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Maintaining Homeostasis • The nervous system and the endocrine system share the responsiblities of maintaining the internal homeostasis of the human body. – – – – Control peripheral effectors Sense the internal and external environment Integrate sensory information Coordinates voluntary and involuntary activity Nervous System • Mass of only 2kg it makes up about 3% of total body weight • Smallest and most complex of 11 body systems • Composed of brain, spinal cord, peripheral nerves, cranial nerves, spinal nerves, ganglia, and sensory receptors Functions • Sensory function – Detection internal and external stimuli – PNS to CNS – sensory or afferent neuron af- toward • Integrative function – Processes sensory information received from afferent neurons by analyzing, storing and making decisions for appropriate responses – Interneurons • Motor function – Responds to integration decisions made by interneurons causing effector cells to respond – CNS to PNS – motor or efferent neuron ef- away Two Subdivisions CNS • Integration, processing and coordination of sensory data and motor commands • Higher functions • Consists of brain & spinal cord PNS • Brings sensory info to CNS • Brings motor commands to peripheral tissue • Includes all tissue outside CNS Cellular Organization of Neural Tissue Neurons • Comprised of three components – Soma or Cell Body • Contains a large nucleus with a prominent nucleolus • Most neurons cannot divide (amitotic) because they lack the centrioles that are essential for mitosis to occur. • Contain clusters of rough ER and free ribosomes called Nissle bodies, where protein synthesis occurs • Newly synthesized substances are conveyed in the axoplasm – Anterograde transport • Movement from cell body to axon terminals – Retrograde transport • Movement of materials from axon terminal back to the cell body – Dendrites • Highly branched process that joins the soma at the cone shaped axon hillock. • Receive information from other neurons – Axon • Long cytoplasmic process attached to soma at the axon hillock – Initial segment • First part of axon • Contains the trigger zone, area where impulses arise at the junction of the axon hillock and the initial segment – Axon terminals • End of axon • May have bulb-shaped structures called synaptic end bulbs containing vesicles to store neurotransmitters for communication between the neuron and another neuron or effector cell • Along with synaptic terminals can conduct an action potential and propagate it toward another neuron, a muscle fiber, or a gland cell Structural Diversity in Neurons Bipolar •Have one main dendrite and one axon •Found in the retina of the eye, inner ear, olfactory area of the brain Unipolar •Sensory neurons that begin in the embryo as bipolar neurons •During development, the axon and dendrite fuse into a single process that divides into two branches •Axon branch that extends into the CNS ends in synaptic end bulbs •Axon branch that extends into the periphery has dendrites at its distal end Multipolar neurons Anaxonic neurons Rare, small, lack features •Usually have several dendrites and one axon •Most abundant in brain and spinal cord Neuroglia of CNS • Makes up about half the volume of the CNS • Do not generate or propagate action potentials therefore they do not function with memory • Can readily multiply and divide • Support, nourish, and protect neurons and maintain homeostasis Astrocytes – – – Largest and most numberous of glial cells in CNS Star-shaped with many process Form three-dimensional framework of CNS, guide neuron development, maintain blood-brain barrier, respond to neural tissue damage Oligodendrocyte – – Smaller than astrocytes, with fewer processes Form supporting network around CNS neurons by producing myelin sheath covering many CNS axons Microglia – – Small wandering cells with few processes that have many small branches Protect CNS cells from disease by engulfing invading microbes and debris of dead cells Ependymal cells – – – Epithelial cells arranged in a single layer; either cuboidal or columnar; many cilliated Line ventricles of the brain and central canal of spinal cord Form cerebrospinal fluid and assist in its circulation Peripheral Nervous System Neuroglia Schwann cells – Flattened cells that encircle PNS axons – Produces part of the myelin sheath or neurilemma of PNS axons – Participate in regeneration of PNS axon Satellite cells – Flattened cells arranged around the cell bodies of neurons in ganglia – Support neurons within PNS ganglia Myelination • Most mammalian neurons are surrounded by a multilayered lipid and protein covering or myelin sheath – Electrically insulates the axon of a neuron – Increases the speed of nerve impulse conduction – Amount of myelin increases from birth to maturity. White & Gray Matter • White matter – Consists primarily of the myelinated axons of many neurons – Surrounds the gray matter in the spinal cord • Gray Matter – Contains primarily neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia – Thin shell of gray matter covers the surface of the cerebrum and cerebellum SensoryReceptors Exteroceptor Provides information about the external environment in the form of touch, temperature, and pressure sensations Proprioceptor Monitors the position and movement of skeletal muscles and joints Interoceptor Monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems and provide sensations of taste, deep pressure, and pain Neurons communicate with other cells • Neurons are electrically excitable cells that communicate with one another – Graded potentials are used for short-distance communication – Action potentials allow communication over both short and long distances within the body Steps for Generation of an Action Potential • Area of an excitable membrane brought to threshold by graded depolarization • Sodium channel activation occurs • Sodium ions enter the cell and depolarization occurs • Sodium channels are inactivated • Voltage-gated potassium channels open and potassium moves out of the cell, initiating repolarization • Sodium channels regain their normal properties • Temporary hyperpolarization occurs Action Potential consists of Depolarizing and Repolarizing Phase Ion Channels • Graded potentials and action potentials occur because the plasma membranes of neurons contain many different kinds of ion channels that open or close in response to specific stimuli – Leakage channels • More potassium ion leakage channels than sodium ion leakage channels are present • membrane has high permeability to potassium than sodium – Voltage-gated channel • Opens in a response to membrane potential or voltage • Participate in action potentials – Ligand-gated channel • Opens and closes in response to a specific chemical stimulus • Chemical ligands include- neurotransmitters, hormones, and particular ions – Mechanically-gated channel • Opens and closes in response to mechanical stimulation in the form of vibration such as sound waves, pressure, or tissue stretching The Synapse • Special junction between neurons or between a neuron and an effector. – Presynaptic neuron • Neuron sending the signal – Postsynaptic neuron • Neuron receiving the message • Dependent upon – Quantity, nature, and frequency of neurotransmitter – Characteristics of the receptor • Site of intercellular communication between neurons Chemical Synapse vs. Electrical Synapse • Space between two cells (synaptic cleft) • Presynaptic nerve releases a neurotransmitter that diffuses through the synaptic cleft and binds to receptors in plasma membrane of postsynaptic neuron • Most common • Impulse conducted directly through gap junction of adjacent cells • Direct signal transduction is spread from cell to cell • Provide faster communication and synchronization of activity for a large number of neurons or muscle fibers • Rare in CNS Damage and Repair • PNS neurons have a greater capacity for repair and regeneration than CNS neurons – Axons and dendrites of PNS neurons that are associated with a neurolemma may undergo repair if the cell body remains intact, if the schwann cells are functions, and if scar tissue formation does not occur too rapidly – Axons in the CNS are myelinated by oligodendrocytes that do not form neurolemmas; hence regeneration can not occur Neural Regeneration • Very limited • In PNS: Wallerian Degeneration Normal function may or may not be reestablished