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20 Cards in this Set
- Front
- Back
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Nervous System (2 parts)
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Central nervous system (CNS) – brain and nerve cord
Spinal cord in vertebrates Peripheral nervous system (PNS) – all neurons and projections of their plasma membranes that are outside of the CNS In certain invertebrates with a simple nervous system, the distinction is less clear or not present |
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Neurons
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Cells that send and receive electrical and chemical signals to and from other neurons or other cells throughout the body
All animals except sponges have neurons Number varies widely as a function of size and behavioral complexity |
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Neuron Structure
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Cell body or soma
Contains nucleus and organelles Dendrites Extensions of plasma membrane May be single or branching Incoming signals Axons Extension of plasma membrane Typically single Sending signals Axon hillock near cell body Axon terminals convey electrical or chemical message to other cells |
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Glia
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Perform various functions
Many times more numerous than neurons Astrocytes – metabolic support Microglia – remove cellular debris Glia may function as stem cells to produce more glial cells and neurons Myelin sheath interrupted by nodes of Ranvier Produced by oligodendrocytes (CNS) and Schwann cells (PNS) |
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3 Main Types of Neurons
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Sensory neurons
Detect information from the outside world or internal body conditions Afferent neurons – transmit to CNS Motor neurons Send signals away from CNS (efferent neurons) to elicit response Interneurons or association neurons Form interconnections between other neurons in the CNS |
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Reflex Arc
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Stimulus from sensory neurons sent to CNS, little or no interpretation (few to no interneurons), signal transmitted to motor neurons to elicit response
Quick and automatic response |
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Electrical Properties
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Membrane potential
Difference in charge inside and outside the cell Plasma membrane barrier separating charges Ion concentrations differ between the inside and outside of the cell Polarized Resting membrane potential When neurons not sending signals |
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3 Factors Contributing to Resting Potential
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1) Na+/K+ -ATPase (sodium-potassium pump
Transports 3 Na+ out for every 2 K+ moved in 2) Ion specific channels allow passive movement of ions More ungated K+ channels than ungated Na+ channels Membrane more permeable to K+ at rest 3) Negatively charged molecules such as proteins more abundant |
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Communicating Between Neurons
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Changes in membrane potential are changes in the degree of polarization
Depolarization – cell membrane less polarized, less negative relative to surrounding solution Gated channels open allowing Na+ to flow in and membrane potential becomes more positive (less negative) Hyperpolarization – cell membrane more polarized, more negative K+ moves out of the cell making the cell membrane less positive (more negative) |
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Gated Ion Channels
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Changes in membrane potential are changes in the degree of polarization
Depolarization – cell membrane less polarized, less negative relative to surrounding solution Gated channels open allowing Na+ to flow in and membrane potential becomes more positive (less negative) Hyperpolarization – cell membrane more polarized, more negative K+ moves out of the cell making the cell membrane less positive (more negative) |
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2 Types of Changes
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1) Graded potentials
Depolarization or hyperpolarization Varies depending on strength of stimulus Occur locally on dendrites or cell body 2) Action potentials Carry electrical signal along an axon Always the large same amplitude depolarization All-or-none – cannot be graded Actively propagated – regenerates itself as it travels Spreads a short distance and dies out Act as triggers for action potential |
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Evolution Of K channels
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Evolution of K+ channels with a slightly slower opening time than Na+ channels was a key event that led to the formation of nervous systems
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Absolute Refractory Period
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While inactivation gate of Na+ closed, cell is unresponsive to another stimulus
Places limits on the frequency of action potentials Also ensures action potential does not move backward toward cell body |
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Conduction
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Graded potentials reach threshold potential at axon hillock
Triggers opening of voltage-gated Na+ channels just beyond hillock region Depolarizes area farther along axon Sequential opening of Na+ channels conducts a wave of depolarization from axon hillock to axon terminal Inactivation gate of Na+ channels prevents backward movement toward cell body |
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Speed Varies Based on
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Axon diameter
Broad axons provide less resistance and action potential moves faster Myelination Myelinated faster then unmyelinated Oligodendrocytes and Schwann cells make myelin sheath Not continuous – gaps at nodes of Ranvier Saltatory conduction – action potential seems to “jump” from node to node |
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Synapses
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Junction where nerve terminal meets a neuron, muscle cell, or gland
Presynaptic cell (sends signal), synaptic cleft and postsynaptic cell (receives signal) 2 types Electrical – electric charge freely flows through gap junctions from cell to cell Chemical – neurotransmitter acts as signal from presynaptic to postsynaptic cell |
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Chemical Synapse
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Presynaptic nerve cell contains vesicles of neurotransmitter
Exocytosis releases neurotransmitter into synaptic cleft Diffuses across cleft Binds to channels or receptors in postsynaptic cell membrane |
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Neurotransmitters
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More than 100 different ones in animals
Categorized by size or structure Excitatory and inhibitory neurotransmitters Like brake and accelerator on a car All nervous systems operate in this way, with combined excitatory and inhibitory actions of neurotransmitters |
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5 Classes of Neurotransmitters
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Acetylcholine
One of most widespread neurotransmitters Released at neuromuscular junctions Excitatory in brain and skeletal muscles but inhibitory in cardiac muscles Biogenic amines Widespread physiological effects and psychoactive Abnormally high or low levels associated with a variety of mental illnesses (schizophrenia, depression) Amino acids Glutamate most widespread excitatory neurotransmitter GABA (gamma aminobutyric acid) most common inhibitory neurotransmitter Neuropeptides Often called neuromodulators – can alter response of postsynaptic neuron to other neurotransmitters Opiate peptides Gaseous neurotransmitters Not sequestered into vesicles Produced locally as required Short-acting - influence other cells by diffusion Several drugs for male sexual dysfunction enhance erections by increasing or mimicking action of NO on smooth muscle Function of CO uncertain |
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Postsynaptic Receptors
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In some cases, same neurotransmitter can have excitatory or inhibitory effects
Response of postsynaptic cell depends on receptor type Ionotropic receptors – ligand-gated ion channels open in response to neurotransmitter Metabotropic receptors – G-protein coupled receptors (GPCRs) initiate changes in postsynaptic cell |
