Summary
Highlights
Psychologists study the biological mechanisms that underlie behavior, including how millions of neurons result in a thought. This chapter explains the biological foundations of behavior, covering genetics, the nervous system, and the endocrine system. It delves into evolutionary principles such as natural selection, the difference between genotype and phenotype, and gene-environment interactions, using sickle cell anemia as an example to illustrate adaptive traits in specific environments.
Genetic variation is crucial for a species' adaptation to its environment. Human genetic variation begins with the fertilization of an egg by sperm, each contributing 23 chromosomes. Genes, made of DNA, determine traits. A person's genotype is their genetic makeup while their phenotype is their observable characteristics, influenced by both genetics and environment. Traits can be dominant or recessive, and some are polygenic. Gene mutations are a source of harmful genes. The chapter also discusses the concepts of 'range of reaction' and 'genetic environmental correlation,' and introduces epigenetics, which studies how the same genotype can lead to different phenotypes. A study on schizophrenia highlights the interplay of genetic risk and environmental factors.
The nervous system comprises glial cells, which support neurons, and neurons, which are interconnected information processors. Neurons have a semi-permeable membrane, dendrites for receiving signals, and an axon for transmitting them. The myelin sheath insulates axons, increasing signal speed, and its loss can lead to disorders like PKU and multiple sclerosis. Neuronal communication involves electrical signals (action potentials) down the axon and chemical signals (neurotransmitters) across the synaptic cleft, forming a lock and key relationship with receptors on adjacent neurons. Agonists mimic neurotransmitters, while antagonists block them, and re-uptake inhibitors prolong their effect, as seen in treatments for Parkinson's and depression.
The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS is further divided into the somatic nervous system (voluntary actions, sensory and motor information) and the autonomic nervous system (involuntary actions). The autonomic system has sympathetic (fight-or-flight response to stress) and parasympathetic (returns body to normal state) divisions, which work together to maintain homeostasis. Chronic stress can have negative health consequences due to prolonged sympathetic activation.
The brain is a complex organ connected to the outside world by the spinal cord, which also handles reflexes. Neuroplasticity allows the nervous system to change and adapt, even after injury. The brain's surface, the cerebral cortex, is divided into two hemispheres (left and right), connected by the corpus callosum. Localization of function means different brain areas are responsible for specific functions, though hemispheres interact for complex behaviors. Brain damage, like strokes, allows researchers to understand these functions.
The forebrain, the largest part of the brain, includes the cerebral cortex and subcortical structures. The cerebral cortex is divided into four lobes: frontal (reasoning, motor control, emotion, language), parietal (sensory information processing), temporal (hearing, memory, emotion, language), and occipital (visual information processing). Damage to specific areas, like Broca's or Wernicke's, results in distinct language deficits. Subcortical structures include the thalamus (sensory relay), and the limbic system (emotion and memory), which contains the hippocampus (learning and memory), amygdala (emotion), and hypothalamus (homeostatic processes, connection to endocrine system). The case of Henry Molaison (H.M.) demonstrated the hippocampus's role in memory formation.
The midbrain contains structures like the reticular formation (sleep/wake cycle, arousal), substantia nigra, and ventral tegmental area (dopamine production, movement, mood, reward). The hindbrain, at the back of the head, includes the medulla (automatic processes), pons (connects hindbrain to other parts, sleep regulation), and cerebellum (balance, coordination, motor skills, procedural memory). Brain imaging techniques such as CT scans, PET scans, MRI, fMRI, and EEG allow clinicians and researchers to visualize brain structures and activity, providing insights into various neurological and psychological conditions.
The endocrine system consists of glands that produce hormones, which are chemical messengers that travel through the bloodstream and affect cells with specific receptors. Hormones have widespread, slower, and longer-lasting effects than neurotransmitters. The pituitary gland, often called the 'master gland,' controls other glands and secretes growth hormone and endorphins. The thyroid gland regulates growth, metabolism, and appetite. Adrenal glands produce stress hormones like adrenaline, and the pancreas regulates blood sugar with insulin and glucagon. Gonads secrete sexual hormones vital for reproduction and sexual motivation. The discussion concludes with a 'dig deeper' section on anabolic steroids, their effects, and ethical considerations in sports.