Functional neuroanatomy



Functional neuroanatomy


This chapter examines the functional anatomy of the brain, focusing on the cerebral cortex, basal ganglia, hippocampus and amygdala. The main functional divisions of the thalamic region, brain stem and cerebellum are also discussed. Sensory and motor pathways of the CNS are described in Chapter 4. The blood supply to the brain is discussed in Chapter 10, in the context of stroke.



Cerebral cortex


This section describes the gyri and sulci of the cerebral hemispheres (Figs 3.13.3) and the main functional areas of the cerebral cortex (Fig. 3.4). Brodmann areas (BA) are indicated in brackets if they have important functional or clinical associations (these are numbered cortical regions, defined by microscopic differences in the structure of the cerebral cortex; see Chapter 5).







Frontal lobe


The frontal lobe is anterior to the central sulcus and above the lateral sulcus. It accounts for around 40% of the cortical surface area and contributes to movement, behaviour, personality and language. It has lateral, medial and inferior surfaces.



Lateral aspect (Figs 3.1 and 3.4A)


The precentral gyrus ribbons forward over the cerebral convexity, immediately anterior to the central sulcus. It corresponds to the primary motor cortex (BA 4) which contains an inverted, point-to-point or somatotopic map of the opposite half of the body (Greek: soma, body; topos, place). The representation of each body part in the motor strip is proportional to the precision of movement control. This means that the areas for the hands, face and tongue are disproportionately large (Fig. 3.5). A useful landmark for identifying the motor cortex is the motor hand area which resembles an inverted capital omega (Fig. 3.6).




The remainder of the lateral frontal lobe consists of the superior, middle and inferior frontal gyri which run from anterior to posterior. The region in front of the motor strip is the lateral premotor area (BA 6) but it does not correspond to any particular gyral or sulcal boundaries. The premotor cortex also contains an inverted body map and is concerned with preparation and execution of movement sequences, particularly those that occur in response to an external trigger (e.g. catching a ball, rather than throwing one). More anteriorly, the frontal eye field (BA 8) is a cortical centre for attention and gaze which directs both eyes towards the contralateral visual field. The frontal eye field and the remainder of the lateral frontal lobe belong to the prefrontal cortex, which is discussed separately below.




Medial aspect (Figs 3.2 and 3.4B)


The superior frontal gyrus and precentral gyrus both continue onto the medial surface of the hemisphere. The boundary between the superior frontal gyrus and the underlying limbic lobe is the cingulate sulcus. This runs parallel to the corpus callosum before turning upwards to the superior margin of the hemisphere as the pars marginalis (marginal part). The central sulcus lies immediately anterior to the pars marginalis and slopes downwards and backwards towards it at an angle of approximately 90 degrees. The paracentral lobule is a U-shaped convolution on the medial surface of the hemisphere that loops underneath the central sulcus. It therefore straddles the boundary between the frontal and parietal lobes. The paracentral lobule includes the primary motor and sensory areas for the lower limbs and genitalia.


The supplementary motor area (SMA) is just in front of the paracentral lobule. It contains a map of both sides of the body and tends to be recruited with its counterpart in the opposite hemisphere (e.g. when the hands are working together to manipulate an object). In contrast to the lateral premotor area, the SMA (or ‘medial premotor area’) is particularly concerned with internally generated (self-initiated) actions, rather than those that occur in response to an external event. The SMA is underactive in Parkinson’s disease, in which voluntary movements are initiated with effort and performed slowly (Ch. 13). Just anterior to the SMA, there is a small medial extension of the frontal eye field.




Prefrontal cortex (Figs 3.3 and 3.4)


The large portion of the frontal lobe anterior to the motor and premotor areas is the prefrontal cortex and is involved in personality, behaviour, language and intellect. It is divided into lateral, orbital and medial parts:



The prefrontal cortex includes Broca’s area (discussed below, in the context of language) and the frontal eye fields (see above). The effects of prefrontal cortex lesions are discussed in Clinical Box 3.1.





Parietal lobe


The parietal lobe is posterior to the central sulcus and above the lateral sulcus. Its posterior boundary is the parieto-occipital sulcus, which is only visible from the medial aspect of the cerebral hemisphere. The parietal lobe is concerned with somatosensory and visuospatial perception. It has lateral and medial surfaces.




Lateral aspect (Figs 3.1 and 3.4A)


The postcentral gyrus is immediately posterior to the central sulcus, behind and parallel to the motor strip. It corresponds to the primary somatosensory cortex (BA 3, 1 and 2). The sensory strip contains an inverted map of the opposite side of the body that mirrors that of the motor strip, but the relative proportions of the body parts reflect the degree of tactile sensitivity.


The remainder of the lateral parietal lobe is divided into superior and inferior parietal lobules by the intraparietal sulcus. This is a deep cleft at right angles to the central sulcus. The somatosensory association cortex (BA 5) is a small area in the superior parietal lobule, just behind the sensory strip. Lesions here may lead to astereognosia: the inability to recognize objects by touch (Greek: a-, without; stereos, solid; gnosis, knowledge). The posterior parietal cortex (BA 7) has close links with the occipital lobe and is concerned with visuospatial perception and attention (Clinical Box 3.2). This includes the representation and manipulation of objects (e.g. using a knife and fork) and the perception of movement (e.g. judging the approach of a moving vehicle). Certain semi-automatic movements are initiated by projections from the parietal cortex to the lateral premotor area (Clinical Box 3.3).




The inferior parietal lobule is a multimodal association area which lies at the junction of the visual, auditory and somatosensory cortices. It consists of the supramarginal gyrus (BA 40) anteriorly and the angular gyrus (BA 39) posteriorly. The inferior parietal lobule contributes to aspects of receptive language such as phonology, reading and spelling, particularly in the language-dominant hemisphere. It is also involved in spatial and symbolic representation of abstract concepts including quantity and number.



Medial aspect (Figs 3.2 and 3.4B)


The superior parietal lobule continues onto the medial surface of the hemisphere as the precuneus. This rectangular-shaped area is involved in mental imagery and recall of personal experiences. Like the medial prefrontal cortex, it is part of the ‘default network’ of the brain and is engaged during activities such as daydreaming and introspection. The postcentral gyrus (‘sensory strip’) also continues onto the medial surface of the hemisphere, making up the posterior part of the paracentral lobule (representing the lower half of the body).




Occipital lobe


The occipital lobe is posterior to the preoccipital notch and the parieto-occipital sulcus. It is concerned entirely with vision and has medial, lateral and inferior surfaces.



Medial aspect (Figs 3.2 and 3.4B)


The calcarine sulcus follows an undulating course from the parieto-occipital sulcus anteriorly to the occipital pole posteriorly. It is a deep cleft which extends to the occipital horn of the lateral ventricle. The wedge-shaped region above the calcarine sulcus is the cuneus (Latin: cuneus, wedge) which represents the lower quadrant of the opposite visual field. The tongue-like lingual gyrus is below the calcarine sulcus (Latin: lingua, tongue) and represents the upper quadrant of the opposite visual field.


Much of the primary visual cortex (BA 17) is hidden from view within the banks of the calcarine sulcus. Central vision is represented towards the occipital pole, peripheral vision more anteriorly. The primary visual cortex is flanked above and below by visual association cortex (BA 18 and 19) located within the cuneus and lingual gyrus.




Central visual pathways (Figs 3.7 and 3.8)


The retina contains a point-to-point (retinotopic) representation of the visual fields which is maintained throughout the central visual pathways. Its retinal ganglion cells give rise to over a million axons that leave the posterior pole of the eye as the optic nerve. The two optic nerves then unite to form the optic chiasm. Posterior to the chiasm, the optic tracts continue on each side to the lateral geniculate nucleus (LGN) of the thalamus where they synapse. Thalamocortical neurons then project to the primary visual cortex, via the optic radiations.





The optic chiasm (Fig. 3.7)


The central visual pathways are crossed. This means that the right visual field is represented in the left occipital lobe and vice versa. Since light travels in straight lines and enters the eye via the small aperture of the pupil, objects in the right visual field project to the left half of each retina (coloured red in Fig. 3.7). Axons originating from the left half of each retina must therefore project to the left cerebral hemisphere. For this to happen, nerve fibres from the inner half of the retina must cross the midline to enter the opposite optic tract. This takes place at the optic chiasm, named for its resemblance to the Greek letter chi.



The primary visual cortex


Visual information is segregated into three separate ‘channels’ (concerned with form, motion and colour) by the primary visual cortex (V1) and visual association cortices (V2, V3… and higher). Two parallel visual ‘streams’ dealing with different aspects of visual perception arise from the occipital lobe. The dorsal or parietal lobe stream is concerned with the location and movement of objects and their positions relative to the body (the ‘where’ pathway). The ventral or temporal lobe stream synthesizes information about form and colour, allowing objects to be recognized (the ‘what’ pathway). Central visual pathway lesions are discussed in Clinical Box 3.4.






Temporal lobe


The temporal lobe lies below the lateral sulcus and is angled downwards and forwards to resemble the thumb of a boxing glove. Its posterior boundary (with the occipital lobe) is the pre-occipital notch. The temporal lobe is involved in hearing, speech comprehension and visual recognition. It has superior, lateral and inferior surfaces.




Superior aspect (Fig 3.10)


The superior surface of the temporal lobe is hidden within the lateral sulcus and includes the transverse temporal gyri. These finger-like convolutions run obliquely (posteriorly and medially) and include the primary auditory cortex (BA 41 and 42). The auditory cortex contains a tonotopic map that represents the audible frequency spectrum (low frequencies laterally, high frequencies medially).



Projections reach the auditory cortex from the medial geniculate nucleus (MGN) of the thalamus via the auditory radiations. The primary auditory cortex receives projections from both ears. This means that a temporal lobe lesion would not be expected to cause contralateral deafness in the same way that an occipital lobe lesion might cause contralateral loss of sight.


The surrounding cortex, which continues onto the lateral surface of the temporal lobe is the auditory association area (BA 22). In the language-dominant hemisphere this is specialized for the comprehension of speech sounds.




Inferior aspect (Fig. 3.3)


On the inferior aspect of the hemisphere, the occipital and temporal lobes form a continuous, uninterrupted surface that is composed of the medial and lateral occipitotemporal gyri. The medial occipitotemporal gyrus is medial to the collateral sulcus. The portion lying within the occipital lobe is also known as the lingual gyrus, whereas the part contained in the temporal lobe is the parahippocampal gyrus. The lateral occipitotemporal gyrus runs alongside, between the collateral sulcus medially and occipitotemporal sulcus laterally. The anterior and posterior ends of the lateral occipitotemporal gyrus are usually tapered to give it a ‘spindle’ shape. It is therefore also known as the fusiform gyrus (Latin: fusiform, shaped like a spindle). It receives projections from the occipital lobe (part of the ‘what pathway’) and appears to be involved in the recognition of complex visual patterns. It contributes to reading (in the language-dominant hemisphere) and face recognition (in the non-dominant hemisphere) (Clinical Box 3.5).



image Clinical Box 3.5:   Visual agnosia


Lateral and inferior temporal lobe lesions may interfere with object recognition, despite otherwise normal vision, which is called visual agnosia (Greek: a-, without; gnosis, knowledge). The two main types are illustrated in Fig. 3.11. Apperceptive agnosia is a problem with the early stages of recognition, interfering with the ability to perceive objects. Associative agnosia involves the later stages of recognition. Objects are seen normally and can be described in detail, but do not look familiar and can only be recognized and named using other senses (e.g. touch, smell). Sometimes agnosia affects specific categories of object (e.g. foodstuffs, living things, tools). Damage to the non-dominant fusiform gyrus, in the inferior temporal region, may cause selective visual agnosia for faces, termed prosopagnosia (Greek: prosop, face; a-, without; gnosis, knowledge).

< div class='tao-gold-member'>

Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Jun 19, 2016 | Posted by in NEUROLOGY | Comments Off on Functional neuroanatomy

Full access? Get Clinical Tree

Get Clinical Tree app for offline access