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Chapter 2

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Part I: Biological Foundations of Behaviour Chapter 2: Functional Neuroanatomy- The Nervous System and Behaviour The Nervous System is Composed of Cells -Neuron or nerve cells: the basic unit of the nervous system, each composed of a cell body, receptive extension(s) (dendrites), and a transmitting extension (axon) -they underlie the simplest and the most complex of our abilities and talents -each neuron receives inputs from many others, integrates those inputs, and then distributes the processed info to other neurons (100-150 billion of these tiny info processing units into an immense array of circuits -glial cells: also called glia or neuroglia. Nonneural brain cells that provide structural, nutritional and other types of support to the brain -Cajal proposed that although neurons come very close to one another (they are contigous), they are not quite continuous with one another. He insisted that at each point of contact between neurons a tiny gap keeps the cells separate. New Perspective: Neuron Doctrine 1.brain is composed of separate neurons and other cells that are independent structurally, metabolically and functionally 2. Information is transmitted from cell to cell across tiny gaps (synapse: the tiny gap between neurons where info is passed from one to the other (human 10^15 synapses)) The neuron has four structural divisions specialized for information processing -Mitochondrion: a cellular central structure of a cell that contains the chromosomes -Cell nucleus: the spherical central structure of a cell that contains the chromosomes -Ribosomes: structures in the cell body where genetic information is translated to produce proteins Functional Zones: 1. Cellular extension called dendrites (receptive surfaces of the neuron) serve as an input zone (receives info from other neurons), receiving info from other neurons. Dendrites may be elaborately branched to accommodate synapes from many other neurons 2. A cell body region (or soma) which contains the cell's nucleus, may receive additional synaptic contacts. In most types of neurons, inputs are combined and transformed in the cell body, so it acts as an integration zone (part of the neuron that initiates nerve electrical activity) 3. A single extension, the axon ( a single extension from the nerve cell that carries nerve impulses from the cell body to other neurons), leads away from the cell body and serves as a conduction zone (nerve's electrical signal may be actively propagated), transmitting the cell's output info in the form of electrical impulses away from the cell body 4. Specialized swellings at the ends of the axon, the axon terminal (synaptic boutons) are a functional output zone. They transmit the neurons activity to other cells at the synapses Neurons can be classified by shape, size or function 1. Multipolar neurons have many dendrites and a single axon, and they are the most common type of neuron 2. Bipolar neurons have a single dendrite at one end of the cell and a single axon at the other end. (sensory systems such as vision) 3. Unipolar neurons have a single extension (or process) usually thought of as an axon, that branches in two directions after leaving the cell body. One end if the input zone with branches like dendrites; the other the output zone. Such cells transmit touch info from the body into the spinal cord -in all 3, the dendrites are in the input zone. -multipolar and bipolar, the cell body also receives synapses (also in input) -many axons are only a few micrometers long but for neurons that connect the spinal cord to the rest of the body, axons may be more than a meter in length -motoneurons: aka motoneuron. A nerve cell that transmits motor messages stimulating a muscle or gland -sensory neuron: a neuron that is directly affected by changes in the environment, such as light, odour or touch; carries messages from the periphery back to the spinal cord and brain -interneuron: a neuron that is neither a sensory neuron nor a motoneuron; it receive input from and sends output to other neurons -make up most of the brain (complete functions of the brain -larger neurons tend to have more complex inputs and outputs, cover greater distances and convey info more rapidly than smaller neurons The neuronal cell body and dendrites receive info across synapses -arrangement of neuron's dendrites- its tree branch like arborization--reflects the complexity of the neuron's info processing function -at each synapse, info is transmitted from the axon terminal of the presynaptic (releases neurotransmitters) neuron to the receptive surface of the postsynaptic neuron (responds to neurotransmitters) Three Principle Components of Synapse 1. The presynaptic membrane (releases neurotransmitters) of the axon terminal of the presynaptic neuron 2. A specialized postsynaptic membrane on the surface of the dendrite or cell body of the postsynaptic neuron 3. A synaptic cleft, the gap of about 20-40 nanometers that separates the presynaptic and postsynaptic membranes. -presynaptic axon terminals contain numerous tiny spheres called synaptic vesicles (30-140 nm), -neurotransmitters: also called synaptic transmitter, chemical transmitters. The chemical released from the presynaptic axon terminal that serves as the basis of communication between neurons. -after crossing the cleft, the released neurotransmitter interacts with postsynaptic receptors: specialized protein molecules that capture and react to molecules of the neurotransmitter -this action results in electrical changes in the postsynaptic cell, if it's a neuron, this electrical event affects the likelihood that the postsynaptic neuron will release its own neurotransmitter. -neurons with elaborate dendrites tend to have more synaptic inputs -neural plasticity: the ability of the nervous system to change in response to experience or the environment The axon integrates and then transmits information -axon hillock: cone shaped area from which the axon originates out of the cell body. Functionally the integration zone of the neuron -it gathers and integrates info from all the synapses on the neuron's dendrites and soma, then converting the processed info into a code of electrical impulses that carries the neuron's message down the axon towards its target -rarely a neuron will have only one axon, but it divides into several branches called axon collaterals, allowing the neuron to influence (innervate: provide neural input) a number of postsynaptic cells -axon: rapid transmission of electrical signals along the outside of the axon, and the much slower transportation of substances inside the axon, to and from the axon terminals Glial cells support and enhance neural activity -glial cells serve many functions, including the breakdown of transmitters, the production of myelin sheaths around axons, the exchange of nutrients and other materials with neurons, the direct regulation of the interconnections and activity of neurons and the removal of cellular debris -glial cells communicate with each other and with neurons and they directly affect neural functioning by providing neurons with raw materials and chemical signals that alter neuronal structure and excitability. Glial cells come in four different types: 1.Astrocyte: a star shaped glial cell with numerous processes (extensions) that run in all directions - receive synapses directly from neurons and also surround and monitor the activity of nearby neuronal synapes. Also involved in the formation of new synapses 2. Microglial cells: microglia. Extremely small glial cells that remove cellular debris from injured or dead cells -key component of neural pain systems -important for maintenance of synapses; interference with this function associated with development of Alzheimer's and other dementias 3. Oligodendrocyte: a type of glial cells that forms myelin in the central nervous system 4. Schwann cell: the glial cell that forms myelin(fatty insulation around an axon, that improves the speed of conduction of nerve impulses) in the peripheral nervous system (myelination) -edema swelling of tissue, esp. in brain, can lead to Multiple Sclerosis The Nervous System Consists of Central and Peripheral Divisions -peripheral nervous system: the portion of the nervous system that includes all the nerves and neurons outside the brain and spinal cord -central nervous system: the portion of the nervous system that includes the brain and the spinal cord The peripheral nervous system has three components -consists of nerves--collection of axons bundled together -motor nerves transmit info from the spinal cord and brain to the mucles, organs, and glands. -sensory nerves arise from sensory surfaces and convey info from the body to the spinal cord and brain Cranial Nerves -connected directly to the brain -12 pairs of cranial nerves-- serve the sensory and motor systems of the head and neck -nerves pass through small openings in the skull, directly entering or leaving the brain without ever joining the spinal cord - olfactory (smell) -optic -oculomotor, trochlear, abducens (muscles that move the eyes) -trigeminal (face, sinuses, teeth; jaw muscles) -facial (tongue, soft palate; facial muscles, salivary glands, tear glands) -vestibulocochlear (inner ear: hearing and balance) -glossopharyngeal (tate and other mouth sensation; throat muscles) -vagus (info from internal organs; internal organs) -spinal accessory (neck muscles) -Hypoglossal (tongue muscles) Spinal Nerves -31 pairs of nerves emerge at regularly spaced intervals through opening in the backbone -one pair serves each side of the body -each spinal nerve consists of the fusion of two distinct branches called roots -dorsal root (back): the branch of a spinal nerve entering the dorsal horn of the spinal cord, that carries sensory info from the peripheral nervous system to the spinal cord -ventral root(front): the branch of a spinal nerve, arising from the ventral horn of the spinal cord, that carries motor messages from the spinal cord to the peripheral nervous system -cervical: top 8 segments of the spinal cord in the neck region -thoracic: 12 spinal segments below the cervical portion of the spinal cord, corresponding to the chest -lumbar: 5 spinal segments that make up the upper part of the lower back -sacral: 5 spinal segments make up lower part of the lower back -coccygeal: lower spinal vertebr
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