4th June 2023

Basic Neuroanatomy

Neuroanatomy of the brain

Neuroanatomy is a broad topic, most of which is outside the remit of MRCP Part 1. However, there are some key areas that are of important relevance, especially in the context of common clinical conditions such as stroke. This section will be focused on clinically orientated anatomy, which will have some crossover with other areas of the MRCP syllabus.

Anatomy of the cerebral cortex

The brain, along with the spinal cord, is the main organ in the central nervous system. The brain is made up of the cerebrum, cerebellum, and brainstem. The cerebrum is made up of two hemispheres, left and right. Each hemisphere is further divided into the cerebral cortex and structures beneath it known as subcortical structures. The cerebral cortex is divided in to six lobes. These are the frontal, temporal, parietal, occipital, insular and limbic lobes (see image below). Each of these lobes is associated with specific functions. As a result, lesions in a particular part of the brain will have specific symptoms and signs.

Lobes of the brain

Image Source: Wikimedia Commons (2014). Blausen 0101 Brain Lateral View, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Blausen_0101_Brain_LateralView.png (Accessed on: 11 January 2023).

Table 1: Lobes of the brain and their function

Functional areas of the brain. Dashed areas shown are commonly left hemisphere dominant

Image Source: Wikimedia Commons (2014). Blausen 0102 Brain Motor & Sensory (flipped), Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Blausen_0102_Brain_Motor%26Sensory_%28flipped%29.png (Accessed on: 11 January 2023).

Knowing the function of the different lobes of the brain helps to localise the site of damage in the CNS based on the patient’s signs and symptoms. It is important to note Wernicke’s and Broca’s areas (image above) as these are important when thinking about the localisation of lesions in patients presenting with aphasia.

This may be receptive aphasia, affecting reading, listening (seen in damage to Wernicke’s area) or expressive aphasia, affecting writing and talking (seen in damage to Broca’s area).

For aphasia to occur, the lesion must be in the dominant hemisphere; right-handed individuals will invariably be left-hemisphere dominant; the majority of left-handed individuals will also be left-hemisphere dominant, however, there is an increased proportion than right-handers of having a dominant right hemisphere. Lesions to the corresponding area of the non-dominant hemisphere will result in visual or auditory inattention to the opposite side.

Topographic representation in the cerebral cortex

The primary sensory and motor cortices of the brain can be mapped out to show the specific areas for which they are responsible. This is important in terms of localising neurological deficits to their corresponding region of the brain. The toes are represented in the upper area of the cerebral hemisphere, with the upper limb, hand, head, and neck regions represented more laterally; this depiction of the distorted body across the cerebral cortex, as seen below, is known as the cortical homunculus.

Topographic representation of the primary sensory cortex.

Image Source: Wikimedia Commons (2013). 1421 Sensory Homunculus, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:1421_Sensory_Homunculus.jpg (Accessed on: 11 January 2023).

Blood supply to the brain

The blood supply to the brain is divided into the anterior and posterior segments. These are supplied by two pairs of arteries, the two internal carotid arteries and two vertebral arteries. The internal carotid artery divides to become the anterior and middle cerebral artery. The anterior and middle cerebral arteries supply the anterior segment of the brain.

The two vertebral arteries join in the brainstem to form the basilar artery. This becomes the posterior cerebral artery at the level of the cerebrum. The posterior cerebral artery supplies the posterior segment of the brain.

Blood supply of the brain subdivided by region.

Image Sources: 

• Wikimedia Commons (2008). Cerebral vascular territories, Licensed under a creative commons licence. Available for free at: https://en.wikipedia.org/wiki/File:Cerebral_vascular_territories.jpg (Accessed on: 11 January 2023).
• Wikimedia Commons (2008). Cerebral vascular territories midline, Licensed under a creative commons licence. Available for free at: https://en.wikipedia.org/wiki/File:Cerebral_vascular_territories_midline.jpg (Accessed on: 11 January 2023).

There is communication between the anterior and posterior segments at the circle of Willis (Fig. 24). This provides collateral communication between the internal carotid and posterior cerebral arteries.

Circle of Willis.

Image Source: Wikimedia Commons (2007). Circle of Willis en, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Circle_of_Willis_en.svg (Accessed on: 11 January 2023).

Neuroanatomy of the spinal cord

Basic spinal cord anatomy

The spinal cord extends from the medulla oblongata in the brainstem to the conus medullaris at the level of L1-L2 in the lumbar spine. It then forms a fibrous extension called the filium terminale, connecting the conus medullaris to the coccyx, stabilising the spinal cord. The filum terminale is surrounded by a bundle of nerves forming the cauda equina.

The spinal cord and brain are surrounded by three layers of membranes known as the meninges. The pia mater is the innermost layer, tightly adherent to the surface of the brain and spinal cord. The arachnoid mater is the middle layer, and the space between the pia and the arachnoid mater is called the subarachnoid space; this space contains CSF. The outermost layer is the dura mater. The epidural space refers to the area between the dura mater and the surrounding vertebral wall, and contains small blood vessels, lymphatics, and adipose tissue.

Ascending and descending tracts

Ascending and descending tracts in the spinal cord are pathways which enable sensory and motor information to be relayed between the brain, spinal cord, and peripheral nervous system. Knowing where these tracts decussate and their location within the white matter is helpful in the localisation of lesions affecting these pathways.

Ascending tracts

The three main sensory modalities entering the spinal cord and their respective tracts are:

1. Pain and temperature – Spinothalamic tract
2. Proprioception and stereognosis – Dorsal columns
3. Light touch – Combination of spinothalamic tract and dorsal columns 

The spinothalamic tract decussates almost immediately after entering the spinal cord; this means that a lesion of the tract will result in loss of pain and temperature sensation contralaterally, below the level of the lesion. The dorsal columns decussate at the junction between the spinal cord and brainstem; therefore, a lesion of the dorsal column causes a decrease in conscious proprioception and stereognosis ipsilaterally below the level of the lesion. All of these sensory pathways eventually decussate and then terminate in the thalamus, which communicates with the sensory cortex.

Descending tracts

The motor (corticospinal) pathway travels from the motor area of the cerebral cortex down through the brain stem, crossing at the level of the medial lemniscus. It synapses in the anterior horn (motor grey matter) of the spinal cord just prior to leaving the cord. Lesions of the corticospinal tract in the spinal cord will result in weakness on the ipsilateral side.

Cross sectional anatomy of the spinal cord.

Image Source: Wikimedia Commons (2010). Spinal cord tracts – English, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Spinal_cord_tracts_-_English.png (Accessed on: 11 January 2023).