The Biological Bases of Behavior

The Biological Bases of Behavior PSYCHOLOGY 1 NEURONS: THE MESSENGERS The brain of an average human being contains as many as 100 billion nerve cells, or neurons. Billions more neurons are found in other parts of the nervous system. Yet a single neuron holds many of the secrets of behavior and mental activity. Neurons vary widely in size and shape, but they are all specialized to receive and transmit information. Neurons: Individual cells that are the smallest units of the nervous system. Dendrites: Short fibers that branch out from the cell body and pick up incoming messages. Axon: Single long fiber extending from the cell body; it carries outgoing message PSYCHOLOGY 2 Parts of the Neuron PSYCHOLOGY 3 PSYCHOLOGY 4 Parts of the Neuron • Soma (cell body): cell metabolism • Dendrites: short fibers on cell body, receive “incoming” chemical messages and electrical signals from other neurons and send these signals to the cell body. • Axon: single fiber extending from cell body, carries “outgoing” chemical message • Axon Terminal: the end of the axon • Myelin Sheath: white fatty covering on longer axons -helps messages travel further and faster. In multiple sclerosis, the Myelin sheath is 5 damaged & the nervelse is impaired What make a neuron different from other cells are the tiny fibers that extend out from the cell body, enabling a neuron to perform its special job: receiving and transmitting messages. The short fibers branching out around the cell body are dendrites. Their role is to pick up incoming messages from other neurons and transmit them to the cell body. The single long fiber extending from the cell body is an axon. The axon’s job is to carry outgoing messages to neighboring neurons or to a muscle or gland. Axons vary in length. Longer axons are usually covered with a myelin sheath, a series of fatty cells which have wrapped around an axon many times. These make the axon look like a necklace of sausage-shaped beads. They serve a similar function as the insulation around electrical wire. At the very end of the axon is the axon ending, which goes by a variety of names such as the synaptic knob, the axon foot, and so on. PSYCHOLOGY 6 Cont. Between the axon ending and the dendrite of the next neuron is a very tiny gap called the synapse (or synaptic gap). For every neuron, there are between 1000 and 10,000 synapses. When the action potential reaches the axon ending, it causes tiny bubbles of chemicals called vesicles to release their contents into the synaptic gap. These chemicals are called neurotransmitters. These sail across the gap to the next neuron, where they find special places on the cell membrane of the next neuron called receptor sites. PSYCHOLOGY 7 Cont. PSYCHOLOGY 8 THE CENTRAL NERVOUS SYSTEM The Organization of the Nervous System Every part of the nervous system is connected to every other part. The central nervous system includes • the brain and • spinal cord, which together contain more than 90 percent of the body’s neurons. The peripheral nervous system consists of nerves that connect the brain and spinal cord to every other part of the body, carrying messages back and forth between the central nervous system and the sense organs, muscles, and glands. PSYCHOLOGY 9 THE HUMAN NERVOUS SYSTEM PSYCHOLOGY 10 Central and peripheral nervous systems. PSYCHOLOGY 11 The Spinal Cord The spinal cord runs from the base of the skull all the way down the spine to the "tail bone." There are motor pathways coming down from the brain and sensory pathways going up to the brain. Besides sending messages up and down to and from the brain, the spinal cord has another very important function: Reflexes. If you hold your finger over a flame for a period of time, the pain will trigger motor neurons to pull your finger away. Reflexes do much more than just get your finger out of the fire: A great deal of movement is accomplished through reflexes. Even brand new babies already have the necessary reflexes for walking: If you hold a baby and gently touch its feet to the floor, it will start making step-like movements! PSYCHOLOGY 12 The brain is the seat of awareness and reason, the place where learning, memory, and emotions are centered. The human brain is the product of millions of years of evolution. As new, more complex structures were added, older structures were retained. One way to understand the brain is to look at three layers that evolved in different stages of evolution: (1)the primitive central core; (2) the limbic system, which evolved later; and (3) (3)the cerebral hemispheres, which are in charge of higher mental processes. PSYCHOLOGY 13 The Primitive Central Core: At the point where the spinal cord enters the skull, it becomes the hindbrain. Because the hindbrain is found in even the most primitive vertebrates, it is believed to have been the earliest part of the brain to evolve. The part of the hindbrain nearest to the spinal cord is the medulla (lower part of the brain). The medulla controls involuntary vital body functions such as breathing, heart rate, and blood pressure. Near the medulla lies the pons (a whitish band of nerve fibers on the surface of the brainstem between medulla & midbrain), which produces chemicals that help maintain our sleep–wake cycle. Brainstem is the part of the brain between the spinal colum & the cerebral hemispheres. At the top and back of the brainstem is the cerebellum (or “little brain”), which is responsible for our sense of balance and for coordinating the body’s actions to ensure that movements go together in efficient sequences. Damage to the cerebellum causes severe problems in movement. PSYCHOLOGY 14 More or less directly over the brainstem are the thalamus. The thalamus is often described as a relay station (passing on something such as a message or information): Almost all sensory information from the lower parts of the central nervous system passes through the thalamus on the way to higher levels of the brain. The thalamus both integrates and shapes incoming signals. Directly below the thalamus is the smaller hypothalamus, which exerts an enormous influence on many kinds of motivation. Portions of the hypothalamus govern hunger, thirst, sexual drive, and body temperature and are directly involved in emotional behavior such as rage, terror, and pleasure. Cont. PSYCHOLOGY 15 Limbic System • An interconnected system of brain nuclei (plural of nucleus) associated with basic needs and emotions such as hunger, pain, pleasure, satisfaction, sex, and instinctive motivation. The most primitive part of the brain, it is situated close to the inner wall of each cerebral hemisphere and includes the brain system concerned with the sense of smell. PSYCHOLOGY 16 The Limbic System: The limbic system is a ring of loosely connected structures located between the central core and the cerebral hemispheres. In evolutionary terms, the limbic system is more recent than the central core and fully developed only in mammals. Animals with primitive limbic systems, such as fish and reptiles. Their patterns of feeding and attacking fixed. Mammals (including humans) are more flexible in their responses to the environment, suggesting that the limbic system suppresses some instinctive behavior. One part of the limbic system, the hippocampus (area of the brain associated with memory) plays an essential role in the formation of new memories. People with severe damage to this area can still remember names, faces, and events that they embedded in memory before they were injured, but cannot remember anything new. PSYCHOLOGY 17 Cont. The amygdala and the hippocampus govern emotions (areas of the brain that is associated with feelings of fear & aggression &n important for visual learning and memory) When portions of these structures are damaged or removed, aggressive animals become tame (friendly) and docile (quiet). In contrast, stimulation of some portions of these structures causes animals to exhibit signs of fear and panic, whereas stimulation of other portions triggers unprovoked attacks. The limbic system appears to play a central role in times of stress, coordinating and integrating the activity of the nervous system. PSYCHOLOGY 18 The Cerebral Cortex The cerebrum is divided into two hemispheres and covered by a thin layer of gray matter called the cerebral cortex. It is the part of the brain that processes thought, vision, language, memory, and emotions. The cerebral cortex accounts for about 80 percent of the weight of the human brain and containing about 70 percent of the neurons in the central nervous system. The cerebral cortex is the most recently evolved part of the nervous system, and more highly developed in humans than in any other animal. Fish have no cerebral cortex, and reptiles and birds have only a primitive cortex. In lower mammals, such as rats, the cerebral cortex is small and relatively smooth. But an intricate pattern of folds, hills, and valleys, called convolutions, enable the cerebral hemispheres to fit inside our relatively small heads. In each person. PSYCHOLOGY 19 Cont. PSYCHOLOGY 20 THE ENDOCRINE SYSTEM The endocrine system is made up of endocrine glands that produce hormones, chemical substances released into the bloodstream to guide such processes as metabolism, growth, and sexual development. Hormones are also involved in regulating emotional life. -The Thyroid Gland The thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability when the thyroid is overactive, and cause drowsiness and a sluggish metabolism when the thyroid is under-active. PSYCHOLOGY 21 Cont. - The Pineal Gland The pineal gland is a pea-sized gland and is located near the center of the brain. It produces melatonin, a hormone that affects the modulation of wake/ sleep patterns- The Pancreas The pancreas lies in a curve between the stomach and the small intestine and controls the level of sugar in the blood by secreting insulin and glucagon. -The Pituitary Gland It produces the largest number of different hormones and therefore has the widest range of effects on the body's functions. The posterior pituitary is controlled by the nervous system. It produces two hormones: vasopressin, which causes blood pressure to rise and regulates the amount of water in the body's cells, and oxytocin, which causes the lactation to begin after childbirth. The anterior pituitary, often called the "master gland," responds to chemical messages from the bloodstream to produce numerous hormones that trigger the action of other endocrine glands. PSYCHOLOGY 22 Cont. -The Adrenal Glands In mammals, are the triangle-shaped endocrine glands that sit on top of the kidneys. They are chiefly responsible for regulating the stress response PSYCHOLOGY 23 The glands of the endocrine system. PSYCHOLOGY 24