Biological and Neuropsychology: Brain Organization, Communication, and Assessment
Split-Brain Surgery and Hemispheric Specialization
Split-brain surgery is a radical medical procedure primarily performed to reduce the severity of epileptic seizures. This intervention involves the surgical severing of the corpus callosum, which is the thick band of nerve fibers that facilitates communication between the two cerebral hemispheres. Once the corpus callosum is severed, the two halves of the brain essentially function independently of one another. Each hemisphere’s primary connections are to the opposite side of the body; therefore, the left hemisphere controls various functions of the right side, and the right hemisphere controls the left side. This division provides a unique opportunity for neuropsychological research into hemispheric specialization.
Roger Sperry conducted pioneering research on split-brain subjects to determine how each hemisphere processes information. In his experiments, stimuli presented in the right visual field are sent to the left hemisphere, while stimuli from the left visual field are transmitted to the right hemisphere. When common objects were flashed in the right visual field, split-brain subjects could name and describe the objects because the left hemisphere is responsible for verbal processing. Conversely, the right hemisphere has its own specialized talents focused on non-verbal processing. Research indicates that while the left hemisphere handles tasks related to language and verbal output, the right hemisphere excels in spatial, non-verbal, and perceptual tasks. This research established the concept of perceptual asymmetries, though academics caution against over-generalizing findings from split-brain subjects to individuals with intact corpus callosa.
The Organization of the Human Nervous System
The human nervous system is subdivided into a hierarchical structure that manages internal and external information. The two main branches are the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and the spinal cord. The brain itself is categorised into the forebrain (which includes the cerebrum, limbic system, thalamus, and hypothalamus), the midbrain (containing the reticular formation), and the hindbrain (comprising the cerebellum, pons, and medulla). The PNS acts as a communication link between the CNS and the rest of the body.
The Peripheral Nervous System is further divided into the Somatic Nervous System and the Autonomic Nervous System (ANS). The Somatic Nervous System handles voluntary movements and sensory information through afferent nerve fibers (carrying signals toward the CNS) and efferent nerve fibers (carrying signals away from the CNS to muscles and glands). The Autonomic Nervous System controls involuntary bodily functions and is partitioned into the Sympathetic and Parasympathetic divisions. These divisions generally act in opposition to maintain homeostasis or respond to stress.
Key Functions of the Parasympathetic and Sympathetic Systems
The Parasympathetic Nervous System is associated with "rest and digest" functions. Its primary activities include stimulating the flow of saliva, slowing the heartbeat, constricting the bronchi in the lungs, stimulating peristalsis and secretion in the digestive tract, stimulating the release of bile, and contracting the bladder. These processes conserve energy and maintain regular bodily maintenance.
In contrast, the Sympathetic Nervous System prepares the body for "fight or flight" responses. Its activation leads to the dilation of the pupils, the inhibition of saliva flow, and a significant acceleration of the heartbeat. It also results in the dilation of the bronchi, the inhibition of peristalsis and secretion, the conversion of glycogen into glucose for quick energy, the secretion of adrenaline and noradrenaline, and the inhibition of bladder contraction. These responses are coordinated through the sympathetic ganglia chain and the solar plexus to mobilize the body's resources for immediate action.
Individual Cells of the Nervous System: Neurons
Neurons are the individual cells that serve as the basic chains of communication in the nervous system. They are specialized to receive, integrate, and transmit information using both electrical and chemical signals. A neuron is composed of several key structures: dendrites, which are the branch-like structures that receive incoming signals; the soma, or cell body, which contains the nucleus and integrates information; and the axon, a long fiber that transmits the electrical impulse over a distance. The axon is often insulated by the myelin sheath, which speeds up signal transmission. Signals reach the terminal buttons at the end of the axon, where chemical substances are released to communicate with the next cell. While information generally travels from the dendrites toward the terminal buttons, some substances travel in the opposite direction toward the soma to provide nourishment.
Transmission of Nerve Impulses and Ions
Nerve impulses are electrical signals known as action potentials that travel along the axon of a neuron. This process involves the movement of specific ions in and out of the cell membrane, creating a wave-like electrical effect. The most important ions involved in this process are sodium (), potassium (), and chloride (). To start an action potential, ions rush into the neuron. Following the peak of the action potential, ions flow out of the cell to reset the electrical state. ions primarily play an inhibitory role, helping to prevent the neuron from firing unnecessarily.
When the action potential arrives at the terminal buttons, the signal is passed to the next cell across a small gap called the synapse or synaptic gap. This transmission is chemical rather than electrical. The chemicals secreted from the terminal buttons are known as neurotransmitters. These molecules cross the synaptic gap and bind to specific receptor sites on the dendrites of the receiving neuron. This binding process can either stimulate or inhibit a further signal in the receiving cell.
Neurotransmitters: Function and Dysfunction
Different neurotransmitters are responsible for various physiological and psychological functions, and their imbalances are linked to specific disorders. Dopamine () is essential for the control of voluntary movement and is characterized as the "reward pathway." Dysfunctions in dopamine levels are significantly linked to Parkinsonism (associated with low doses, visualized as "Small Dopey parking a car") and Schizophrenia (associated with high doses, visualized as "Tall Dopey skiing"). Norepinephrine () contributes to the regulation of mood and arousal. Imbalances in this system are linked to depressive disorders, and substances like cocaine can elevate NE activity.
Serotonin is involved in the regulation of sleep, wakefulness, and aggression. It is metaphorically described as "Sir Rotten" when it causes a bad mood; its dysfunction is associated with depressive disorders, obsessive-compulsive disorder (OCD), and eating disorders. Antidepressants like Prozac specifically target serotonin circuits. Acetylcholine () is released by motor neurons to control skeletal muscles and also contributes to attention, arousal, and memory regulation. The degradation of acetylcholine systems is a hallmark of Alzheimer’s disease. Other key neurotransmitters include Gamma Amino Butyric Acid (), which acts as a widely distributed inhibitory transmitter regulating anxiety and arousal, and Glutamate, a widely distributed excitatory transmitter involved in learning and memory, which is also linked to Schizophrenia.
Multiple Sclerosis (MS) and Research Case Studies
Multiple Sclerosis (MS) is a debilitating, degenerative, and lifelong neurological disease that affects the central nervous system. It is characterized by damage to the myelin sheath of nerves, which disrupts the electrical signals (the exposed fibers lead to signal degradation). Research by Prof. Chrisma Pretorius and Ninon Joubert explored the experiences of individuals with MS in the Western Cape, South Africa. Their qualitative study revealed that patients face daily challenges regarding the diagnosis process, the "invisible" nature of their illness, and difficulties with medical aid schemes. However, resources such as social support, mobility aids, religion, and knowledge about the disease help individuals cope.
Further research by Daniel R. du Plooy and Prof. Pretorius examined the experiences of MS caregivers, noting that the task is physically and emotionally exhausting. Caregivers cope with symptom management, limited social interaction, financial strain, and future unpredictability. Additionally, Jacqui Steadman and Prof. Pretorius investigated the impact of online Facebook support groups for people with MS. Their findings suggested that even non-active members (passive users) benefited from emotional support, informational support, and social companionship provided by these online networks, despite barriers like "emotions lost online" or the exposure to negative aspects of the disease.
The Endocrine System
The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various bodily functions. The Hypothalamus acts as a link between the nervous system and the endocrine system, controlling the Pituitary Gland. The Pituitary Gland is often referred to as the "master gland" because it regulates the activity of other endocrine glands. Other major glands include the Thyroid, which regulates metabolism; the Adrenal Glands (comprising the adrenal cortex and adrenal medulla), which respond to stress; and the Pancreas, which manages insulin levels and blood sugar. Finally, the female ovaries and male testes (gonads) manage sexual behavior, reproductive hormones, and physical growth.
Research Methods: Looking Inside the Brain
Neuroscientists use various specialized techniques to study the brain. Traditional methods include electrical recordings, lesioning (damaging specific brain tissue), and electrical stimulation. Contemporary research utilizes advanced brain-imaging techniques. Computerized Tomographic (CT) Scanning, developed in the 1970s, uses X-ray images of cross-sections of the brain to create a three-dimensional model. It is the least expensive and most widely used method in research. Magnetic Resonance Imaging (MRI), emerging in the 1980s, uses magnetism and radio frequency coils to provide detailed images of brain structure. Functional MRI (fMRI) is a variation that allows researchers to see the brain in action (e.g., seeing activity when a patient is listening or looking). Another functional method is Positron Emission Tomography (PET), which tracks metabolic activity.
Clinical Neuropsychology and Professional Roles
A neuropsychologist is a professional who evaluates and assesses patients with various conditions such as Multiple Sclerosis, Dementia, Parkinson’s disease, strokes, seizure disorders, Traumatic Brain Injuries (TBI), and developmental or learning disorders. Their assessments serve to confirm or clarify a diagnosis (differential diagnosis), quantify cognitive and behavioral strengths and weaknesses, and track changes in functioning over time. They also assist workers applying for medical boarding (disability). In South Africa, the path to becoming a neuropsychologist involves a Bachelor’s degree in psychology, an Honours degree, an accredited MA in Neuropsychology, an HPCSA-approved internship, and passing a board exam.
Symptom Validity and Malingering
A unique challenge in neuropsychological assessment is symptom validity testing. Unlike traditional therapy clients, patients involved in personal injury or insurance claims may have external incentives to exaggerate or distort their symptoms for monetary compensation. This is known as malingering, defined as the intentional production of false or grossly exaggerated physical or psychological symptoms motivated by external incentives. Neuropsychologists use specific tools to detect this, such as the Rey 15-Item Test, which checks for consistency in memory performance, and the Rey Complex Figure test, which evaluates visuospatial construction and memory.