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2.7. The Cerebral Cortex
Overview
The cerebral cortex is the outermost layer of the cerebrum, and it is composed of grey matter. Note that the nervous system is made up of two types of matter: white matter and grey matter. White matter is so-called because it is made up of the myelinated axons of neurons. Grey matter is so-called because it is made up of the non-myelinated cell bodies of neurons.
As already mentioned in Section 2.1. Overview of the Brain, the cerebral cortex is subdivided into hemispheres (right and left) and each hemisphere is then divided into six lobes (frontal lobe, parietal lobe, occipital lobe, temporal lobe, limbic lobe and insular cortex/insula). The cerebral cortex contains many important processing and communication centres. Out of all the primates, humans have the largest cerebral cortex, which reflects our unique cognitive abilities.
The brain has a noticeable appearance with prominent raised folds and grooves. In neuroscience, the raised folds are called gyri, the shallow grooves are called sulci and the deep grooves are called fissures. In some cases, functions of the brain are localised in particular gyri, and you will see this below. Note that, in Figure 2.7.1., the coloured areas represent functional areas (also called functional cortices) and the black lines represent sulci and fissures. Note also that the motor and sensory areas in the left cerebral hemisphere are largely responsible for the motor and sensory functions in the right side of the body and vice versa. This is because the fibres cross over (also called decussate) at various points on the way to the brain.
Now, let’s take a look at the various areas of cerebral cortex and their associated functions that have been identified by researchers, and then look briefly at the white matter tracts that lie beneath the cortex.
As already mentioned in Section 2.1. Overview of the Brain, the cerebral cortex is subdivided into hemispheres (right and left) and each hemisphere is then divided into six lobes (frontal lobe, parietal lobe, occipital lobe, temporal lobe, limbic lobe and insular cortex/insula). The cerebral cortex contains many important processing and communication centres. Out of all the primates, humans have the largest cerebral cortex, which reflects our unique cognitive abilities.
The brain has a noticeable appearance with prominent raised folds and grooves. In neuroscience, the raised folds are called gyri, the shallow grooves are called sulci and the deep grooves are called fissures. In some cases, functions of the brain are localised in particular gyri, and you will see this below. Note that, in Figure 2.7.1., the coloured areas represent functional areas (also called functional cortices) and the black lines represent sulci and fissures. Note also that the motor and sensory areas in the left cerebral hemisphere are largely responsible for the motor and sensory functions in the right side of the body and vice versa. This is because the fibres cross over (also called decussate) at various points on the way to the brain.
Now, let’s take a look at the various areas of cerebral cortex and their associated functions that have been identified by researchers, and then look briefly at the white matter tracts that lie beneath the cortex.
Grey Matter
Primary motor cortex
This cortex initiates complex patterns of muscle contractions that we use, for example, to move our arms, hand and fingers when chopping up vegetables for a meal. The primary motor cortex lies directly within the precentral gyrus, which itself lies anterior to the central sulcus. In this cortex a mini map of the body exists so the brain can identify and enable the contraction of a specific muscle of the body.
Primary somatosensory cortex
This cortex receives and begins to interpret somatic sensory information from the peripheral nervous system, for example, the temperature of the water that we are going to boil the vegetables in. The primary somatosensory cortex lies directly within the postcentral gyrus, which itself lies posterior to the central sulcus. In this cortex a mini map of the body exists so the brain can identify exactly where a sensation comes from.
Premotor cortex
This cortex is in addition to the primary motor cortex and is thought to help prepare the body for movement and postural changes.
Sensory association cortex
This cortex is in addition to the primary somatosensory cortex and is also responsible for processing sensations and, in the dominant lobe, helps us to use language to communicate.
Prefrontal cortex
This cortex allows us to control our behaviour, plan and make decisions, and use our intellectual abilities. Alcohol affects the functioning of the prefrontal cortex, causing people to lose their social inhibitions.
Broca's area
Broca’s area is found in the dominant hemisphere and is known as the motor speech area. It allows us to choose the correct words and vocalise them.
Wernicke's area
Wernicke’s area is found in the dominant hemisphere too and is known as the sensory speech area. It allows us to understand words that are spoken to us.
Primary auditory cortex
This cortex allows us to perceive sounds that are received as action potentials from the cochlea of the inner ear.
Primary visual cortex and visual association cortex
The primary visual cortex allows us to interpret images. We receive an upside-down image from the retina and this area of the brain flips it the right way around. The visual association cortex allows us to recognise and understand images that were sent from the retina.
This cortex initiates complex patterns of muscle contractions that we use, for example, to move our arms, hand and fingers when chopping up vegetables for a meal. The primary motor cortex lies directly within the precentral gyrus, which itself lies anterior to the central sulcus. In this cortex a mini map of the body exists so the brain can identify and enable the contraction of a specific muscle of the body.
Primary somatosensory cortex
This cortex receives and begins to interpret somatic sensory information from the peripheral nervous system, for example, the temperature of the water that we are going to boil the vegetables in. The primary somatosensory cortex lies directly within the postcentral gyrus, which itself lies posterior to the central sulcus. In this cortex a mini map of the body exists so the brain can identify exactly where a sensation comes from.
Premotor cortex
This cortex is in addition to the primary motor cortex and is thought to help prepare the body for movement and postural changes.
Sensory association cortex
This cortex is in addition to the primary somatosensory cortex and is also responsible for processing sensations and, in the dominant lobe, helps us to use language to communicate.
Prefrontal cortex
This cortex allows us to control our behaviour, plan and make decisions, and use our intellectual abilities. Alcohol affects the functioning of the prefrontal cortex, causing people to lose their social inhibitions.
Broca's area
Broca’s area is found in the dominant hemisphere and is known as the motor speech area. It allows us to choose the correct words and vocalise them.
Wernicke's area
Wernicke’s area is found in the dominant hemisphere too and is known as the sensory speech area. It allows us to understand words that are spoken to us.
Primary auditory cortex
This cortex allows us to perceive sounds that are received as action potentials from the cochlea of the inner ear.
Primary visual cortex and visual association cortex
The primary visual cortex allows us to interpret images. We receive an upside-down image from the retina and this area of the brain flips it the right way around. The visual association cortex allows us to recognise and understand images that were sent from the retina.
White Matter
The areas of white matter connect different parts of the brain to each other. There are different categories of white matter, which are referred to as tracts:
Association tracts
These connect different areas of grey matter contained in the same cerebral hemisphere. For example, association fibres link association cortices with the hippocampus (see Section 2.8. Limbic System)
Commissural tracts
These connect areas of grey matter in one cerebral hemisphere with the same area in the opposite cerebral hemisphere. The largest of these is the corpus callosum, and this can be seen in Figure 2.7.2. It is estimated that it contains 200 million axons. The fibres of the corpus callosum that bend forward to the tips of the frontal lobes are called forceps minor, and the fibres that bend backwards to the tips of the occipital lobe are called forceps major.
Projection tracts
These connect areas of grey matter in the cerebrum with lower areas of the brain, such as the thalamus and brainstem.
Association tracts
These connect different areas of grey matter contained in the same cerebral hemisphere. For example, association fibres link association cortices with the hippocampus (see Section 2.8. Limbic System)
Commissural tracts
These connect areas of grey matter in one cerebral hemisphere with the same area in the opposite cerebral hemisphere. The largest of these is the corpus callosum, and this can be seen in Figure 2.7.2. It is estimated that it contains 200 million axons. The fibres of the corpus callosum that bend forward to the tips of the frontal lobes are called forceps minor, and the fibres that bend backwards to the tips of the occipital lobe are called forceps major.
Projection tracts
These connect areas of grey matter in the cerebrum with lower areas of the brain, such as the thalamus and brainstem.
Clinical Top Tip:
Aphasia
As mentioned above, Broca’s area is the motor speech area. If Broca’s area or its communicating neuron tracts are damaged, a patient will struggle to produce language in both speech and writing. They are aware of their inability to produce language and may be frustrated at this. This is known as Broca’s aphasia (aphasia = difficulty with communication). Wernicke’s aphasia also exists, and in this condition, a patient will struggle to understand what is being said to them and chooses to respond with inappropriate words in conversation. In this condition, they are unaware of their inability to understand and choose appropriate language.
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