Unit 1: The Brain and the brain function
Paul Broca (1861) performed an autopsy on the brain of a patient, nicknamed Tan, who had lost the capacity to speak, although his mouth and his vocal cords weren’t damaged and he could still understand language.
Tan’s brain showed deterioration of part of the frontal lobe of the left cerebral hemisphere, as did the brains of several similar cases.
This connected destruction of the part of the left frontal lobe known as Broca’s area to loss of the ability to speak, known as expressive aphasia.
Carl Wernicke similarly found another brain area involved in understanding language in the left temporal lobe.
Destruction of Wernicke’s area results in loss of the ability to comprehend written and spoken language, known as receptive aphasia.
Lesions, precise destruction of brain tissue, enabled more systematic study of the loss of function resulting from surgical removal (also called ablation), cutting of neural connections, or destruction by chemical applications.
Studies by Roger Sperry and Michael Gazzaniga of patients with these “split brains” have revealed that the left and right hemispheres do not perform exactly the same functions (brain lateralization) that the hemispheres specialize in.
Computerized axial tomography (CAT or CT) creates a computerized image using X-rays passed through various angles of the brain showing two-dimensional “slices” that can be arranged to show the extent of a lesion.
In magnetic resonance imaging (MRI), a magnetic field and pulses of radio waves cause the emission of faint radio frequency signals that depend upon the density of the tissue.
Measuring Brain Function
An EEG (electroencephalogram) is an amplified tracing of brain activity produced when electrodes positioned over the scalp transmit signals about the brain’s electrical activity (“brain waves”) to an electroencephalograph machine.
The amplified tracings are called evoked potentials when the recorded change in voltage results from a response to a specific stimulus presented to the subject.
Positron emission tomography (PET) produces color computer graphics that depend on the amount of metabolic activity in the imaged brain region.
Functional MRI (fMRI) shows the brain at work at higher resolution than the PET scanner.
Changes in oxygen in the blood of an active brain area alters its magnetic qualities, which is recorded by the fMRI scanner.
A magnetic source image (MSI), which is produced by magnetoencephalography (MEG scan), is similar to an EEG, but the MEG scans are able to detect the slight magnetic field caused by the electric potentials in the brain.
Central nervous system: consists of your brain and your spinal cord.
Peripheral nervous system : includes two major subdivisions: your somatic nervous system and your autonomic nervous system.
Your peripheral nervous system lies outside the midline portion of your nervous system carrying sensory information to and motor information away from your central nervous system via spinal and cranial nerves.
Somatic nervous system: has motor neurons that stimulate skeletal (voluntary) muscle.
Autonomic nervous system: has motor neurons that stimulate smooth (involuntary) and heart muscle.
Your autonomic nervous system is subdivided into the antagonistic sympathetic nervous system and parasympathetic nervous system.
Sympathetic stimulation results in responses that help your body deal with stressful events including dilation of your pupils, release of glucose from your liver, dilation of bronchi, inhibition of digestive functions, acceleration of heart rate, secretion of adrenaline from your adrenal glands, acceleration of breathing rate, and inhibition of secretion of your tear glands.
Parasympathetic stimulation calms your body following sympathetic stimulation by restoring digestive processes (salivation, peristalsis, enzyme secretion), returning pupils to normal pupil size, stimulating tear glands, and restoring normal bladder contractions.
Spinal cord, protected by membranes called meninges and your spinal column of bony vertebrae, starts at the base of your back and extends upward to the base of your skull where it joins your brain.
The Brain
According to one evolutionary model (triune brain), the human brain has three major divisions, overlapping layers with the most recent neural systems nearest the front and top.
The reptilian brain, which maintains homeostasis and instinctive behaviors, roughly corresponds to the brainstem, which includes the medulla, pons, and cerebellum.
The old mammalian brain roughly corresponds to the limbic system that includes the septum, hippocampus, amygdala, cingulate cortex, hypothalamus, and the thalamus, which are all important in controlling emotional behavior, some aspects of memory, and vision.
The new mammalian brain or neocortex, synonymous with the cerebral cortex, accounts for about 80 percent of brain volume and is associated with the higher functions of judgment, decision making, abstract thought, foresight, hindsight and insight, language, and computing, as well as sensation and perception.
The surface of your cortex has peaks called gyri and valleys called sulci, which form convolutions that increase the surface area of your cortex.
Deeper valleys are called fissures.
The last evolutionary development of the brain is the localization of functions on different sides of your brain.
Localization and Lateralization of the Brain’s Function
Association areas are regions of the cerebral cortex that do not have specific sensory or motor functions but are involved in higher mental functions, such as thinking, planning, remembering, and communicating.
Medulla oblongata—regulates heart rhythm, blood flow, breathing rate, digestion, vomiting.
Pons—includes portion of reticular activating system or reticular formation critical for arousal and wakefulness; sends information to and from medulla, cerebellum, and cerebral cortex.
Cerebellum—controls posture, equilibrium, and movement.
Basal ganglia—regulates initiation of movements, balance, eye movements, and posture, and functions in processing of implicit memories.
Thalamus—relays visual, auditory, taste, and somatosensory information to/from appropriate areas of cerebral cortex.
Hypothalamus—controls feeding behavior, drinking behavior, body temperature, sexual behavior, threshold for rage behavior, activation of the sympathetic and parasympathetic systems, and secretion of hormones of the pituitary.
Hippocampus—enables formation of new long-term memories.
Cerebral cortex—center for higher-order processes such as thinking, planning, judgment; receives and processes sensory information and directs movement.
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Plasticity: Although specific regions of the brain are associated with specific functions, if one region is damaged, the brain can reorganize to take over its function.
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Glial cells guide the growth of developing neurons, help provide nutrition for and get rid of wastes of neurons, and form an insulating sheath around neurons that speeds conduction.
The neuron is the basic unit of structure and function of your nervous system.
The cell body (a.k.a. cyton or soma) contains cytoplasm and the nucleus, which directs synthesis of such substances as neurotransmitters.
The dendrites are branching tubular processes capable of receiving information.
The axon emerges from the cyton as a single conducting fiber (longer than a dendrite) that branches and ends in tips called terminal buttons, axon terminals, or synaptic knobs.
The axon is usually covered by an insulating myelin sheath (formed by glial cells).
Neurogenesis, the growth of new neurons, takes place throughout life.
Neurotransmitters are chemicals stored in structures of the terminal buttons called synaptic vesicles.
Dopamine stimulates the hypothalamus to synthesize hormones and affects alertness and movement.
Glutamate is a major excitatory neurotransmitter involved in information processing throughout the cortex and especially memory formation in the hippocampus.
Serotonin is associated with sexual activity, concentration and attention, moods, and emotions.
Opioid peptides such as endorphins are often considered the brain’s own painkillers. Gamma-aminobutyric acid (GABA) inhibits firing of neurons.
Norepinephrine, also known as noradrenaline, is associated with attentiveness, sleeping, dreaming, and learning.
Agonists may mimic a neurotransmitter and bind to its receptor site to produce the effect of the neurotransmitter.
Antagonists block a receptor site, inhibiting the effect of the neurotransmitter or agonist.