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Nervous System – General Nervous System Structure cells, tissues and organs of body are all working for organisms survival every cell in our body responds to stimuli by changing its metabolism in one way or another need to integrate all body activities for homeostasis but cells of the nervous system are highly specialized for receiving stimuli and conducting impulses to various parts of the body need good communication and control: Nervous System Endocrine System in humans, these nerve cells have become organized into the most complex and least understood of the body’s systems Neuroendocrine System receptor ! integration ! effector CNS: General Functions of the Nervous System 1. receive and process sensory information from internal and external environment PNS: cranial nerves spinal nerves two major cell/tissue types in Nervous System: 2. maintain homeostasis by transmitting the appropriate responses through muscles and glands neurons – impulse conduction communicates by: electrochemical impulses (=nerve impulses) cell-to-cell chemicals (=neurotransmitters) 3. Integrate rapid reflex responses with slower hormonal responses ~1 Trillion neurons generally no mitosis 4. generate complex neural pathways of all higher brain functions: neuroglia (=glial cells) – support, protection, insulation, aid in function of neurons self awareness thinking, learning speech, communication emotions Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 brain spinal cord [need specialized cells because of unique sensitivity of neurons to their environment] 1 10-50 times more neuroglia than neurons some mitosis Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 2 two types; axons and dendrites Dendrites Neurons shorter; branching highly specialized to: receptor regions respond to stimuli conduct messages in the form of nerve impulses ! each neuron receives info from dozens to 10’s of 1000’s of other neurons generally don’t divide after birth specialized for information collection !live up to 100 years (eg. dendritic spines) thinly insulated very high metabolic rate: convey messages toward cell body require glucose, can’t use alternate fuels = graded potentials (not nerve impulses) require lots of O2 – only aerobic metabolism can’t survive more than a few minutes without O2 large surface area for reception of signals from other neurons all neurons have cell body and 1 or more processes contain all organelles (except nucleus) as in cell body Axons cell body: each neuron has a single axon contains: most cytoplasm nucleus most organelles no centrioles (don’t divide) neurofibrils long, slender process up to 3-4 feet long (eg. motor neuron of toe) processes: Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 3 Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 4 axons may give off a few collateral branches (=axon collaterals) sensory neurons in CNS are multipolar b. interneurons (association) sometimes with branching axon collaterals in CNS where integration occurs 99% of neurons in body lots of variation in structure thick insulation at terminus, axon branches profusely c. motor neurons (efferent) (up to 10,000 branches) outside CNS multipolar all cell bodies of somatic and some autonomic are inside CNS each branch ends in enlarged bulb = synaptic knob (=axonal terminal) 2. Structure has all organelles except rough ER ! gets proteins via microtubules and microfilaments a. unipolar (=pseudounipolar) difference between nerve and neuron: single short process that splits into two longer processes that together act as an axon neuron = individual nerve cell originate as bipolar neurons nerve = bundle of axons outside CNS surrounded by layers of connective tissue functionally one long fiber carries impulse that bypasses cell body the dendrites are considered the receptive branches at the beginning of the single process neurons can be classified by: 1. function 2. structure (# of processes) especially in ganglia of PNS most are sensory neurons 1. Function b. bipolar a. sensory neurons (afferent) 2 processes; 1 axon, 1 dendrite only in some sense organs outside CNS, almost all are unipolar, a few are bipolar Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 5 Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 5. Schwann cells eg. olfactory cells in nose, some retinal neurons, sensory neurons of inner ear 1. Astrocytes c. multipolar have numerous branches producing a starlike shape !3 processes; 1 axon, many dendrites most common largest and most abundant type most neurons in CNS (interneurons) ! comprise >90% of the tissue in some parts of the brain also some sensory and some motor neurons in PNS astrocytes cover the entire brain surface and most of the nonsynaptic regions of the neurons in the gray matter of CNS d. anaxonic neurons have multiple dendrites but no axons do not produce action potentials also most functionally diverse type found in brain and retina form supportive framework for nervous tissue in very general terms, shape is related to function: sensory neurons interneurons motor neurons 6 direct the formation of tight webs of cells around brain’s capillaries many unipolar =blood/brain barrier & bipolar because of “irritability” of nervous tissue and sensitivity to 02, glucose etc neurons are isolated into their own “fluid compartment” mostly multipolar Neuroglia 1. astrocytes 2. microglia 3. ependymal cells 4. oligodendrocytes Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 this blockage of free exchange between capillaries and tissues is unique for nervous tissue 7 Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 8 ! prevents sudden and extreme fluctuations in composition of tissue fluid in CNS ciliated cells ! resemble cuboidal epithelium line ventricles and spinal canal ! protects irreplaceable neurons from damage help to produce and circulate CerebroSpinal Fluid capillaries in brain are much less leaky than normal capillaries 4. Oligodendrocytes (CNS) smaller cells, fewer (up to 15) processes ! tight junctions: materials must pass through cells clustered around nerve cell bodies astrocytes form an additional layer around these capillaries to further restrict exchange each process reaches out to nerve fiber and wraps around it to produce myelin sheath (electrical insulation) around neurons in CNS [myelin=fatty substance] ! astrocytes help regulate flow into CSF myelin (in CNS and PNS) can be: small molecules (O2 , CO2 , alcohol) diffuse rapidly thick = “myelinated fibers”, “white matter” thin = “unmyelinated fibers”, “gray matter” larger molecules penetrate slowly or not at all 2. Microglia (CNS) small macrophage cells in inflamed or degenerating brain tissue they carry out phagocytosis of microbes and cellular debris 3. Ependymal Cells (CNS) Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 (oligodendroglia) 9 5. Schwann Cells (PNS) Multiple Sclerosis autoimmune disease possibly triggered by a virus in genetically susceptible individuals oligodendrocytes and myelin sheaths of CNS deteriorate and are replaced by hardened scar tissue occur esp between 20-40 yrs of age nerve fibers are severed & myelin sheaths in CNS are gradually destroyed ! short circuits; loss of impulse conduction affects mostly young adults common symptoms:visual problems, muscle weakness, clumsiness eventual paralysis Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 10 Synapses found only in PNS meeting point between a neuron and any other cells = synapse form a segmental wrapping around nerve fibers each segment is produced by 1 Schwann cell gaps between cells = Nodes of Ranvier neurons generally are not directly connected to each other but are separated by a small gap form neurilemma and myelin sheath in PNS neurons synapses are the functional connection between neurons and a few other cells (eg. muscles, glands) outermost coil of Schwann cell with most of cytoplasm & organelles forms neurilemma CNS: neuron ! neuron PNS: ! only in PNS neurons sensory cell!neuron neuron ! neuron neuron ! muscle fiber [=neuromuscular jct] neuron ! gland [=neuroglandular jct] neuron ! epithelial cells ! plays essential role in regeneration of cut or injured neurons [CNS neurons don’t regenerate] each neuron synapses with 1000 – 10,000 axonal terminals ! ~1 quadrillion synapses in human brain at birth brain has ~50 trillion synapses ! 1 month later has 1 quadrillion synapses at synapse the electrical signal is converted to a chemical signal that must diffuse across the synapse to have an effect on the next neuron synapses make neural integration possible Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 11 Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 12 ! each synapse is a “decision making” device that determines whether the second cell will respond to the signal from the first General Function of a Synapse: 1. nerve impulse reaches end of axon at synapse and triggers release of chemical (=neurotransmitter) ! exocytosis 2. NT diffuses across synapse and binds to receptor proteins in cell membrane of target cell 3. triggers response in target cell whole process takes 0.3 – 5.0 ms Human Anatomy & Physiology: Nervous System – General Ziser Lecture Notes 2010.4 13