Unit 3 The Neurons PDF

Unit 3 The neurons Likitha S (Department of Psychology) The Neurons: The Basis of Information Processing There are approximately 100 billion neurons in the human brain. Neurons are larger than glial cells however and make up about 50 per ce...

Unit 3 The neurons Likitha S (Department of Psychology) The Neurons: The Basis of Information Processing There are approximately 100 billion neurons in the human brain. Neurons are larger than glial cells however and make up about 50 per cent of the volume of the brain Neurons only make up approximately 10 per cent of the cells in the brain. The rest are known as glial cells, and these provide a supporting role for the neurons themselves. The idea that the neuron is the unit of brain was suggested by the Spanish Nobel prize winner Santiago Ramóny Cajal from work carried out between 1887 and 1903. Likitha S (Department of Psychology) The Neurons: The Basis of Information Processing As the information-processing units of the nervous system, neurons acquire information, store it as memory, interpret it, and pass the information along to other neurons to produce behavior. In doing so, they regulate body processes such as breathing, heartbeat, and body temperature, to which we seldom give a thought. Scientists think that neurons work together in groups of many hundreds to many thousands to produce most behavior. It is important, then, to understand not only how neurons function but also how they interconnect and influence one another. Likitha S (Department of Psychology)  Functional groups of neurons, or neural networks, connect wide areas of the brain and spinal cord. The loss of a neuron or two from a network is no more noticeable than the loss of one or two voices from a cheering crowd. It is the crowd that produces the overall sound, not each person. An ongoing effort aims to map the structural connectivity—the physical wiring, or connectome—of the entire human brain. Each neuron’s appearance is distinctive, but neurons are also the essence of plasticity. Likitha S (Department of Psychology)  If one views living brain tissue through a microscope, the neurons reveal themselves to be surprisingly active, producing new branches, losing old ones, and making and losing connections with each other as you watch. This dynamic activity underlies both the constancies and the changes in our behavior. Likitha S (Department of Psychology)  Structure and Function of the Neuron A neuron contains a nucleus, a membrane, mitochondria, ribosomes, and the other structures typical of animal cells. The distinctive feature of neurons is their shape. The larger neurons have these major components: dendrites, a soma (cell body), an axon, and presynaptic terminals. Likitha S (Department of Psychology)  The tiniest neurons lack axons and some lack well-defined dendrites. A motor neuron has its soma in the spinal cord. It receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle. A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation, such as touch information from the skin. Different kinds of sensory neurons have different structures Likitha S (Department of Psychology)  A neuron conducting touch information from the skin to the spinal cord. Tiny branches lead directly from the receptors into the axon, and the cell’s soma is located on a little stalk off the main trunk. Likitha S (Department of Psychology)  Dendrites are branching fibers that get narrower near their ends. (The term dendrite comes from a Greek root word meaning “tree”; a dendrite is shaped like a tree.) The dendrite’s surface is lined with specialized synaptic receptors, at which the dendrite receives information from other neurons A neuron may have up to 20 dendrites, each dendrite may have one to many branches, and the spines on the branches may number in the thousands. Dendrites collect information from other cells, and the spines are the points of contact with those neurons. Likitha S (Department of Psychology)  The greater the surface area of a dendrite, the more information it can receive. Some dendrites branch widely and therefore have a large surface area. Some also contain dendritic spines, the short outgrowths that increase the surface area available for synapses. The shape of dendrites varies enormously from one neuron to another and can even vary from one time to another for a given neuron. The shape of the dendrite has much to do with how the dendrite combines different kinds of input Likitha S (Department of Psychology)  The cell body, or soma (Greek for “body”; pl.: somata), contains the nucleus, ribosomes, mitochondria, and other structures found in most cells. Much of the metabolic work of the neuron occurs here. Cell bodies of neurons range in diameter from 0.005 mm to 0.1 mm in mammals and up to a full millimeter in certain invertebrates. Like the dendrites, the cell body is covered with synapses on its surface in many neurons Likitha S (Department of Psychology)  The axon is a thin fiber of constant diameter, in most cases longer than the dendrites. (The term axon comes from a Greek word meaning “axis.”) The axon is the information sender of the neuron, conveying an impulse toward either other neurons or a gland or muscle. Many vertebrate axons are covered with an insulating material called a myelin sheath with interruptions known as nodes of Ranvier. Invertebrate axons do not have myelin sheaths. An axon has many branches, each of which swells at its tip, forming a presynaptic terminal, also known as an end bulb or bouton (French for “button”). This is the point from which the axon releases chemicals that cross through the junction between one neuron and the next. Likitha S (Department of Psychology)  The extent of a cell’s branches and its spine number correspond to its information-processing capacity. Each neuron has but a single axon to carry messages to other neurons. The axon begins at one end of the cell body, at an expansion known as the axon hillock (little hill) The axon may branch out into one or many axon collaterals, which usually emerge from it at right angle Likitha S (Department of Psychology)  A neuron can have any number of dendrites, but no more than one axon, which may have branches. Axons can range to a meter or more in length, as in the case of axons from your spinal cord to your feet. In most cases, branches of the axon depart from its trunk far from the cell body, near the terminals. Other terms associated with neurons are afferent, efferent, and intrinsic. An afferent axon brings information into a structure; an efferent axon carries information away from a structure. Likitha S (Department of Psychology)  Every sensory neuron is an afferent to the rest of the nervous system; every motor neuron is an efferent from the nervous system. Within the nervous system, a given neuron is an efferent from the standpoint of one structure and an afferent from the standpoint of another. (You can remember that efferent starts with e as in exit; afferent starts with a as in admission.) For example, an axon that is efferent from the thalamus may be afferent to the cerebral cortex Likitha S (Department of Psychology)  If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an interneuron or intrinsic neuron of that structure. For example, an intrinsic neuron of the thalamus has all its dendrites or axons within the thalamus; it communicates only with other cells of the thalamus. Likitha S (Department of Psychology) A Schwann cell forms a myelin sheath by wrapping its plasma membrane concentrically around the inner axon  These are the gaps formed between the myelin sheath where the axons are left uncovered. Because the myelin sheath is largely composed of an insulating fatty substance, the nodes of Ranvier allow the generation of a fast electrical impulse along the axon Recap: Dendrites: The dendrites are points on a neuron where information from other neurons are received. Axon : carries messages received by the dendrites to other neurons. Terminal buttons: the part of the axon, like a small bulge at the end, that sends messages to other neurons Myelin sheath: a protective coating of fat and protein that wraps around the axon. The myelin sheath also serves to increase the velocity with which electrical impulses travel through axons. Those axons that carry the most important and urgent information have the greatest concentration of myelin. Thank you Likitha S (Department of Psychology) [email protected]