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Cells of the Nervous System Two Major Cell/Tissue Types in Nervous System

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Cells of the Nervous System Two Major Cell/Tissue Types in Nervous System Cells of the Nervous System two major cell/tissue types in Nervous System: neurons – impulse conduction 100 Billion generally no mitosis neuroglia – support, protection, insulation, etc [need specialized cells because of unique sensitivity of neurons to their environment] 900 Billion some mitosis Neuroglia 1. astrocytes 2. oligodendroglia 3. microglia 4. ependymal cells 5. Schwann cells 1. Astrocytes largest and most abundant type form tight webs around brains capillaries =blood/brain barrier small molecules (O2, CO2, alcohol) diffuse rapidly larger molecules penetrate slowly or not at all this blockage of free exchange between capillaries and tissues is unique for nervous tissue => prevents sudden and extreme fluctuations in composition of tissue fluid in CNS => protects irreplaceable neurons from damage 2. Oligodendrocytes (oligodendroglia) smaller cells, fewer processes clustered around nerve cell bodies help hold nerve fibers together produce myelin sheath (electrical insulation) around neurons in CNS [myelin=fatty substance] 3. Microglia small stationary cells in inflamed or degenerating brain tissue they: enlarge move about 1 carry out phagocytosis of microbes and cellular debris 4. Ependymal Cells ciliated cells line ventricles and spinal canal help to circulate CerebroSpinal Fluid 5. Schwann Cells found only in PNS form a segmental wrapping around nerve fibers each segment is produced by 1 Schwann cell gaps between cells = Nodes of Ranvier form neurilemma and myelin sheath in PNS neurons myelin (in CNS and PNS) can be: thick = “myelinated fibers”, “white matter” thin = “unmyelinated fibers”, “gray matter” outermost coil of Schwann cell with most of cytoplasm & organelles forms neurilemma only in PNS neurons plays essential role in regeneration of cut or injured neurons [CNS neurons don’t regenerate] Neurons highly specialized to respond to stimuli conduct messages in the form of nerve impulses generally don’t divide after birth live up to 100 years very high metabolic rate require glucose, can’t use alternate fuels require lots of O2 – only aerobic metabolism can’t survive more than a few minutes without O2 all have cell body and 1 or more processes cell body: contains most cytoplasm nucleus most organelles 2 no centrioles (don’t divide) neurofibrils in CNS neuron cell bodies are clustered together = nuclei in PNS neuron cell bodies are clustered together = ganglia processes: two types; axons and dendrites Dendrites shorter branching receptor regions contain all organelles (except nucleus) as in cell body large surface area for reception of signals from other neurons specialized for information collection (eg. dendritic spines) convey messages toward cell body = graded potentials (not nerve impulses) Axons each neuron has a single axon long, slender process up to 3-4 feet long (eg. motor neuron of toe) axons may give off a few collateral branches (=axon collaterals) at terminus, axon branches profusely (up to 10,000 branches) each branch ends in enlarged bulb = synaptic knob (=axonal terminal) has all organelles except rough ER gets proteins via microtubules and microfilaments axons quickly decay if cut or damaged axons covered with myelin insulation axons in PNS also have neurilemma tracts = bundles of axons in CNS nerves = bundles of axons in PNS 3 neurons can be classified by: 1. structure (# of processes) 2. function 1. structure a. multipolar ≥3 processes; 1 axon, many dendrites most common most neurons in CNS (interneurons) also some sensory and some motor neurons in PNS b. bipolar 2 processes; 1 axon, 1 dendrite only in some sense organs act as receptor cells in eye and nose c. unipolar (=pseudounipolar) single short process that splits into two longer processes that together act as an axon originate as bipolar neurons especially in ganglia of PNS most are sensory neurons 2. Function a. sensory (afferent) outside CNS, almost all are unipolar, a few are bipolar sensory neurons in CNS are multipolar b. motor (efferent) multipolar all cell bodies of somatic and some autonomic are inside CNS c. interneurons (association) in CNS where integration occurs 99% of neurons in body lots of variation in structure 4.
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