Describe the gross anatomy of the brain

The brain performs complex sensory, motor and higher functions, coordinating the activity of other body systems. It has a high metabolic activity, receiving 15% of the resting cardiac output, a much greater proportion than would be predicted on the basis of its weight. It can be divided into five regions:

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  The telencephalon (cerebrum) is the largest part of the human brain, occupying the anterior and middle cranial fossae. It comprises three sub-regions:

  
  
  
  
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     – The right and left cerebral hemispheres perform higher functions of memory, thinking, planning and language, in addition to being essential for sensory perception and the initiation of voluntary movement.

     
     
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     – The basal ganglia, a collection of nuclei located deep within the cerebral hemispheres, has classically been regarded as part of the extrapyramidal system which coordinates fine motor control, muscle tone and posture.

     
     
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     – The limbic system, a collection of structures located on either side of the thalamus, is involved in a number of higher functions, including long-term memory, emotion and behaviour.

  The right and left cerebral hemispheres are connected by a thick bundle of myelinated nerve axons called the corpus callosum. The cerebral cortex – the outer layer of the cerebrum – is made up of grey matter, reflecting the relatively large number of nerve cell bodies. The deeper layers of the cerebrum are composed of white matter, reflecting the greater proportion of myelinated nerve axons. The cerebral cortex has a large surface area shaped into numerous folds with ridges (gyri) and troughs (sulci). Each cerebral hemisphere has four lobes:
  
  
  
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     – Frontal lobe. The majority of the frontal lobe is involved in higher functions: problem-solving, reasoning, planning, language generation and complex social and sexual behaviour. The premotor and primary motor cortices, located in the posterior frontal lobe, are involved in the planning and initiation of movement.

     
     
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     – Parietal lobe, the area of the brain involved in sensory integration. The post-central gyrus – the most anterior part of the parietal lobe – contains the primary somatosensory cortex. The left and right parietal lobes have slightly different functions. The dominant hemisphere (the left hemisphere in 97% of the population) is concerned with structure and order; for example, reading and mathematics involve ordering letters and numbers, respectively. The non-dominant hemisphere is concerned with spatial awareness.

     
     
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     – Temporal lobe, which controls hearing, language and memory. The left temporal lobe contains the primary auditory cortex. Wernicke’s area – an area of cortex important in receptive language – lies between the temporal and frontal lobes of the dominant hemisphere.

     
     
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     – Occipital lobe, the area of the brain that interprets visual stimuli.

  
  
  
  
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  The diencephalon, which includes:

  
  
  
  
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     – The thalamus, which acts as a relay station. Sensory afferent neurons, with the exception of the olfactory neurons, synapse in the lateral thalamic nuclei before relaying to the cerebral cortex. The medial structures of the thalamus have roles in pain perception, awareness and the regulation of sleep.

     
     
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     – The hypothalamus, a specialised area of the brain that regulates autonomic function and links the nervous system to the endocrine system. The hypothalamus exerts control over the pituitary gland, a major endocrine gland (see Chapter 80). In addition, the hypothalamus has roles in appetite and satiety, thirst and control of osmolarity, thermoregulation and circadian rhythm.

     
     
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     – The subthalamus contains the subthalamic nucleus, which is thought to be part of the basal ganglia.

     
     
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     – The metathalamus contains the lateral geniculate nucleus, which relays visual information from the optic nerve to the primary visual cortex, and the medial geniculate nucleus, which relays auditory information to the primary auditory cortex.

  
  
  
  
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  The mesencephalon (midbrain) joins the hindbrain to the cerebral hemispheres. The midbrain contains the cerebral aqueduct of Sylvius, which connects the third ventricle to the fourth ventricle (see Chapter 46). The midbrain also contains the third (oculomotor) and fourth (trochlear) cranial nerve nuclei and the red nuclei, which relay extrapyramidal tracts from the cerebellum and cerebral cortex to the spinal cord. The medulla oblongata, pons and midbrain are collectively referred to as the brainstem.

  
  
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  The cerebellum (meaning ‘little brain’) occupies the posterior cranial fossa. Although it makes up only 10% of the brain’s volume, the cerebellum contains over 50% of the brain’s neurons, reflecting its central role in the refinement of movement. The cerebellum does not initiate movements (this is the role of the motor cortex). Instead, it modifies movements to ensure they are smooth, coordinated and accurate. The cerebellum is also responsible for learning motor movements: it builds a ‘working model’ of the environment based on experience. Damage to the cerebellum therefore does not result in paralysis, but in ataxia and poor motor learning. The cerebellum is divided into two main parts:

  
  
  
  
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     – The vermis, the central part, is concerned with motor and postural control of the trunk.

     
     
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     – The cerebellar hemispheres, located on either side of the vermis, are concerned with coordinated motor control of the limbs. Damage to the right cerebellar hemisphere causes limb ataxia on the right side (i.e. the ipsilateral side) and vice versa. This is in contrast to damage to the right side of the motor cortex, which causes a left-sided hemiplegia.

  On each side, afferent and efferent nerve impulses are carried between the cerebellum and the brainstem through three nerve bundles:
  
  
  
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     – The superior and middle cerebellar peduncles connect the cerebellum to the pons.

     
     
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     – The inferior cerebellar peduncle connects the cerebellum to the medulla.

  
  
  
  
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  The rhombencephalon consists of the following structures:

  
  
  
  
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     – The pons (‘bridge’) connects the cerebellum to the brainstem and the medulla oblongata to the midbrain. The nuclei of cranial nerves V to VIII are also located in the pons. The pneumotaxic and apneustic centres – nuclei that form part of the respiratory centre – are located at the border between the pons and medulla.

     
     
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     – The medulla oblongata connects the brain to the spinal cord. Most descending motor tracts of the pyramidal system decussate (cross over to the contralateral side) in the medulla, at the bulges known as the pyramids. Ascending sensory tracts of the dorsal column–medial lemniscal pathway also decussate in the medulla. The medulla contains the nuclei involved in the physiological functions most essential to life: the respiratory and vasomotor centres and extrinsic regulation of the heart through the autonomic nervous system. The medulla controls many stereotyped reflexes, including the vomiting, swallowing, sneezing, gag and cough reflexes.