I. Introduction A. The endocrine and nervous systems can both do long distance signaling.!1. Endocrine: Slow. Signal molecules are hormones carried in blood and target cells can be anywhere!2. Nervous system: Fast. Signal molecs are neurotransmitters released at synpases where the signaling cell and target must be in close proximity (axons up to 1 m long though so they can send a signal pretty far)!B. Nervous system!1. Creates, analyzes, identifies, and integrates info --> maintains homeostasis and behavior patterns!2. Is an integrated communication network made by billions of neurons + supporting cells!- info transmitted as electrical impulses that move along circuits of neurons!- info usually travels in one direction --> separate circuits for sensory and motor signaling!- several neurons used to send info!3. Neural pathways!- Sensory input: sensory receptors monitor changes (stimuli), gather info --> create a sensory input!- Sensory stimuli collected from receptors --> sensory (afferent) neuron conducts impulse to CNS --> stimuli are integrated w/other info in the brain/spinal cord --> motor (efferent) neurons conduct nerve impulse to an effector --> effector (muscles, glands) responds (i.e. secretion, contraction)!- Integration: SI is processed and interpreted, decide what to do about it - Motor output: activate effector organs (muscles, glands). The response.!C. Anatomical divisions of the nervous system!1. CNS (Central Nervous System)!- brain and spinal cord!- incl. portions of sensory and motor neurons + all of interneurons!- receives and integrates sensory signals to initiate a response!2. PNS (peripheral nervous system)!- peripheral nerves (everything else)!- incl. sensory and autonomic ganglia (groups of nerve cell bodies)!- sense stimuli and carry out response!- incl. sensory and motor neurons!D. Functional divisions of the nervous system!1. Somatic: voluntary motor innerv., effectors= skeletal m. 2. Autonomic: involuntary motor innerv., effectors= smooth m., glands, Purkinje fibers - Sympathetic: fight or flight - Parasympathetic: rest and digest!- Enteric: autonomic nerve plexuses in gut which can function indep. from the CNS!!II. Neurons are designed to receive signals and conduct electrical impulses. !A. > 10 billion neurons in the human nervous system!1. Excitable/irritable cells, rapid change in membrane potential due to stimuli!B. Components of neurons!1. Cell body (soma, perikaryon): !- has the nucleus (which is euchrom= open), but large nucleolus !- synthesizing center for cell!- round in PNS, polygonal in CNS!- lipofuscin granules (tertiary lysosomes)!- Nissl Substance= basophilic staining RER stacks!- axon hillock= where axon arises from, lacks Nissl sub. and large organelles, contains MTs, neurofils, mitochondria, and vesicles moving!2. Dendrites - can have 1 or more/cell!- projections that receive stimuli!- often branched = arborized!- few organelles at distal ends, filled w/neurofils and MTs!- spines on their surface form synpases w/axon of other neurons!3. Axons (nerve fibers) = electrical wire - transmit signals to other cells via synapse!- 1 axon/neuron from hillock!- very long (up to 1m), up to 99% of cell V!- no Nissl substance and can't syn. proteins!- axolemma= plasma membrane - axoplasm= cytoplasm!- initial segment= between axon hillock and myelin sheath, where axon potential (AP) is generated by summation of excitatory and inhibitory stimuli!- branching at end (arboization) so it can synapse w/many target cells!- boutons/end bulbs form synapses w/target cells!- surrounded by supporting cells: oligodendrocytes in CNS and Schwann cells in PNS!- can be myelinated/unmeylinated C. Classification of neurons!1. Based on function!- Sensory (afferent) neurons: receive sensory stimuli, convey impulses from receptors to CNS - Motor (efferent) neurons: convey impulses from CNS to effectors - Interneurons: 99% of neurons, form conxns between sensory and motor neurons!2. Based on their shape and # of processes!- Bipolar neurons: 1 dendrite, 1 axon, receptors for special senses (taste, sight, etc) - Unipolar (pseudounipolar) neurons: 1 process that bifurcates, most are sensory, signals travel thru to CNS w/o passing thru soma - Multipolar neurons: 1 axon and 2 or more dendrites, motor neurons in PNS and interneurons in CNS!D. Role of the cytoskeleton!1. Supports and defines neuron shape!2. Actin rich cortex around inside edge of cell!3. Dendrites: a few neurofils + MTs in 2 directions!motor neuron in spinal corrd4. Axons!- Many neurofils-->determines diameter of axon-->speed of AP!- Many MTs: + end at terminal, - end towards cell body!- MTs transport orgs/particles along axon= axonal transport - Anterograde: kinesin motors bring new components from cell body to axon terminal - Retrograde: cytoplasmic dynein brings recycled comps, growth factors, toxins/disease from axon term to cell body !- Fast transport system: most transport, 20-400 mm/day - Slow transport system: anterograde of cytoskeletal components (1/2 mm/day), corresponds to rate of axon growth, limits axon regeneration!E. Supporting cells:!1. PNS: Schwann cells that support axons, satellite/capsule cells that surround some in ganglia. CNS: glial cells (glue) or neuroglia, astrocytes, oligodendrocytes, microglia, ependymal cells!2. Schwann Cells supports neuron in PNS!- Myelinate some axos wrapping pm of Schwann cell many times around a large (>1 micron) axon --> myelin sheath= insulating layer!- Groups of smaller axons surrounded by 1 layer of Schwann cell membrane!3. Myelinated nerve fibers in PNS!- Myelin sheath: pm of S cell wrapped in concentric layers around nerve fiber --> ensures rapid conduction and separates from CT!- S cell cytoplasm is not really in sheath escept in Schmidt-Lanterman clefts - rest of cyto is in inner collar near axon and outer collar of perinuclear cytoplasm, also perinodal cytoplasm at nodes of Ranvier, cyto is continuous!- myelin sheaths formed around axons >1 micron diameter and the larger the axon, the more layers!- everything surrounded by external lamina of Schwann cell!- many S cells may be needed to myelinate one axon!- length covered by 1 S cell = internodal segment!- gap = Nodes of Ranvier, not myelinated, surrounded by inner collar, more accesible!4. Unmyelinated nerve fibers in PNS!- many axons are enveloped w/in simple folds of 1 S cell!- multiple axons may occupy a single fold !5. Oligodendrocytes myelinate nerve fibers/axons in CNS!- predom. glial
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