Cerebrum

The cerebrum is the largest part of the human brain and its surface is rippled with shallow grooves and ridges that afford the characteristic walnut-like appearance – these undulations increase the surface area of the cerebrum to provide greater processing power. The surface crust of the cerebrum is about 3 mm thick and is called the cerebral cortex. A deep midline groove separates the cerebrum into equal halves called cerebral hemispheres. Communication between hemispheres is achieved via a number of commissures or connecting bridges of nerve fibres, the biggest of these being the corpus callosum. The hemispheres can be further subdivided into lobes, of which there are four external and one internal. The four external are called the frontal, parietal, temporal and occipital lobes; the internal one, tucked in deep between the frontal and temporal lobes, is called the insula (see Fig 2.2).

Figure 2.2 Lobes of the human brain.

Adapted from Human Anatomy and Physiology (7th edn), 2007, by E N Marieb and K Hoehn, Pearson-Benjamin Cummings, Fig 12.6, p 435, with permission

The cerebral cortex is organised in such a way as to allow specialisation of function. Discrete regions of the cortex control the primary processing of vision, taste, hearing, general sensations and muscle movements. Furthermore, as new information is received it can be referenced against past experiences stored in association areas. The frontal lobes are particularly important in the processing of emotions and behaviour, decision making, personality and temperament, self-discipline and cognition. Another form of specialisation occurs at the hemispheric level: one hemisphere may be more dominant over the other in the control of some functions. Classic examples of this cerebral dominance include handedness (90% of humans are right-handed), language control (left hemisphere is usually dominant) and spatial perception (right hemisphere is usually dominant). This phenomenon does not mean that the non-dominant hemisphere makes no contribution to these functions, nor does it mean that in a typical right-handed person their left hemisphere is dominant for all of these functions – it is very much function-specific. In some brain disorders, such as schizophrenia and depression, an imbalance between the hemispheres may contribute to the manifestation of the condition (see Chs 3 and 4).

Diencephalon

The diencephalon contains the thalamus and hypothalamus. The thalamus is a relay centre for both ascending (sensory) and descending (motor) information. Sensations from the body enter the thalamus and are sorted as to the origin of the transmission and the type of sensation received (e.g., taste, touch, vibration or thermal). They can then be sent to the appropriate processing centre in the cortex, where sensory perception is completed. The hypothalamus has a significant role to play in homeostasis with respect to appetite regulation, fluid and electrolyte balance and body temperature control. It is the interface between the nervous and the endocrine systems, in that it is physically connected to the pituitary gland by a stalk and has a great influence on hormone release from this gland. The hypothalamus also plays a role in autonomic nervous system control. The link between the hypothalamus and the autonomic nervous system, pituitary gland and other brain regions affords it a unique role in the initiation of emotional and behavioural responses.

Brain stem and cerebellum

The brain stem consists of three regions: midbrain, pons and medulla. It is associated with the control of a number of important reflexes (e.g. head movements, tracking stimuli, vision, coughing and vomiting), as well as vital functions such as respiration, blood pressure and heart rate. The major motor pathway serving the skeletal muscles passes through the posterior portion of the medulla. This region is called the medullary pyramids because of the characteristic appearance of the posterior surface of the medulla. Another name for this motor tract is the pyramidal pathway.

The cerebellum has a major role in muscle movement. It assists in the planning of movements, provides feedback to the cortex as to the efficiency of the initiated movements, is involved in the control of posture and muscle tone and ensures that movements are smooth and coordinated.

Nerve cells and pathways

Communication around the nervous system occurs via the activation of nerve pathways. The structural and functional unit of these pathways is the neurone or nerve cell. There are billions of neurones in the human brain. Neurones receive messages from other nerve cells in the form of electrical impulses and respond by either sending or not sending impulses along to the next neurone in the pathway. Neurone function is also influenced by chemical modulators within the tissue fluid bathing the nerve cells. Circuits of nerve pathways connect regions of the brain; many circuits form nerve networks that control brain functions. Brain circuits and networks can be switched on or off as they process inputs, integrate information, make decisions and activate the most appropriate responses. Thought processes and life experiences can reinforce and enhance the dominance of particular brain circuits at the cost of others. Indeed, our brains are quite malleable. Throughout our lives, neural connections are changing; new connections between neurones are made and connections in existing circuits are lost. This is the basis of learning and memory formation. It is a requirement in the normal development and maturation of humans to adulthood, but this process continues throughout adult life as we learn to adapt to changing life circumstances.

Neurotransmission

The interface or connection between sending and receiving neurones is called the synapse. It has been suggested that there are 1 × 10¹⁵ synapses in the human brain. The synapse consists of the axon terminal of the sending neurone (the presynaptic neurone), the cell membrane of the receiving neurone (the postsynaptic neurone) and a small gap between the two called the synaptic cleft (see Fig 2.3). As the presynaptic and postsynaptic neurones do not actually make direct contact, the communication is conveyed postsynaptically via the release of chemical messengers or neurotransmitters.

Figure 2.3 The synapse.

A representation of a typical synapse.

Reproduced from Fundamentals of Anatomy and Physiology (7th edn), 2006, by F H Martini, Pearson Education, Inc (publishing as Benjamin Cummings), Fig 12.2, with permission