11 06 2014 Chapter 12 Nervous Tissue Overview of the Nervous System Endocrine and Nervous systems maintain internal coordination o Endocrine System Communicates by means of chemical messengers hormones secreted into the blood o Nervous System Employs electrical and chemical means to send messages from cell to cell Nervous System carries out its tasks in 3 basic steps o 1 Receives sensory information about changes in the body and the external environment via sensory receptors and sends coded messages up spinal cord to the brain Ex The sound of a dog barking o 2 Processes sensory information in the brain and spinal cord relates it to past experiences and determines what response is appropriate to the circumstances Ex Deciding how close the dog barking is and whether it is a threat to you o 3 Receives motor commands from brain and sends it down spinal cord to be sent out throughout the body i e muscles and glands and carry out a response Ex Ignoring the dog barking running away etc reaction Subdivisions of the Nervous System Central Nervous System CNS Brain and spinal cord o Enclosed by cranium and vertebral column Peripheral Nervous System PNS Nerves and ganglia o Nerve A bundle of nerve fibers axons wrapped in fibrous connective tissue o Ganglion A knot like swelling in a nerve where neuron cell bodies are concentrated o Divided into two divisions Sensory division and motor division Sensory Afferent Division Carries sensory signal from various receptors to the CNS and informs the CNS of stimuli within or around the body Visceral Sensory Division Carries signals from the viscera of the thoracic and abdominal cavities Internal organs i e heart lungs stomach and urinary bladder Somatic Sensory Division Carries signals from receptors in the skin muscles bones and joints Everywhere else anything that you can feel without involvement of internal organs o Ex Feeling texture softness roughness pain temperature etc Motor Efferent Division Carries signals from the CNS to gland and muscle cells that carry out the body s response Effectors Cells and organs that respond to commands from the CNS Visceral Motor Division Autonomic Nervous System ANS Carries signals to glands cardiac muscle and smooth muscle Involuntary muscle contraction Visceral Reflexes Responses of ANS and its receptors Two division o 1 Sympathetic Division Arousing the body for action Ex Fright Increase in heart rate increase in respiration inhibition of digestive and urinary systems o 2 Parasympathetic Division Calming the body and bringing it back to normal Slows heart rate and breathing Stimulates digestive and urinary system Somatic Motor Division Carries signals to skeletal muscles Output produces both voluntary and involuntary muscular contraction Somatic Reflexes Involuntary muscle contractions Nervous Tissue Two cell types in nervous tissue o 1 Neurons Excitable o 2 Glial Cells Support cells for nervous system Provide structural immune protective support Universal Properties of Nerves Neurons o 1 Excitability Irritability Ability to respond to stimuli environmental changes o 2 Conductivity Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations o 3 Secretion When electrical signal reaches end of nerve fiber a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell Classes of Neurons 1 Sensory Afferent Neurons Specialized to detect stimuli o Transmit information to the CNS o Begin in almost every organ of the body and end in CNS o Afferent Conducting signals toward CNS 2 Interneurons Association Neurons Lie entirely within the CNS o Receive signals from many neurons and carry out the integrative function Integrative Function Process store and retrieve information and make decisions that determine how the body will respond to stimuli o 90 of all neurons are interneurons o Lie between and interconnect incoming sensory pathways outgoing motor pathways of CNS 3 Motor Efferent Neurons Send information command out to effectors i e muscles and glands o Motor because most of them lead to muscles o Efferent neurons Conduct signals away from CNS Anatomy of a Neuron Cell body Neurosoma Perikaryon Control center of the neuron o Has a single centrally located nucleus with a large nucleolus o Cytoplasm contains Mitochondria Lysosomes Golgi complex Numerous inclusions glycogen granules lipid droplets melanin and lipofuscin golden brown pigment produced when lysosomes digest worn out organelles Lipofuscin accumulates with age Wear and tear granules Most abundant in old neurons Extensive rough endoplasmic reticulum Cytoskeleton Consists of dense mesh of neurofibrils bundles of actin filaments Compartmentalizes rough ER into dark staining Nissl bodies contain rough ER and free ribosomes Neurofilaments structural support think skeleton o No centrioles no further cell divisions Dendrite s Vast number of branches coming from a few thick branches from the soma o Resemble bare tree branches o Primary site for receiving signals from other neurons o The more dendrites a neuron has the more information it can receive and incorporate into decision making o Provide precise pathway for the reception and processing of neural information Axon Originates from a mound on one side of the soma called the axon hillock o Axon Hillock ONLY location of initiation of action potential in neurons o Cylindrical relatively unbranched for most of its length Axon Collaterals Branches of axon Branch extensively on distal end o Where information is sent from neuron Specialized for rapid conduction of nerve signals to points remote to the soma o Axolemma Plasma membrane surrounding axon o Axoplasm Cytoplasm of axon o Only one axon per neuron o Schwann cells and myelin sheath enclose axon o Terminal arborization Located on distal end of axon extensive complex of fine branches Synaptic Knob Terminal Button Little swelling that forms a junction synapse with the next cell Contains synaptic vesicles full of neurotransmitter Classes of Neurons Multipolar Neuron One axon and many dendrites that come immediately off of soma o Most common type of neuron o Ex Most neurons in brain and spinal cord CNS Bipolar Neuron One axon and one dendrite o Dendrite leaves the soma as one extension then may branch out o Ex Olfactory cells retina inner ear Unipolar Neuron One extension off of soma which then branches into axon and dendrites o Ex Sensory nerves from skin and organs to spinal