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BMS 250 Lecture
Chapter 12
| Term | Definition |
|---|---|
| Functions of the nervous system | collect and evaluate information, initiate a response to stimulus |
| How can the nervous system be classified? | structurally (where organs are located) and functionally (what type of info is carried) |
| Structural organization of the nervous system | CNS and PNS |
| Central Nervous System (CNS) | brain & spinal cord |
| Peripheral Nervous System (PNS) | nerves & ganglia |
| Ganglia | clusters of neuronal bodies in the PNS |
| Functional classification of the nervous system | sensory/afferent nervous system and motor/efferent nervous system |
| Parts of the sensory/afferent nervous system | somatic sensory and visceral sensory |
| Somatic sensory | detects stimuli we consciously perceive |
| Visceral sensory | detects stimuli we do not consciously perceive |
| Parts of the motor/efferent nervous system | somatic motor and autonomic motor |
| Somatic motor | voluntary movement |
| Autonomic motor | involuntary movements |
| Parts of the autonomic motor system | sympathetic and parasympathetic |
| Shingles | sensory nervous system disease causing intense pain- virus remain dormant in ganglia and reactivates later |
| Polio | motor nervous system disease causing paralysis- virus from contaminated food that affects the anterior bone of the spinal cord and affects motor nerves in the spinal cord |
| Nerve | a bundle of parallel axons in the peripheral nervous system composed of the epineurium, perineurium, endoneurium |
| Epineurium | wraps the whole outer nerve; composed of dense irregular CT |
| Perineurium | wraps a fascicle (bundles of axons); formed of dense irregular CT |
| Endoneurium | wraps an individual axon; composed of areolar CT |
| Classification of nerves | functionally (type of neurons contained within) and structurally (where nerves extend from) |
| Functional classifications of nerves | sensory, motor, and mixed nerves |
| Sensory nerves | contains sensory neurons to the CNS |
| Motor nerves | contains motor neurons from the CNS |
| Mixed nerves | contains both sensory and motor neurons |
| Structural classification of nerves | cranial and spinal nerves |
| Cranial nerves | extend from the brain |
| Spinal nerves | extend from the spinal cord |
| What nervous system conducts signals to the CNS and carries sensory information? | the afferent nervous system |
| What is the portion of the nervous system that conducts impulses from the CNS to skeletal muscle? | somatic motor |
| Neurons | excitable cells of nervous tissue that initiate and propagate electrical signals |
| Structure of neurons | dendrites, soma, axons |
| Dendrites | short, small tapering processes that branch off soma and transmit graded potential toward soma |
| When there is a greater number of dendrites... | there is a greater input to the soma |
| Soma | cell body, defined by the presence of the nucleus, transmits graded potentials to axon hillock, performs basic cellular functions (directs metabolism, controls protein synthesis |
| Axon | single, longer process extending away from soma (axon hillock), transmits action potentials, branches into axon collaterals, expands at extreme ends (synaptic knobs) |
| Characteristics of neurons | excitability, conductivity, secretion of neurotransmitters, extreme longevity, amitotic |
| Amitotic | lack ability to form new cells by mitosis |
| Classification of neurons | structurally (number of neuronal processes emanating from the soma) and functionally (direction of action potentials relative to CNS) |
| Structural classifications of neurons | anaxonic, unipolar, bipolar, multipolar |
| Anaxonic | only dendrites, no axon |
| Unipolar | only one process leaving soma that branches off into axons and dendrites, branches like a "T" |
| Bipolar | two processes extend from soma, one dendrite, one axon |
| Multipolar | one axon, many dendrites |
| Functional classifications of neurons | sensory, interneuron, motor |
| Sensory neuron | picks up stimuli; contains soma, dendrite, and synaptic terminal (unipolar or bipolar) |
| Interneuron | found within CNS, obtains info from sensory neurons and integrates it into the CNS and takes it into another neuron; contains various dendrites (anaxonic or multipolar) |
| Motor neuron | cell body communicates with interneuron and takes information away from CNS (multipolar) |
| Synapses | the location where the axon of one neuron contacts another neuron or an effector |
| Two types of synapses | chemical and electrical synapses |
| Chemical synapses | there is a delay (0.3-0.5ms) in the transmission of information, contains presynaptic neuron, synaptic cleft, postsynaptic neuron, neurotransmitters released from synaptic vesicles and bind post synaptic receptor |
| Presynaptic neuron | produces the signal |
| Synaptic cleft | narrow gap between the two neurons |
| Postsynaptic neuron | receives the signal |
| Electrical synapse | no delay in the transmission of information; physically bound by gap junctions that allow ions to flow bidirectionally between the cells; contains a presynaptic neuron and postsynaptic neuron |
| Glial cells | support cells of the nervous system found in both the PNS and CNS, nonexcitable and capable of mitosis |
| Glial cells in the CNS | astrocytes, ependymal cells, microglia, oligodendrocytes |
| Glial cells in the PNS | satellite cells, neurolemmocytes (Schwann cells) |
| Astrocytes | star-like shape because of their processes, functions: anchor neurons and blood vessels in place, regulate chemical composition of extracellular environment, repair damaged tissue, help form the blood-brain barrier |
| Blood-brain barrier (BBB) | strictly regulates entry of substances into brain, keeps most toxins out and lets nutrients in, astrocyte processes wrap around capillaries in brain |
| Ependymal cells | line internal cavities (ventricles) of brain, produce and secrete cerebrospinal fluid |
| Microglia | immune cells of the nervous system that phagocytize pathogens, dead neurons, cellular debris, and stimulate inflammation |
| Oligodendrocytes | form myelin sheath (insulation that allows for faster propagation of action potentials); have long, slender cytoplasmic processes |
| Satellite cells | wrap around soma to insulate PNS somas, regulate nutrient and waste exchange |
| Neurolemmocytes (Schwann cells) | form myelin sheath around PNS axons |
| Myelination | the process by which part of an axon is wrapped with myelin (insulating cover formed by repeating concentric layers of plasma membrane of certain glial cells- oligodendrocytes and neurolemmocytes) |
| Myelination of PNS axons | neurolemmocyte wraps around a portion of the axon, overlapping inner layers of plasma membrane form myelin sheath, cytoplasm and nucleus of neurolemmocyte are pushed to periphery, myelinations one 1mm segment of one axon in the PNS |
| Node of Ranvier | neurofibril node |
| Myelination of CNS axons | same process of PNS, except myelinates one 1mm segment of many axons in CNS |
| Difference between nerves and a neuron | nerves are organs (collection of nervous and CT) |
| What makes the transmission of electrical signals possible? | the distribution of pumps and channels in the neuronal plasma membrane |
| Pumps | maintain concentration gradients by moving substances against their gradient, requires cellular energy |
| Channels | allows substances to move down their concentration gradient, doesn't require cellular energy |
| Leak channels | always open, allows continuous diffusion of one type of ion |
| Chemically-gated channels/ligand-gated channels | closed at rest, open briefly in response to neurotransmitter binding, allow diffusion of one type of ion |
| Voltage-gated channels | closed at rest, open briefly in response to changes in electrical charge across membrane, allows diffusion of one type of ion |
| Sodium/Potassium (Na+/K+) pumps | maintain the resting membrane potential, account for 2/3 of neuron's energy expenditure, moves 3 Na+ to ECF and 2 K+ to ICF |
| Calcium (Ca2+) pumps | establish a concentration gradient for Ca2+ in the axon terminal (important for synaptic transmission) moves Ca2+ to the ECF |
| Function segments of a neuron | receptive, initial, conductive, transmissive segment |
| Receptive segment | dendrites and soma |
| Initial segment | axon hillock |
| Conductive segment | axon |
| Transmissive segment | axon terminals |
| Pumps/Channels in the plasma membrane of the entire neuron | Na+/K+ pumps, Na+ leak channels, K+ leak channels |
| Pumps/Channels in the receptive segment | chemically-gated cation channel, chemically-gated K+ channels, chemically-gated Cl- channels |
| Pumps/Channels in the initial and conductive segments | voltage-gated Na+ channels, voltage-gated K+ channels |
| Pumps/Channels in the transmissive segment | voltage-gated Ca2+ channels, Ca2+ pumps |
| Electrophysiology | study of electrical changes across plasma membranes |
| Electrical gradient | there is an unequal distribution of ions across the plasma membrane; "separation of charge" |
| Electrical potential | an electrical gradient represents potential energy or electrical potential |
| As ions move down their gradient, what happens to potential energy? | it becomes kinetic energy or an electrical current |
| Resting Membrane Potential (RMP) | the potential difference across a cell's plasma membrane when it is not being stimulated (at rest); cell is polarized (-70mV) |
| Establishing and maintaining resting membrane potential in neurons relies on | electrochemical gradients for Na+ and K+ ions, diffusion of Na+ and K+ ions through ion leak channels, and Na+/K+ pumps to maintain RMP |
| What diffuses more easily through leak channels: K+ ions or Na+ ions | K+ ions |
| Establishing RMP in neurons | K+ moves through leak channels more easily than Na+, K+ efflux outpaces Na+ influx, the neuron loses positive ions |
| Physiological events in the receptive segment | dendrites and soma, binding of neurotransmitter released from presynaptic neurons, production of graded potentials |
| Physiological events in the initial segment | axon hillock, summation of graded potentials, initiation of action potentials |
| Physiological events in the conductive segment | axon, propagation of action potentials |
| Physiological processes of transmissive segment | axon terminal, action potential causes release of neurotransmitter |
| Depolarization | once a neurotransmitter binds to the channels, it opens and Na+ moves into ICF, gain of positive charge makes the cytosol less negative |
| Hyperpolarization | K+ moves into ECF, loss of positive charge makes the cytosol more negative |
| Hyperpolarization | Cl- moves to ICF, gain of negative charge makes the cytosol more negative |
| Repolarization | the Na+/K+ pump returns the membrane potential to RMP/polarized state |
| Electrochemical gradient for the flow of Na+ ions via a Na+ leak channel | concentration gradient for Na+: favors Na+ movement to ICF, electrical gradient for Na+ favors Na+ movement to ICF; combines these forces form the electrochemical gradient that favors Na+ movement to the ICF |
| Electrochemical gradient for the flow of K+ ions via a K+ leak channel | concentration gradient for K+: favors K+ movement to ECF, electrical gradient for K+: favors movement to ICF; combined these forces form the electrochemical gradient that favors K+ movement to ECF |
| Two types of electrical changes that occur across a neuron's plasma membrane | graded potentials, action potentials |
| Graded potentials/postsynaptic potentials/local potentials | occurs along receptive segment, results from opening of chemically-gated channels, may cause depolarization or hyperpolarization, size of change in membrane potential varies, travels only a short distance |
| Types of graded potentials | excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP) |
| Excitatory postsynaptic potential (EPSP) | neurotransmitter binds receptor/opens chemically-gated cation channels, Na+ diffuses into cell, changes in potential: depolarization, change in membrane potential is small and only lasts a few ms |
| Inhibitory postsynaptic potential (IPSP) | neurotransmitter binds receptor/opens chemically-gated K+ channels, K+ diffuses out of cell, change in potential: hyperpolarization, change in membrane potential is small and only lasts a few seconds (also occurs with Cl- diffusing out of cell) |
| Effects of graded potentials | numerous postsynaptic potentials are generated simultaneously, the outcome of all these EPSP's and IPSP's is determined in the initial segment by summation |
| Summation | the changes in membrane potential generated by all the graded potentials added together at the initial segment |
| An action potential will be generated if... | graded potentials arriving at the axon hillock move the membrane potential to threshold potential (-55mV) |
| What is threshold potential? | -55mV |
| Two types of summation | spatial and temporal summation |
| Spatial summation | multiple presynaptic neurons release neurotransmitter at various locations on the receptive segment |
| Temporal summation | a single presynaptic neuron repeatedly releases neurotransmitters at the same location on the receptive segment within a short period of time |
| When do both types of summation occur? | simultaneously |
| Effects of summation/when threshold is reached | voltage-gated channels at the axon hillock open, initiating an action potential |
| Action Potential | begins in initial segment, occurs along conductive segment, results from sequential opening and closing of voltage-gated channels, causes large stereotypical change in membrane potential, "all or none" |
| Stages of action potential | RMP, voltage-gated Na+ channels open-summating graded potentials, depolarization, repolarization, hyperpolarization, return to RMP |
| Depolarization | Na+ influx causes "rising phase", mediated by voltage-gated Na2+ channels, Na+ enters from adjacent areas, & membrane potential changes from -70mV to -55mV, reaching threshold: VGNCs open & membrane potential depolarizes (-55mV to +30mV), then VGNCs close |
| Repolarization | K+ efflux, mediated by VGK+ channels, VGKCs open slowly to coincide w/ peak depolarization, K+ exits the cell & the membrane repolarizes (+30mV to-70mV), VGKCs remain open longer than needed & membrane hyperpolarizes (-70mV to -80mV), VGKCs close |
| Action potentials propagate in... | one direction; repeated steps along adjacent regions of the axon downstream from the soma; due to properties of voltage-gated Na+ channels |
| The "all" property of action potentials | voltage changes reach threshold (-55mV), V-gated channels open, action potentials propagates along the axon in the same way every time |
| The "none" property of action potentials | voltage charges are subthreshold, v-gated channels remain closed, no action potentials |
| Where does a graded potential occur? | receptive segment |
| What channels are involved in graded potentials? | chemically-gated |
| How do the membrane potential change in graded potential? | can be depolarization (EPSP) or hyperpolarization (IPSP) |
| Degree of membrane potential change in graded potentials? | dependent on magnitude of stimulus |
| Distance traveled in graded potentials? | short distance ("local"), signal degrades as it travels |
| Where do action potentials occur? | initial and conductive segments |
| Channels involved in action potential | voltage-gated |
| How does the membrane potential change in action potentials? | always the same sequence: depolarization, repolarization, hyperpolarization, return the RMP |
| Degree of membrane potential change in action potentials? | all-or-none |
| Distance traveled in action potentials | along the entire length of axon (hillock to terminal) |
| States of voltage-gated Na+ channels | resting (closed), activation (open), inactivation (closed) |
| Resting state of voltage-gated Na+ channels | closed, inactivation gate open, activation gate closed |
| Activation state of voltage-gated Na+ channels | inactivation gate open, activation gate open |
| Inactivation state of voltage-gated Na+ channels | inactivation gate closed, activation gate open |
| Refractory period | brief time period after an action potential (AP) when its impossible or difficult to fire another AP |
| Absolute refractory | no amount of stimulation can generate an AP, VGNCs are in the inactivation state, occurs in stages 3 and 4 of action potentials (depolarization and repolarization) |
| Relative refractory | an AP may be generated, but requires a greater-than-normal stimulus, VGNCs have returned to resting state, neuron is hyperpolarized due to open VGKCs, occurs in stage 5 of action potentials (hyperpolarization) |
| How an action potential propagates depends on... | whether or not the axon is myelinated (continuous conduction or saltatory conduction) |
| Continuous conduction | unmyelinated axons, sequential opening of VGNCs and VGKCs along entire length of axon |
| Saltatory conduction | myelinated axons, action potentials “jump” from one neurofibril node to the next neurofibril node; more efficient since less energy is required by Na+/K+ pumps to maintain RMP |
| Action potential velocity is influenced by... | myelination (faster velocity), axon diameter (larger diameter=faster velocity) |
| What does the amplitude of action potential depend on? | nothing, it is always the same |
| What does the frequency of action potential depend on? | stimulus strength (stronger stimulus=more frequent action potentials) |
| Synaptic transmission | occurs in transmissive segments, initiated by the arrival of an action potential at the axon terminal, causes release of neurotransmitter from synaptic knobs, depends on concentration gradients for Ca2+ |
| Steps of synaptic transmission | AP arrives at synapse and triggers VGCCs to open, Ca2+ enters the synaptic knob &binds synaptic vesicles which fuse with plasma membrane & NT is released by vesicular exocytosis, NT diffuses across synaptic cleft & binds receptors of neuron or effector |
| Neurotransmitters | chemical messengers, stored in synaptic vesicles in the axon terminal, transmit nerve signal across the chemical synapse from one neuron to its target |
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kkade
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