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A&P Lec Chap 12
A&P Lecture WK 7 Chap 12 Nervous Tissue
| Term | Definition |
|---|---|
| Endocrine system and nervous system maintain | internal coordination |
| Endocrine system: | communicates by means of chemical messengers (hormones) secreted into to the blood |
| Nervous system: | utilizes neurons (nerve cells) to send messages from cell to cell by electrical and chemical means; occurs in three steps |
| nervous system communication steps | 1. receives stimuli from an external environment and transmits messages to 2. central nervous system (CNS) (brain or spinal cord) 3. CNS processes the info and determines a response CNS issues commands to muscle and gland cells to carry out response |
| Nervous system has two major anatomical subdivisions: | central nervous system (CNS) and peripheral nervous system (PNS) |
| Central nervous system CNS: | brain and spinal cord |
| Peripheral nervous system PNS: | nerves and ganglia |
| Nerves: | a bundle of nerve fibers (axon) wrapped in fibrous connective tissue |
| Ganglion: | a knot like swelling in a nerve where neuron cell bodies of PNS are concentrated - swelling of nervous tissue |
| steps of PNS system | sensory input, integration, motor output |
| Sensory input: | anything that is the stimulus such as an environmental stimulus, like being thirsty and needing to drink water |
| Integration: | then it sends it to the central nervous system to find the correct response |
| Motor output: | is the response that occurs, like getting and drinking water |
| Peripheral nervous system is functionally divided into: | sensory and motor divisions each with somatic and visceral subdivisions |
| Sensory (afferent): | carries signals from receptors (sense organs) to CNS |
| Somatic sensory division: | carries signals from receptors in the skin, muscles, bones, and joints |
| Visceral sensory division: | carries signals from the viscera (heart, lungs, stomach, and urinary bladder) |
| Motor (efferent) division: | carries signals from CNS to effectors |
| Effectors: | glands and muscles that carry out the body's response |
| Somatic motor division: | carries signals to skeletal muscles; causes voluntary muscle contraction and automatic reflexes |
| Visceral motor division (autonomic nervous system ANS): | carries signals to glands, cardiac and smooth muscle; no voluntary control; responses called visceral reflexes |
| Sympathetic division of ANS: | stimulates and prepares the body for action (flight or fight) |
| Parasympathetic division of ANS: | has a calming effect on the body An enteric plexus within digestive tract wall enables coordination and communication within digestive tract |
| Visceral is guts/organs: | somatic is skeletal - big body movements |
| Three properties allow neurons to communicate with other cells - neurons and tissues are excitable: | excitability, conductivity and secretion - If the neuron doesn’t do all three of these it does not function correctly |
| Excitability: | ability to respond to stimuli |
| Conductivity: | produce electrical signals that are conducted to other cells |
| Secretion: | when signal reaches end of axon, the neuron secretes a neurotransmitter that stimulates the next cell |
| Three functional classes of neurons: | sensory neurons, interneurons, and motor neurons |
| Sensory (afferent neurons): | detect stimuli and transmit info about them toward the CNS Begin in every organ of body and terminate in CNS Is the sensory input step Conduct signals from receptors to the CNS |
| Interneurons: | receive signals from other neurons., process this info, and make resulting decisions - integration step - Confined to CNS - Lie entirely within CNS connecting motor and sensory pathways; most common functional type (about 90% of all neurons) |
| Motor (efferent) neurons: | send signals out to muscles and gland cells (the effectors) Conduct signals from CNS to effectors such as muscles and glands |
| Components of a neurons: | Cell body: Has mitochondria, lysosomes, golgi complex, inclusions, and rough ER Has no centrioles; mature neurons cannot undergo mitosis after adolescence |
| Cell body: | also called neurosoma, soma, or perikaryon, contains nucleus and many organelles |
| Neurons can have many | neurites (extensions) reaching out to other cells |
| Dendrites: | most numerous neurites, resemble branching of a tree, primary sites for receiving signals from other neurons - Neurons can have one dendrite or thousands of dendrites |
| axon(nerve fiber): | long, cylindrical extension ; relatively unbranched but may give off axon collaterals; specialized for rapid conduction of nerve signals - A neuron never has more than one axon; some neurons have none |
| Axons originate at Axon hillock: | which are a critical part so that the signal goes through, a mount on one side of the cell body |
| Each branch of arborization ends in | a bulbous axon terminal (terminal bouton), which forms a synapse with next cell |
| Neuroglia or glial cells: non-neuronal supportive cells | Bind neurons together Form supportive tissue framework In fetus, they guide migrating neurons to their destination Cover mature neurons (except at synapses) Prevents neurons from touching each other Gives precision to conduction pathways |
| Four types of glial occur in the CNS: | oligodendrocytes, ependymal cells, microglia, and astrocytes - found in the brain and spinal cord |
| Oligodendrocyte: | form myelin sheaths in CNS Insulates neurons with myelin |
| Ependymal cells: | line internal cavities of brain; secrete and circulate cerebrospinal fluid (CSF) Create cerebrospinal fluid |
| Microglia: “macrophages”; | engulf debris, provide defense against pathogens Immune cells in the CNS Clean up messes/damages |
| Astrocytes: | most abundant type; wide variety of functions Part of the blood brain barrier: helps block specific things Feed the neuron Help neurons grow |
| Peripheral glia cells, found only in the peripheral nervous system PNS: | Schwann cells and satellite cells |
| Schwann cell or neurolemmocytes: | envelop axons of PNS from myelin sheath and assist in regeneration of damaged fibers Insulates PNS neurons with myelin |
| Satellite cells: | surround nerve cell bodies in ganglia of PNS provide insulation around cell body and regulate chemical environment Insulate and regulate environment around peripheral nervous system cell bodies |
| Tumors are | masses of rapidly dividing cells - can only be a glial cell and must be able to go through mitosis Mature neurons have lotto or no capacity for mitosis and seldom form tumors |
| Brain tumors arise from: | Meninges (protective membranes of CNS) Metastasis from nonneuronal tumors in other organs Glial cells that are mitotically active throughout life |
| Gliomass: | tumors of glial cells; grow rapidly and are highly malignant Blood-brain barrier decreases effectiveness of chemotherapy Treatment consists of radiation or surgery |
| Myelin sheath: | spiral layers of insulation around an axon - essentially plasma membrane of cell Formed by schwann cells in PNS, oligodendrocytes in CNS 20% protein and 80% lipid - Insulates axon to travel faster |
| Myelination: | production of the myelin sheath begins during fetal development, proceeds rapidly during infancy, complete by late adolescence |
| Myelination in the PNS: | A Schwann cell spirals repeatedly around a small section of a single axon Lays down as many as 100 layers of its membrane with no cytoplasm between the layers These layers are the myelin sheath |
| Myelination in the CNS: | Each oligodendrocyte extends several processes that wrap around small portions of many axons in its immediate vicinity |
| During myelination in CNS: | nucleus cannot migrate around the axon like a Schwann cell does Must push newer layers of myelin under the older ones, so myelination spirals inward toward axon |
| In both the PNS and CNS the myelin sheath is segmented: | Many Schwann cells (PNS) or oligodendrocytes (CNS) are needed to myelinate one axon |
| Myelin sheath gap (node of ranvier): | gap between segments where the action potential is occurring Where depolarization occurs |
| Intermodal segments: | myelin-covered segments in between action potentials |
| Trigger zone: | axon hillock and initial segment, plays important role in initiating nerve signal |
| Many axons in the CNS and PNS are unmyelinated: In PNS, | Schwann cells hold small unmyelinated axons in surface grooves - Membrane folds once around each axon; does not spiral repeatedly around it - This wrap serves as neurolemma - Basal lamina surrounds Schwann cell along with its axons |
| Speed at which a nerve signal travels down an axon depends on two factors: | diameters and presence of myelin - size matters and myelination matters |
| Diameter: | larger axons have more surface area and conduct signals more rapidly |
| Presence or absence of myelin; | myelin speeds signal conduction |
| Regeneration of damaged PNS nerve fiber (axon) can occur if | nerve cell bodies are intact and at least some neurilemma remains |
| PNS Steps of regeneration: regeneration is slow and not perfect but is possible | 1. Axon distal to injury degenerates, macrophages clean up tissue debris 2. Cell body swells, ER breaks up, nucleus moves off center 3. Axon stump sprouts multiple growth processes |
| Neurons can regenerate only IF: | Lives in PNS Cell body is remains intact The myelin sheath (post trauma) must remain |
| PNS Steps of regeneration: after Axon stump sprouts multiple growth processes | 4. Schwann cell neurolemma, endoneurium, basal lamina form a regeneration tube 5. Guides regrowth to original destination |
| Describe some conditions in which a neuron can regenerate (neuron is a single cell, while a nerve is a bundle, major damage that cannot be regenerated): | Lives in PNS Cell body is remains intact The myelin sheath (post trauma) must remain |
| In PNS Regeneration is not fast, perfect, or always possible | - Slow regrowth; process may take 2 years - Some axons connect with wrong muscle fibers; some die - Damaged CNS axons usually unable to regenerate |
| Resting membrane potential (RMP): | charge difference across plasma membrane - Typically −70 millivolts (mV) in an unstimulated, “resting” neuron - Negative value indicates more negatively charged particles on inside of membrane compared to outside |
| Electrical currents in the body created by flow of | ions (Na+, K+) through gated channels in the membrane |
| Ionic basis of the resting membrane potential (RMP): | Ions are unequally distributed between extracellular fluid (ECF) and intracellular fluid (ICF) |
| RMP resting membrane potential results from the combined effect of: | - The diffusion of ions down their concentration gradients through the membrane - Selective permeability of the membrane, allowing some ions to pass more easily than others - Electrical attraction of cations and anions to each other |
| Potassium (K+) has greatest influence on RMP because | - It is more concentrated in ICF compared to ECF - Cell membrane more permeable to K+ (through leak channels) than to other ions- - As K+ leaks out, inside of membrane becomes more negative - Resulting electrical attraction brings K+ back in |
| Equilibrium is reached: | no net movement of K+ occurs when tendency for K+ to exit (down its concentration gradient) equals tendency for to K+ enter (by electrical attraction) |
| Sodium also influences the RMP: | - Na+ is more concentrated in ECF compared to ICF, but membrane is much less permeable to Na+ than K+ - Na+ diffuses into cell, down its concentration gradient/attracted electrically, but it’s a relatively small amount |
| sodium in RMP | Cancels some of the negative charge, reducing the voltage across the membrane |
| Sodium-potassium pump (Na+/K+) compensates for the continual leakage of Na+ and K+ since Na+ moves out of cell and brings K+ into the cell: | this maintains their concentration gradients Works continuously and requires ATP 70% of the energy requirement of the nervous system |
| Sensory neurons can be stimulated by | chemicals, light, heat, or mechanical forces |
| Stimulation triggers what? | local, temporary change in membrane potential Ex: chemical stimulant binds to a receptor on the neuron causing depolarization as Na+ moves in |
| Local potential: | Temporary, short-range change in voltage |
| Characteristics of local potentials | : graded, decremental, reversible, depolarization, hyperpolarization |
| Graded: | vary in magnitude with stimulus strength - Stronger stimuli open more channels, and they stay open longer |
| Decremental: | get weaker the farther they spread from the point of stimulation - Might not make it to axon hillock - Stimulus gets smaller like a rock in a puddle |
| Reversible: | if stimulation ceases, membrane voltage quickly returns to normal resting potential - Can be either excitatory or inhibitory |
| Depolarization is excitatory: | makes a neuron more likely to fire an action potential) - Green light more likely to excite/reach threshold/depolarize - Sodium moving out |
| Hyperpolarization (membrane more negative) is inhibitory— | makes a neuron less likely to produce an action potential - a neuron less likely to produce an action potential - Red light less likely to excite/reach threshold/depolarize - Potassium ions moving out |
| Action potential: | rapid up-and-down change in voltage produced by the coordinated opening and closing of voltage-gated ion channels Only occurs if it can reach the (trigger zone of axon) |
| in action potential If excitatory local potential reaches trigger zone and is still strong enough | , it opens enough voltage-gated Na+ channels to generate an action potential |
| Steps of an action potential: | 1. Local potential spreads to axon hillock 2. Voltage at axon hillock must reach threshold |
| Steps of an action potential: after voltage at axon hillock reaches threshold | 3. Voltage-gated Na+ channels open quickly - Voltage-gated K+ channels open more slowly - Na+ enters and depolarizes membrane further - Na+ channels become inactivated, begin closing |
| Steps of an action potential: after Na+ depolarizes and channel begins closing | 4. Voltage peaks by the time inflow ceases - Membrane polarity has reversed—now more positive on the inside and negative on the outside 5. The voltage-gated flows out of cell and membrane becomes more negative again— repolarization |
| Steps of an action potential: after depolarization occurs | 6. K+ continues to exit and produces a negative overshoot hyperpolarization) more negative than the original RMP 7. Membrane voltage returns to RMP as Na+ leaks into the cell |
| Threshold: | the minimum voltage to open voltage-gated channels |
| The concentrations of Na+ and K+ on either side of the membrane do not change | significantly during an action potential - Movement of only a few ions can have a large effect on the membrane potential - Only about one in a million ions crosses the membrane during an action potential |
| Only the thin layer of ions close to the membrane is | affected - Even after thousands of action potentials, the cytosol still has a higher concentration of K+ and a lower concentration of Na+ than the ECF |
| Characteristics of action potentials: | all or none law, non decremental, irreversible, |
| All-or-none law: | if threshold reached, neuron fires up to maximum voltage; if threshold not reached, it does not fire |
| Non decremental: | do not get weaker with distance |
| Irreversible: | once started, an action potential travels all the way down the axon cannot be stopped |
| Refractory period: period of resistance to stimulation; has two phases: | Absolute refractory period and relative refractory period |
| Absolute refractory period: | no stimulus of any strength will trigger another AP - already conducting an AP - occurs during depolarization and repolarization - Caused by inactivation of voltage-gated Na+ channels |
| Relative refractory period: | an unusually strong stimulus is needed to trigger a new AP - During hyperpolarization, a larger depolarization (local potential) is required to reach threshold - Only if the stimulus is super strong |
| Because neurons are excitable they need more than 1 what? | stimulus or the stimuli to be more powerful and it must reach the axon hillock/threshold to conduct a signal |
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