The Fight or Flight Response
The physiological changes associated with the fight or flight response allow the body to access energy reserves and enhance sensory capabilities. This response facilitates immediate action to either fight off a threat or escape to safety. The implications of this response were critical for the survival of our ancestors who faced tangible physical dangers. However, in contemporary society, many stress-inducing situations are psychological rather than physical.
For instance, one may feel intense anxiety during a public speaking event or just before an important exam, where genuine physical danger is absent. Yet, individuals have evolved to trigger the fight or flight response as a reaction to these perceived threats. In modern contexts, such responses may not be adaptive and can lead to detrimental health outcomes if experienced consistently in situations that do not require physical confrontation or flight. (Chandola, Brunner, & Marmot, 2006; Glaser & Kiecolt-Glaser, 2005)
Consequences of chronic stress exposure might include increased susceptibility to heart disease and impaired immune function. Some of the propensity for this stress reactivity may be rooted in early traumatic experiences. Once the perceived threat is resolved, the parasympathetic nervous system activates, restoring the body to a state of relaxation. This involves a normalization of heart rate and blood pressure, constriction of pupils, restoration of bladder control, and conversion of glucose into glycogen for storage by the liver.
The Brain and Spinal Cord
Learning Objectives
By the end of this section, you will be able to:
Explain the functions of the spinal cord.
Identify the hemispheres and lobes of the brain.
Describe the types of techniques available to clinicians and researchers to image or scan the brain.
Overview of the Brain
The brain is an incredibly intricate organ composed of billions of interconnected neurons and glial cells. It is a bilateral structure, divided into distinct lobes that serve particular functions. Despite the specialization of functions associated with each lobe, all brain areas collaborate to underpin our thoughts and actions.
The Spinal Cord
The spinal cord acts as the brain's connection to the outside world, functioning as a relay station for messages. It not only transmits messages to and from the brain but also executes automatic processes known as reflexes. The spinal cord commences as a bundle of nerves at the brain stem, which governs essential life processes such as breathing and digestion, and extends to just below the ribs, where it terminates, contrary to common belief that it reaches the base of the spine.
Structurally, the spinal cord consists of 30 segments, each corresponding to a vertebra and connecting to specific body parts through the peripheral nervous system. Nerves extend from each vertebra: sensory nerves transmit incoming messages, while motor nerves convey commands to muscles and organs. Some sensory responses are processed directly by the spinal cord without involving the brain, allowing for immediate reactions to stimuli. For example, reflex actions such as withdrawing from a hot surface or the knee-jerk reaction can occur faster because they do not require brain processing time.
Despite the protective vertebral structure and cerebrospinal fluid cushioning the spinal cord, injuries can lead to paralysis depending on where the damage occurs; injury to higher spinal segments results in greater loss of function than injuries lower down.
Neuroplasticity
An illustrative case of neuroplasticity is that of journalist Bob Woodruff, who sustained a traumatic brain injury from a bomb explosion during reporting in Iraq. Woodruff experienced significant cognitive deficits, particularly concerning memory and language. Through rigorous cognitive and speech therapy, he exhibited remarkable recovery, underscoring the brain's ability to adapt and reorganize. Neuroplasticity refers to the nervous system's capacity to change, which can happen through personal experiences or in response to injury. It includes mechanisms such as the formation of new synapses, pruning unused synapses, alterations in glial cells, and even the generation of new neurons.
The Two Hemispheres
The brain’s cerebral cortex is characterized by an uneven surface comprising folds (gyri) and grooves (sulci). The most significant of these grooves is the longitudinal fissure, which bifurcates the brain into left and right hemispheres.
Research has shown some degree of specialization, or lateralization, in function between the two hemispheres. The left hemisphere predominantly governs the right side of the body and is associated with language functions, memory association, selective attention, and positive emotions. Conversely, the right hemisphere controls the left side and is linked to pitches, arousal, and negative emotions (Ehret, 2006). Although there are distinctions between hemispheric functions, it is essential to note that the interaction between both hemispheres is crucial for integrated behavior (Banich & Heller, 1998).
The two hemispheres communicate via the corpus callosum, a band of neural fibers consisting of about 200 million axons. In specific cases, such as severe epilepsy, the corpus callosum may be severed as a treatment, leading to unique behaviors in ‘split-brain’ patients. For instance, such patients may struggle to verbally identify objects presented to their left field of vision due to the right hemisphere's largely nonverbal processing, yet may successfully reproduce the object with their left hand, which is controlled by the right hemisphere. Operationally, research on individuals with strokes has informed our understanding of the relationship between brain structure and behavior.
Forebrain Structures
The forebrain is the largest brain region and comprises the cerebral cortex (involved in complex cognitive tasks) and various subcortical structures (thalamus, hypothalamus, pituitary gland, and the limbic system). The cerebral cortex can subdivide into four lobes: frontal, parietal, temporal, and occipital.
Lobes of the Brain
Frontal Lobe
Located at the front, extending back to the central sulcus, this lobe is involved in reasoning, motor control, emotion, and language. It hosts the motor cortex (planning and coordinating movement) and the prefrontal cortex (higher cognitive functions). Broca’s area is within this lobe and is crucial for language production (damage leads to difficulty spoken language).Parietal Lobe
Positioned behind the frontal lobe, this lobe deals with processing sensory information, including touch, temperature, and pain. It houses the somatosensory cortex, where different areas correspond to sensations from specific body parts. For example, areas of the cortex related to fingers are larger than those corresponding to toes, reflecting the density of nerve endings.Temporal Lobe
Located near the temples, it is associated with hearing, memory, emotion, and certain language aspects. The auditory cortex here processes sound, whereas Wernicke’s area pertains to speech comprehension (damage leads to an inability to comprehend language while still allowing for fluent speech).Occipital Lobe
Situated at the brain's back and responsible for visual processing. The primary visual cortex located here interprets incoming visual information with spatial arrangements corresponding to visual fields.
Other Forebrain Structures
The thalamus acts as a sensory relay for all senses, except smell, directing them to relevant brain areas for processing. The limbic system includes the hippocampus (memory and learning), amygdala (emotion), and hypothalamus (homeostasis and endocrine regulation).
The Case of Henry Molaison (H.M.)
A pivotal case in neuroscience, H.M. underwent surgery to treat severe seizures that led to the removal of parts of his limbic system (hippocampus and amygdala). Post-surgery, he suffered significant memory deficits, illustrating the hippocampus' critical role in the formation of explicit memories. H.M. could not remember new experiences and could not recognize new faces, despite retaining the ability to learn new skills without conscious recall.
Midbrain and Hindbrain Structures
The midbrain connects the forebrain to the hindbrain and contains the reticular formation, pivotal for sleep/wake cycles and arousal, as well as regions producing dopamine critical for movement and mood regulation. The hindbrain, comprising structures like the medulla (regulating autonomic functions), pons (bridging brain connections and regulating sleep), and cerebellum (coordinating movement and motor skills), plays a crucial role in basic life functions.
Issues in Brain Death
Decisions surrounding medical ethics arise when dealing with patients declared brain dead yet sustained by medical technology. The case of Terri Schiavo showcased the complexities of determining care for individuals lacking significant brain function, generated extensive legal and ethical discussions, and called attention to the need for clear directives for end-of-life care.
Brain Imaging Techniques
Increasingly, brain imaging technology enhances our understanding of brain functions and injuries, employing techniques such as:
CT Scan - Employs X-rays to create images of body areas, useful for identifying tumors and atrophy.
PET Scan - Involves injecting a tracer to visualize brain activity uniquely, indicating metabolically active regions.
MRI - Utilizes strong magnetic fields to create detailed images of brain structures.
fMRI - An extension of MRI tracking changes in blood flow and oxygen levels to illustrate brain activity over time.
EEG - Records electrical activity of the brain via electrodes, retaining millisecond-level accuracy for analyzing sleep patterns and disorders.
The Endocrine System
The endocrine system is composed of various glands that secrete hormones, chemicals that influence many bodily functions. The interaction between the hypothalamus in the CNS and the pituitary gland in the endocrine system dictates many of these hormonal functions.
Major Endocrine Glands and Their Functions
Pituitary Gland - Known as the master gland, regulates other endocrine glands, secreting growth hormones, hormones from the hypothalamus, and controlling various bodily functions.
Thyroid Gland - Regulates metabolism and energy levels through thryoxine and triiodothyronine secretion.
Adrenal Glands - Secrete hormones involved in the stress response, such as epinephrine and norepinephrine.
Pancreas - Generates insulin and glucagon for blood sugar management.
Gonads (Ovaries/Testes) - Produce sex hormones vital for reproduction, as well as mediating sexual behavior.
Consequences of Hormonal Imbalance
Hormonal imbalances can lead to various health disorders, highlighting the importance of understanding the roles of different hormones in maintaining bodily functions.