Transcript the brain
THE BRAIN • It consumes about 20% of the energy used by the body. In infants, it consumes about 60%. This generates a lot of heat, which must be removed to prevent damage. • It has a million billion synaptic connections, making it one of the most densely connected network systems among natural and fabricated systems found on earth. ANATOMY • Myth: Humans use only 10% or 1% of their brain. Though the brain still holds mysteries that are being studied, every part of the brain is known to have a function. • A possible origin of this myth is the fact that only about 10% of the neurons in the brain are firing at any given time. • However, if all of your neurons began firing at once, you would not become smarter, but instead suffer an epileptic seizure. MENINGES • The brain is the best protected organ in the body. • The first layer of protection is the skull, which acts as armor shielding the brain from blows. • Next come the meninges, three membranes that surround the brain to keep it from being damaged by contact with the inside of the skull. • It is these membranes that become infected when someone gets meningitis, and it is because the meninges are in direct contact with the brain that meningitis is so dangerous. • For even more protection, the brain (and the spinal cord) are bathed in cerebro-spinal fluid. • This fluid circulates through a series of communicating cavities called ventricles. • Cerebro-spinal fluid also circulates between the pia mater and the arachnoid mater of the meninges. • In addition to cushioning blows, this fluid reduces the pressure at the base of the brain by causing the nerve tissue to “float”. • As the Cerebro-spinal fluid flows downward, it carries away toxic wastes and moves hormones between widely separated regions of the brain. The cerebrum • Located in the anterior portion of the forebrain. • Divided into two hemispheres that are connected by the corpus callosum. • Cerebral cortex: surface of the cerebrum Very convoluted. Spread out: 2500 sq cm 10 billion neurons and 50 trillion synapses • The convolutions have "ridges" which are called gyri (singular: gyrus), and "valleys" which are called sulci (singular: sulcus). • Some of the sulci are quite pronounced and long, and serve as convenient boundaries between four areas of the cerebrum called lobes. • How we know what we are doing within our environment (Consciousness). • How we initiate activity in response to our environment • Judgments we make about what occurs in our daily activities. • Controls our emotional response. • Controls our expressive language. Assigns meaning to the words we choose. Involves word associations. • Memory for habits and motor activities. Broca’s Area • In most people the Broca's area is in the lower part of the left frontal lobe. • It is one of the main language areas in the cerebral cortex because it controls the motor aspects of speech. • Broca Aphasia : • Persons with a Broca aphasia can usually understand what words mean, but have trouble performing the motor or output aspects of speech. • Other names for this disorder are 'expressive' and 'motor' aphasia. Wernicke's area • Wernicke's area is associated with the processing of words that we hear being spoken • Broca's area and Wernicke's area are connected by a large bundle of nerve fibres called the arcuate fasciculus. Language model • When you hear a word spoken, the auditory signal is processed first in your brain’s primary auditory cortex, which then sends it on to the neighbouring Wernicke’s area. • Wernicke’s area associates the structure of this signal with the representation of a word stored in your memory, thus enabling you to retrieve the meaning of the particular word. • In contrast, when you read a word out loud, the information is perceived first by your visual cortex, which then transfers it to the angular gyrus, from which it is sent on to Wernicke’s area. • The mental lexicon in Wernicke’s area recognizes this word and correctly interprets it according to the context. • For you then to pronounce this word yourself, this information must be transmitted via the arcuate fasciculus to a destination in Broca’s area, which plans the pronunciation process. • Lastly, this information is routed to the motor cortex, which controls the muscles that you use to pronounce the word. Wernicke’s aphasia • Lesions that cause Wernicke’s aphasia (also known as sensory aphasia or receptive aphasia) reduce understanding of spoken and written language • Responses to simple instructions, such as “Place object A on top of object B” show that patients do not understand what is being asked of them. They can read an instruction correctly, but cannot perform the action indicated by the meaning of the words. • When Wernicke’s aphasics speak, their language is littered with jargon, made-up terms, and other incomprehensible words. Their grammar is often intact, but they misuse so many words that conversation with them is very difficult. • Patients with Wernicke’s aphasia may be unaware that their speech is disorganized or that they are having any trouble in understanding other people when they speak. Motor Cortex • At the back portion of the frontal lobe, along the sulcus that separates it from the parietal lobe, is an area called the motor cortex. • stimulating areas of the motor cortex with tiny electrical probes caused movements. The lowest portions of the motor cortex, closest to the temples, control the muscles of the mouth and face. The portions of the motor cortex near the top of the head control the legs and feet. The corpus callosum • The corpus callosum is composed of a band of nearly 800 million nerve fibers adjoining the left and right cerebral hemispheres. • This information superhighway transmits neural information from one hemisphere to the other producing all integrated thought patterns. • In the central nervous system, the “grey matter” is composed of the neurons’ cell bodies and their dense network of dendrites. • The grey matter includes the centre of the spinal cord and the thin outer layer of the cerebral hemispheres, commonly known as the cortex. • The white matter consists of the myelin sheathing that covers the axons of the neurons to enable them to conduct nerve impulses more rapidly. • These myelinated axons are grouped into bundles (the equivalent of nerves) that make connections with other groups of neurons. Right and Left Hemispheres • Left Brain • Logical, sequential Rational Analytical synthesizing Objective Looks at parts Right Brain Random Intuitive Holistic, Subjective Looks at wholes • Location for visual attention. • Location for touch perception. • Goal directed voluntary movements. Manipulation of objects. • Integration of different senses that allows for understanding a single concept. • Vision and visual processing • Hearing ability • Memory aquisition • Some visual perceptions • Categorization of objects. • Breathing and Heart Rate • Swallowing Reflexes to seeing and hearing (Startle Response). • Controls sweating, blood pressure, digestion, temperature (Autonomic Nervous System). • Affects level of alertness. Ability to sleep. • Sense of balance • Consists of the midbrain, medulla oblongata, and the pons • Coordination of voluntary movement • Balance and equilibrium • Some memory for reflex motor acts. THE EVOLUTIONARY LAYERS OF THE HUMAN BRAIN • Triune Brain theory ( 1970’s) • 1. The reptilian brain, the oldest of the three, controls the body's vital functions such as heart rate, breathing, body temperature and balance. • Our reptilian brain includes the main structures found in a reptile's brain: the brainstem and the cerebellum. • The reptilian brain is reliable but tends to be somewhat rigid and compulsive. • The neocortex first assumed importance in primates and culminated in the human brain with its two large cerebral hemispheres that play such a dominant role. • These hemispheres have been responsible for the development of human language, abstract thought, imagination, and consciousness. • The neocortex is flexible and has almost infinite learning abilities. • 2. The limbic brain emerged in the first mammals. It can record memories of behaviours that produced agreeable and disagreeable experiences, so it is responsible for what are called emotions in human beings. • The main structures of the limbic brain are the hippocampus, the amygdala, and the hypothalamus. • The limbic brain is the seat of the value judgments that we make, often unconsciously, that exert such a strong influence on our behaviour. Hippocampus • The hippocampus is the brain's main entry point for memory. It is here that the initial encoding of memory elements gets processed for later recall. • Has genetically-controlled specifications for exactly where in the brain each important element of a memory will be stored. • Is involved in the recognition of novelty and in processing spatial relations, such as the route to school and home, or to one's office • Damage to the hippocampus can render an individual incapable of forming and storing any new memories or retrieving previously learned information. The consequence is various forms of amnesia. Amygdala • The amygdala is considered the brain’s primary emotional center (it is more like an emotional thermostat) • It communicates with all other sensory input systems and the cerebral cortex • Recall, retention, and long-term memory are all enhanced by the almost "hair-trigger" firing of the amygdala, which performs a key role in processing nearly all emotional events. • Emotions assist in deciding what to pay attention to, which impacts what will ultimately be remembered. • If the amygdala is removed surgically or sustains damage, an individual would process events devoid of any emotional input. • Connections between the amygdala and the cortex allow emotions to influence or sometimes to prevent nearly all learning as well as long-term memories. • The brain comprises several different kinds of memory. The hippocampus and the cortex make explicit, conscious memories possible. For its part, the amygdala enables one of the forms of implicit memory: emotional memories associated with fear. • Various aspects of an especially emotional situation such as a car accident will be processed both by the hippocampus and by the amygdala, working in parallel. Thanks to the hippocampus, you will remember whom you were with, what you did, and the fact that it was a particularly painful situation. However, it is because of the amygdala that when you remember the event, your palms will sweat, your heart will race, and your muscles will tense The cingulate gyrus • The cingulate gyrus influences impulsivity, attention, and the control of emotional behaviors. • An immature anterior cingulate gyrus is now highly suspect in incidences of ADD and ADHD. • During puberty, the cingulate gyrus and the corpus callosum mature. At that time, the connections from the frontal lobe back to the amygdala come into better balance with the number of connections going the opposite direction. The Reticular Activating system • The reticular activating system (RAS) is involved in most central nervous system activity, including control of wakefulness, sleep and part of our ability to direct attention toward specific areas of our conscious minds. • The RAS is a network of interlacing nerve cells and fibers that receives input from multiple sensory pathways. It extends from the spinal cord to the lower brain stem, upward through the mesencephalon and thalamus, and then is distributed throughout the cerebral cortex. • RAS fibers affect the autonomic and motor systems. They integrate the regulation of cardiovascular, respiratory and motor response to external stimuli. • The RAS diffusely distributes incoming sensory stimulation throughout the CNS, upregulating and readying the system to respond more specifically to input.