Principles of Neuroscience: The Brain - The Most Powerful Tool

Neuroscience centres around the brain. A true understanding of the brain in all its complexity can be said to be the ultimate aim of neuroscience. The myriad functions of the brain, how the brain impacts and controls the body, and the relationship between the brain and mind are just some of the questions that continue to baffle neuroscientists. Although headway has been made in understanding many of the functions of specific regions of the brain (mainly by studying the effect of localised brain injuries on patients), there is still comparatively little understanding of the brain. This is due in part to the limited tools that can currently be used to study the brain (Bassett & Gazzaniga, 2011). The brain is a large and complex system of interacting networks, and multiple networks can be involved in each brain function (Bassett & Gazzaniga, 2011). The further an animal is on the evolutionary scale, the more complex their brain will be. Male brains are on average 138g heavier than female brains, but there is no difference in brain functionality or intelligence between these sexes (Brodal, 2004; Jawabri & Sharma, 2023).

For centuries, it was believed that there must be a single decision-making location inside the brain that controls the mind and body, but this is not the case. (Singer, 2007). The brain is composed of billions of neurons that communicate with each other through synapses (Jawabri & Sharma, 2023). These neurons form organised functional networks that can interact to carry out cognitive functions in the brain. Adult brains are structurally and functionally organised, with specialised regions of the brain optimised to process specific information. These specialised areas then interact with other brain regions through functional networks, enabling parts of the brain to collaborate and support a variety of cognitive functions. Although the mechanisms that organise and build these functional networks are not well understood, studies using functional magnetic resonance imaging have investigated the interaction of different brain regions by analysing how certain brain regions are activated to show how involved these regions are in various cognitive tasks. Such studies have helped to gain a better understanding of brain development and typical and atypical brain functions (Fair et al., 2009).

Figure 1: The Three Sections of the Brain (Unknown, n.d.).

The brain is often referred to as ‘grey matter’. This is because the outer layer of the brain, the cortex, primarily consists of nerve cell bodies, which are grey. The inner layer, the white matter, is primarily made up of myelinated axons and appears white. The axons of the white matter originate from the cell bodies of the grey matter, so the white matter allows the grey matter to communicate and form connections (Van Der Knaap & Valk, 2011). A decrease in grey matter has been shown to occur with age, most likely due to age-related loss of myelination, loss of synapses and neuronal shrinkage. White matter increases slightly into adulthood and then begins to decrease in some regions with age (Giorgio et al., 2010). The brain can be divided into three sections: the cerebrum, the brain stem and the cerebellum.

The Cerebrum

The cerebrum is the culmination of millions of years of evolution (Rhoton, 2007). The cerebrum can be divided into two cerebral hemispheres. Although the two cerebral hemispheres may appear similar on the surface, they are asymmetrical. Hemispherical asymmetry refers to differences in the characteristics, structure and functions between the cerebral hemispheres (Hugdahl & Westerhausen, 2010). Broca’s area and Wernicke’s area demonstrate this asymmetry. By observing speech impairments in patients with lesions to the left hemisphere (specifically, in the inferior frontal gurus), Paul Broca was able to determine that language is governed by the left hemisphere. The part of the brain where these language-impairing lesions occurred was termed ‘Broca’s area’ because of his findings. Broca’s area, in the inferior frontal gyrus of the left hemisphere, is the main region of language production in the brain (Hugdahl & Westerhausen, 2010; Esteves et al., 2020). Furthermore, lesions to the left posterior temporal were discovered by Carl Wernicke to cause difficulties in comprehending language. This language comprehension region is named Wernicke’s area. Broca and Wernicke’s findings have advanced the theory that the left cerebral hemisphere is dominant in language-related functions (Esteves et al., 2020).

Figure 2: Wernicke's area and Broca's area (Unknown, n.d.).

The neocortex is, literally, the ‘new’ cortex. It is the part of the cerebral cortex that has most recently evolved (Rakic, 2009). Mammals are the only animals that have developed a neocortex, the multilayered region of the brain responsible for sensation, cognition, and consciousness (Diaz & Gleeson, 2009). Reptiles have a thin dorsal cortex. In mammals, this evolved into the thicker, layered, columnar neocortex (Kaas, 2006). Due to the highly organised, folded structure of the neocortex, the size of the cortical sheet and cortical field number is much larger than in other animals. Paleontological studies show that the neocortices of early mammalian fossils had few cortical fields. The cortical fields in the neocortices of primates then expanded rapidly, with increased and more complex interconnectivity within the neocortex (Karlen & Krubitzer, 2008). The neocortex consists of over ten billion neurons, including oligodendrocytes and glia, which form trillions of connections with cells throughout the nervous system. These synapses allow for the highly complex activity of the neocortex (Diaz & Gleeson, 2009). Unlike other animals, mammals are capable of complex behaviours such as cognition, perceptions and controlled, intentional movement. As species with large, complex neocortices can perform more complex behaviours, it is understood that these complex mammalian abilities are possible due to the neocortex (Karlen & Krubitzer, 2008). In humans, the neocortex is at its most evolved and complicated. The neocortex processes most higher learning and is responsible for the higher perceived intelligence of humans compared to other mammals. Some scientists also claim that the neocortex is responsible for the conscious mind and morality of humans (Karten, 2015). The neocortex of humans is more evolved than that of other mammalian species. Human neocortices are elaborated compared to other mammals, such as increased folded surface giving a larger surface area, unique neuronal origin points and distinct cell migratory pathways. This evolved neocortex is responsible for the advanced cognitive and emotional abilities of humans (Rakic, 2009).

The cortex has four lobes: the frontal lobe, parietal lobe, temporal lobe and occipital lobe, which are each involved in different neurological functions (Jawabri & Sharma, 2023). The frontal lobes are the biggest of the cerebrum, accounting for two-thirds of the size of the brain. The frontal lobes are involved in motor function and language (in Broca’s area), as well as cognitive processes, including executive function, attention, and memory (specifically, remembering plans made). Processes in the frontal lobes can also contribute to mood, personality, self-awareness, and social and moral reasoning (Chayer & Freedman, 2001; Miller & Cummings, 2017). The frontal lobes also include the somatosensory cortex and motor cortex. The motor cortex is at the top of a hierarchal system of multiple neural structures involved in movement. This is due to the frontal lobe's functions in memory and movement. The somatosensory cortex plans movement, while the motor cortex sends signals for the execution of movement (Jawabri & Sharma, 2023; Miller & Cummings, 2017).

Figure 3: The Four Lobes of the Brain (Unknown, n.d.).

The parietal lobe is found posterior to the frontal lobe. There are two functional regions of the parietal lobe, the anterior parietal and posterior parietal lobes. The anterior parietal lobe encompasses the primary sensory cortex, which receives and interprets sensory signals of touch, position, vibration, pressure, pain and temperature. The posterior parietal lobe encompasses the somatosensory association cortex and secondary somatosensory cortex, which receives somatosensory inputs and combines this information with, for example, visual or auditory inputs to produce higher-order functions like sensorimotor planning, learning, language, spatial recognition and stereognosis, which is telling the difference between objects (Jawabri & Sharma, 2023). The temporal lobe is posterior to the frontal lobe and inferior to the parietal lobe. This lobe contains the primary auditory cortex, which processes sound and tones. Wernicke’s area is also found in the temporal lobe and is required for language comprehension. Another region of the temporal lobe is the middle temporal gyrus, which has roles in semantic memory (common knowledge memory) (Jawabri & Sharma, 2023). The Occipital lobe is the smallest cortical lobe, located posterior to the parietal and temporal lobes. This lobe is the brain’s visual processing hub and is involved in visuospatial processing, depth perception, colour determination, facial recognition, and memory formation. The primary visual cortex, within the occipital lobe, receives the visual inputs from the retina, by way of the thalamus, and transmits the signal through the dorsal or ventral pathways. For information on an object’s location, the signal is transmitted through the dorsal stream to the parietal lobe; object recognition information is transmitted through the ventral stream to the temporal lobe (Rehman & Khalili, 2019).

The Case of Phineas Gage

A significant role of the frontal lobe is in shaping personality. One of the most famous cases in neurology is that of a man named Phineas Gage, who suffered a drastic change in personality following a serious injury to his prefrontal cortex when, in 1848, an accidental explosion sent a 43-inch-long tamping iron (used to compact explosives on the railway construction Gage was working on) into the left side of his skull. The tamping iron entered his head under his left cheekbone and exited through the top of his head, leaving a hole three inches in diameter hole through his brain. Gage survived and could walk and speak, and his cognitive function was preserved. However, it was reported that his personality was completely altered. Before the accident, Gage was described as a polite, sociable, and gentle young man. However, following the injury to his prefrontal cortex, he became irrational, insensitive, profane, and socially inappropriate. Gage lived for another 12 years when he began having seizures and then died four months later. Studies of Gage’s skull in the 1990s revealed the parts of Gage’s brain damaged were the same as those damaged in frontal lobe syndrome. Although his life was irrevocably changed, Phineas Gage’s injury led to unprecedented advances in the understanding of the brain (Haas, 2001; Jawabri & Sharma, 2023). Injury to the prefrontal cortex can result in different types of personality changes, including executive disturbances, disturbed social behaviour, emotional dysregulation, hypo-emotionality/de-energisation, and distress. How the patient makes decisions can also change. Decision-making involves reasoning, learning, and creativity. Damage to the prefrontal cortex can obstruct these mechanisms and lead to the patient making choices they would not have before their accidents (Jawabri & Sharma, 2023).

Figure 4: Phineas Gage's skull (right) and an illustration of the tampering iron going through his skull (left) (Shelley, 2016).

The Brain Stem

The brainstem is the most primitive part of the brain. The brainstem is at the caudal end of the spinal cord and links the cerebral hemispheres to the medulla and cerebellum. It also connects with the spinal cord, basal ganglia and diencephalon (which comprises of the thalamus and hypothalamus) of the brain. The brainstem consists of the midbrain, pons and medulla oblongata. This structure is key to survival as it is required for essential functions like cardiac regulation, breathing, heartbeat and blood pressure, consciousness, and the sleep cycle (Fernández-Gil et al., 2010; Sciacca et al., 2019). The brainstem’s role in consciousness is evident as injury or damage to the upper brainstem results in a coma and a vegetative state. Furthermore, in studies where lesions or electrical stimulation were applied to the reticular formation region of the brainstem, an electrophysiological pattern of wakefulness and attentiveness was observed (Parvizi & Damasio, 2001). Fibre tracts from the cerebral cortex pass through the white matter of the brain stem, allowing for voluntary motor function to occur. Peripheral and spinal cord nerves also pass through somatosensory pathways towards the cortex (Fernández-Gil et al., 2010). Except for the olfactory (I), optic (II) and part of the accessory (XI) nerves, all cranial nerve nuclei are in the brainstem (Sciacca et al., 2019).

The midbrain connects the pons to the diencephalon and communicates through the superior cerebellar peduncles to the cerebellum via the superior cerebellar peduncles (Fernández-Gil et al., 2010). The midbrain contains the cerebral aqueduct of Sylvius, which is surrounded by periaqueductal grey matter and is involved in pain modulation, controlling emotional responses like fear and anxiety, vocalisation, and cardiovascular control (Sciacca et al., 2019).

Figure 5: The Brainstem (Unknown, n.d.).

The pons is found between the midbrain and medulla oblongata. The name ‘pons’ comes from its appearance, which resembles a bridge between the two cerebellar hemispheres. The pons has several transverse fibres that merge to form the middle cerebellar peduncles (Fernández-Gil et al., 2010). The pons have tracts that carry signals from the motor cortex to the cerebellum, medulla, and thalamus (Thau et al., 2022). It also includes part of the auditory pathway called the trapezoid body, which plays a role in localising sound (Sciacca et al., 2019). The medulla oblongata connects the spinal cord and the pons. The medulla oblongata is evolutionarily conserved: the structure and function of this brain structure have been consistent throughout evolution and can be found in all vertebrates. Sensory, proprioceptive, and motor inputs all relay through the medulla oblongata, which contains nuclei (densely populated heterogeneous neuronal cells) that have roles in a variety of functions essential to life. These functions include the regulation of various cardiovascular, respiratory, and autonomic functions. Medullary nuclei house ascending sensory and motor tracts involved in different processes and neuronal circuits. Defects of the medulla oblongata can result in neurodevelopmental disorders, like congenital central hypoventilation syndrome, Wold–Hirschhorn syndrome, Rett syndrome, and Pitt–Hopkins syndrome, depending on the nuclei affected (Diek et al., 2022).

The Cerebellum

Cerebellum means ‘little brain’, and it is about one-tenth the size of the cerebrum, though it encompasses around 80% of total brain neurons. The cerebellum controls coordinated and smooth voluntary movement. The cerebellum is a three-layered structure located at the back of the brain, inferior to the occipital and temporal lobes and above the spinal cord. It has three distinct regions, the vestibulocerebellum, spinocerebellum, and cerebrocerebellum, which are distinguishable due to their afferent and efferent connections and functions. The structure of the cerebellum is made up of regular units of neurons that all have the same cerebellar microcircuitry. The functional units of the cerebellum operate independently of each other. The cerebellum makes indirect and direct connections with the brainstem, spine, and many different cerebral subcortical and cortical regions using Purkinje cells and cerebellar peduncle cells. The white matter of the superior cerebellar peduncle projects to the midbrain to coordinate arm and leg movement. Proprioceptors connecting the inferior cerebellar peduncle and medulla maintain balance and posture, and the one-directional afferent fibres connecting the pons of the middle cerebellar peduncle communicate voluntary motor actions to the cerebellum. The cerebellum receives constant, high-level communications from the cerebrum about intended movements. The cerebellar cortex processes this to send calculated signals to the motor cortex for voluntary muscle contraction. By calculating the force and direction required, the cerebellum makes sure this muscle contraction is precise and coordinated. For a long time, it was believed that the cerebellum was exclusively involved in motor functions, but recent studies have shown it is also involved in cognition (Roostaei et al., 2014; Thau et al., 2022).

Figure 6: The Cerebellum (Unknown, n.d.).

Conclusion

The brain is the most complicated and powerful organ in the body. It decodes all the sensory information we receive and gives meaning to the world around us. Each lobe and major region of the brain plays an essential role in brain function, and the loss of any region through injury or defect can have severe consequences. Although no single cranial structure is known to be solely responsible for consciousness or cognition, billions of neurons in multiple distinct brain regions work together through tens of billions of synaptic connections to carry out a myriad of brain functions. It is known that the brain is responsible for cognition, decision-making, memory, speech, higher cortical functions and movement, among other functions, but we haven’t even scratched the surface of what the brain is capable of. Although human brains seem to be the most advanced and intelligent system on the planet, making them the best tool for deciphering the brain. However, the brain is still a long way from fully understanding itself.

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