Chapter 12: Nervous TissueOverview of the Nervous SystemEndocrine and Nervous systems maintain internal coordinationEndocrine System: Communicates by means of chemical messengers (hormones) secreted into the bloodNervous System: Employs electrical and chemical means to send messages from cell to cellNervous System carries out its tasks in 3 basic steps: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 brainEx: The sound of a dog barking2. Processes sensory information in the brain and spinal cord, relates it to past experiences, and determines what response is appropriate to the circumstancesEx: Deciding how close the dog barking is and whether it is a threat to you3. 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 responseEx: Ignoring the dog barking, running away, etc. (reaction)Subdivisions of the Nervous SystemCentral Nervous System (CNS): Brain and spinal cordEnclosed by cranium and vertebral columnPeripheral Nervous System (PNS): Nerves and gangliaNerve: A bundle of nerve fibers (axons) wrapped in fibrous connective tissueGanglion: A knot-like swelling in a nerve where neuron cell bodies are concentratedDivided into two divisions—Sensory division and motor divisionSensory (Afferent) Division: Carries sensory signal from various receptors to the CNS and informs the CNS of stimuli within or around the bodyVisceral Sensory Division: Carries signals from the viscera of the thoracic and abdominal cavitiesInternal organs (i.e. heart, lungs, stomach, and urinary bladder)Somatic Sensory Division: Carries signals from receptors in the skin, muscles, bones and jointsEverywhere else (anything that you can feel without involvement of internal organs)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 responseEffectors: Cells and organs that respond to commands from the CNSVisceral Motor Division (Autonomic Nervous System/ANS): Carries signals to glands, cardiac muscle, and smooth muscleInvoluntary muscle contractionVisceral Reflexes: Responses of ANS and its receptorsTwo division:1. Sympathetic Division: Arousing the body for actionEx: FrightIncrease in heart rate, increase in respiration, inhibition of digestive and urinary systems2. Parasympathetic Division: Calming the body and bringing it back to normalSlows heart rate and breathingStimulates digestive and urinary systemSomatic Motor Division: Carries signals to skeletal musclesOutput produces both voluntary and involuntary muscular contractionSomatic Reflexes: Involuntary muscle contractionsNervous TissueTwo cell types in nervous tissue:1. Neurons: Excitable2. Glial Cells: Support cells for nervous system—Provide structural, immune, protective supportUniversal Properties of Nerves/Neurons:1. Excitability (Irritability): Ability to respond to stimuli (environmental changes)2. Conductivity—Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations3. Secretion—When electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cellClasses of Neurons1. Sensory (Afferent) Neurons: Specialized to detect stimuliTransmit information to the CNSBegin in almost every organ of the body and end in CNSAfferent: Conducting signals toward CNS2. Interneurons (Association) Neurons: Lie entirely within the CNSReceive signals from many neurons and carry out the integrative functionIntegrative Function: Process, store and retrieve information and make decisions that determine how the body will respond to stimuli90% of all neurons are interneuronsLie between and interconnect incoming sensory pathways/outgoing motor pathways of CNS3. Motor (Efferent) Neurons: Send information/command out to effectors (i.e. muscles and glands)Motor because most of them lead to musclesEfferent neurons: Conduct signals away from CNSAnatomy of a NeuronCell body (Neurosoma/Perikaryon): Control center of the neuronHas a single, centrally located nucleus with a large nucleolusCytoplasm contains:MitochondriaLysosomesGolgi complexNumerous inclusions—glycogen granules, lipid droplets, melanin, and lipofuscin (golden brown pigment produced when lysosomes digest worn-out organelles)Lipofuscin accumulates with ageWear-and-tear granulesMost abundant in old neuronsExtensive rough endoplasmic reticulumCytoskeleton: 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)No centrioles—no further cell divisionsDendrite(s): Vast number of branches coming from a few thick branches from the somaResemble bare tree branchesPrimary site for receiving signals from other neuronsThe more dendrites a neuron has, the more information it can receive and incorporate into decision makingProvide precise pathway for the reception and processing of neural informationAxon: Originates from a mound on one side of the soma called the axon hillockAxon Hillock: ONLY location of initiation of action potential in neuronsCylindrical, relatively unbranched for most of its lengthAxon Collaterals: Branches of axonBranch extensively on distal endWhere information is sent from neuronSpecialized for rapid conduction of nerve signals to points remote to the somaAxolemma: Plasma membrane surrounding axonAxoplasm: Cytoplasm of axonOnly one axon per neuronSchwann cells and myelin sheath enclose axonTerminal arborization: Located on distal end of axon, extensive complex of fine branchesSynaptic Knob (Terminal Button): Little swelling that forms a junction (synapse) with the next cellContains synaptic vesicles full of neurotransmitterClasses of NeuronsMultipolar Neuron: One axon and many dendrites that come immediately off of somaMost common type of neuronEx: Most neurons in brain and spinal cord (CNS)Bipolar Neuron: One axon and one dendriteDendrite leaves the soma as one extension, then may branch outEx: Olfactory cells, retina, inner earUnipolar Neuron: One extension off of soma which then branches into axon and dendritesEx: Sensory nerves from skin and organs to spinal cordAnaxonic Neuron: No axon, only