

The **parietal lobe** is located between the frontal and occipital lobes and underlying the parietal bone.

The lobe extends from the central sulcus anteriorly, which separates it from the frontal lobe, to the parieto-occipital sulcus posteriorly, which separates it from the occipital lobe. The lateral sulcus corresponds to its inferolateral boundary, separating it from the temporal lobe. Medially, it is confined by the medial longitudinal fissure which divides both cerebral hemispheres.

Function: sensory cortex and parietal association cortex

Sulci and gyri

* lateral surface
	+ **postcentral gyrus (primary sensory area):** with a topographical map of the contralateral body. Face, tongue, lips are inferior; trunk, upper limb superolateral; lower limb on medial aspect
	+ **superior parietal lobule:** lateral surface behind the post central sulcus. Part of the parietal association cortex involved in recognition and integration of sensory stimuli.
	+ **inferior parietal lobule (Wernicke's area)**
		- supramarginal gyrus at the end of the sylvian fissure, visual word recognition
		- angular gyrus: ventral to the supramarginal gyrus, semantic language, arithmetic
* medial surface
	+ **paracentral lobule:**
	+ **precuneus:** medial parietal lobe, between marginal sulcus and parietooccipital sulcus
	+ **parietooccipital sulcus:** marks boundary between cuneus & precuneus as well as parietal & occipital lobes
	+ **Cingulate gyrus**
		- posterior cingulate cortex (ventral & dorsal) for self-referential cognition, declarative memory, and semantic function
		- retrosplenial cingulate cortex: episodic memory and spatial navigation
	+ **Cingulate sulcus**
		- Separate paracentral lobule from the middle portion of cingulate gyrus
	+ **Subparietal sulcus:** a continuation of cingulate sulcus, separates precuneus from posterior cingulate gyrus

Relations

* anterior: frontal lobe
* posterior: occipital lobe
* superior: parietal bone
* inferior: temporal lobe

Blood supply

* middle cerebral artery (MCA)
* anterior cerebral artery (ACA): medial parietal lobe
* posterior cerebral artery (PCA):  posterior medial parietal lobe

OCCIPITAL LOBE

The **occipital lobe** is the visual processing center. The primary visual center is located on the medial side of the occipital lobe within the calcarine sulcus

Function: visual cortex and occipital association cortex

Gyri and sulci

* **Occipitotemporal sulcus** separates inferior temporal gyrus, laterally, from lateral occipitotemporal gyrus, medially
* **Medial occipitotemporal gyrus** (fusiform gyrus) & **lateral occipitotemporal gyrus**
	+ Part of ventral stream of visual processing, implicated in processing of faces
	+ Separated by the collateral sulcus
* **Lateral occipital sulcus:** lies on dorsolateral surface
* **Lunate sulcus:** in lateral occipital lobe
* **Transverse occipital sulcus:** is anterosuperior limit of occipital lobe
* **Cuneus:** Receives visual information from contralateral superior retina representing inferior visual field
	+ Medial occipital lobe, above calcarine fissure
* **Calcarine sulcus** separates cuneus from fusiform & lingual gyri
	+ Primary visual cortex along its banks
* **Lingual gyrus:** Corresponds to inferior 1/2 of primary visual cortex, represents contralateral upper quadrant of binocular visual field
	+ Medial occipital lobe, below calcarine fissure
* Superior, middle, inferior occipital gyri

Relations:

* Posterior to parietooccipital sulcus
* Primary visual cortex on medial occipital lobe
* Cuneus on medial surface

The lobe rest on the tentorium cerebelli. At the front edge of the occipital are several lateral occipital gyri, which are separated by lateral occipital sulcus.

The occipital aspects along the medial surface of the hemisphere are divided by the calcarine sulcus. Above the medial, Y-shaped sulcus lies the cuneus, and the area below the sulcus is the lingual gyrus.

**Normal Variants**

Basal ganglia calcification

Asymmetric cerebral hemispheres

Agenesis of the corpus callosum

The corpus callosum normally forms from anterior to posterior

* Partial: hypoplasia – the anterior portion is formed and the posterior portion is not
* Complete: corpus callosum totally absent
* Atypical multitude of possibilities and occurs when the anterior to posterior order of development is not followed