Nervous System Flashcards
Neuron Structure and Organization
• Cell Body (Soma): Contains nucleus, ER, ribosomes, and all other organelles, except centrioles (as neurons are amitotic).
• Dendrites: Appendages emanating directly from soma that receive incoming messages from other cells. The information received from the dendrites is transmitted through the cell body before it reaches the action hillock, which integrates incoming signals.
• Axon Hillock: Sums up excitatory and inhibitory signals from dendrites; if the result is excitatory enough to reach threshold, an Action Potential (transmission of electrical impulses down an axon) is triggered.
• Axon: Long appendage that terminates in close proximity to target structure (muscle, gland, neuron).
• Nerve Terminals: Enlarged flattened structures at the end of an axon that transmit signals to the next neuron via the release of Neurotransmitters (chemical messengers that bind to receptors on postsynaptic membrane).
• Synaptic Cleft: Space between terminal portion of axon and adjacent neuron into which neurotransmitters are released.
• Synapse: Nerve terminal, synaptic cleft, and postsynaptic membrane collectively.
• Myelin Sheath: Fatty membrane that insulates nerve fibers to prevent signal loss and maintains electrical signal within one neuron without crossing over. Myelin also increases speed of conduction in axon. Oligodendrocytes produce myelin in CNS; Schwann Cells produce myelin in PNS.
• Nodes of Ranvier: Small breaks in the
myelin sheath that expose areas of axon membrane and that are critical for rapid signal conduction.
- In CNS, axons bundled together form Tracts and cell bodies of neurons in same tract are grouped into Nuclei.
- In PNS, axons bundled together form Nerves, which can be Sensory, Motor, or Mixed (carries both sensory and motor information); cell bodies of neurons of same type are grouped into Ganglia.
Neuroglia (Glial Cells)
- Oligodendrocytes (CNS) and Schwann Cells (PNS): Produce myelin around axons.
- Astrocytes: Nourish neurons and form blood-brain barrier (controls transmission of solutes from bloodstream into nervous tissue).
- Ependymal Cells: Line ventricles of brain and produce CSF to physically support brain and serve as shock absorber.
- Microglia: Phagocytes that ingest and break down waste products and pathogens in CNS.
Resting Membrane Potential
- Resting Membrane Potential: Net electric potential difference (-70mV) between the Equilibrium Potential of Potassium and the Equilibrium Potential of Sodium existing across the cell membrane created by greater movement of Potassium (K+) out of cell via facilitated diffusion through Potassium Leak Channels and slightly lower movement of Sodium (Na+) into cell via facilitated diffusion through Sodium Leak Channels, which makes the inside of the neuron negative relative to the outside. The resting potential is closer to potassium‘s equilibrium potential because of cell is slightly more permeable to potassium.
- Na/K ATPase: Continually pumps 3 sodium ions back out of the cell for every 2 potassium ions it pumps back into the cell against their concentration gradients via active transport using ATP to maintain their respective gradients and thus the resting membrane potential.
Action Potentials
- Excitatory Inputs (that cause depolarization or raise membrane potential above resting potential) and Inhibitory Inputs (that cause Hyperpolarization or lower membrane potential below resting potential) are integrated together by axon hillock; if enough excitatory input is received to depolarize membrane to Threshold (around -50mV), action potential is triggered. Therefore, not every stimulus generates AP, only those that reach threshold.
- Summation can be Temporal Summation (integration of multiple signals during relatively short period of time; multiple small excitatory signals fired at nearly the same moment could bring postsynaptic cell to threshold) or Spatial Summation (additive effects are based on the number and location of the incoming signals; many large inhibitory signals fired directly on soma cause more profound hyperpolarization of axon hillock than the depolarization caused by few excitatory signals fired on dendrites).
- Action Potentials: All-or-nothing electrical impulses that trigger the release of neurotransmitters into synaptic cleft.
- Once membrane is brought to threshold, Voltage-Gated Sodium Channels open in the membrane (in response to depolarization, further depolarizes membrane) to permit passage of sodium ions into the cell due to strong Electrochemical Gradient (electrically, interior of cell is more negative than exterior, favoring movement of positively charged sodium cations into cell; chemically, higher sodium concentration outside cell favors movement of sodium into cell). Cell rapidly depolarizes (membrane potential becomes more positive) due to influx of sodium through these ion channels. Once Vm reaches +35mV, voltage-gated sodium channels become Inactivated; must be brought back to resting potential to be Deinactivated. VGSC are closed before threshold (-50mV), open from -50mV to +35mV, and inactive from +35mV to resting potential (-70mV).
- Once VGSC become inactivated at +35mV, Voltage-Gated Potassium Channels open, allowing potassium to flow out of cell (depolarization by sodium creates electrochemical gradient favoring efflux of potassium); flow of positively charged potassium cations out of cell restores negative membrane potential via Repolarization.
- Potassium efflux causes overshoot of resting membrane potential, hyperpolarizing the neuron, which makes it refractory to further APs. Refractory Periods can be Absolute Refractory Period (no amount of stimulation can generate another action potential) or Relative Refractory Period (greater than normal stimulation is required to generate another action potential).
- Na/K ATPase restores sodium and potassium gradients partially dissipated by APs, in addition to restoring resting membrane potential.
Impulse Propagation
- Action potentials are propagated down the axon when proximal sodium channels open and depolarize the surrounding regions of the membrane, which brings distal sodium channels to threshold and induces them to open as well; because of the refractory character of these channels, the action potential can move in only one direction.
- Propagation (speed) of AP increases when axon is shorter and has greater cross-sectional area (due to less resistance in both cases). Speed of transmission is maximized by myelin, which insulates membrane and allows transport to occur only at Nodes of Ranvier; permits Saltatory Conduction (signal hops from node to node).
- Increased intensity of a stimulus does not result in an increased potential difference of the action potential, but rather an increased frequency of firing.
Synapse
• Presynaptic Neuron releases Neurotransmitters into Synaptic Cleft to interact with receptors on Postsynaptic Neuron or Effector (gland or muscle).
- When AP reaches nerve terminal, Voltage-Gated Calcium Channels open, allowing calcium to flow into the cell; calcium influx triggers fusion of neurotransmitter-containing vesicles with cell membrane at the synapse, causing exocytosis of neurotransmitters.
- Neurotransmitters released into the synapse diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane. If the receptor is a Ligand-Gated Ion Channel, the postsynaptic cell will either be depolarized or hyperpolarized. If the receptor is a G protein-coupled receptor, it will cause either changes in the level of cAMP or an influx of calcium.
- Neurotransmitters are removed from the synaptic cleft (as constant signaling is undesirable) via three mechanisms: Enzymatic Breakdown in synaptic cleft (such as acetylcholine); Reuptake into presynaptic neuron via Reuptake Carriers (such as serotonin, dopamine, and norepinephrine); Diffusion out of synaptic cleft (such as gaseous nitric oxide).
Types of Neurons
- Sensory Neurons: Afferent, transmit information from sensory receptors to spinal cord and brain.
- Motor Neurons: Efferent, transmit information from brain and spinal cord to muscles and glands.
- Interneurons: Most numerous type of neuron, found between other neurons predominantly in brain and spinal cord; often linked to reflexive behavior.
CNS and PNS
- Brain consists of deeper lying White Matter (axons encased in myelin sheaths) and more superficial Grey Matter (unmyelinated cell bodies and dendrites).
- Spinal cord divided into Cervical, Thoracic, Lumbar, and Sacral regions; consists of deeper lying Grey Matter and more superficial White Matter (unlike brain). Sensory neurons bring in information on the dorsal side of spinal cord through Dorsal Root Ganglia (sensory neuron cell bodies); motor neurons exit spinal cord ventrally.
- Somatic Nervous System: Consists of sensory and motor neurons. Neuron goes from spinal cord to target cell without synapsing.
- Autonomic Nervous System: Subdivided into Sympathetic Nervous System (fight-or-flight) and Parasympathetic Nervous System (rest-and-digest). Manages involuntary muscle activity to regulate heartbeat, respiration, digestion, and temperature control (piloerection and perspiration). Preganglionic Neuron originating in CNS synapses on cell body of Postganglionic Neuron in PNS, which then stimulates target cell.
• Reflex Arcs: Neural circuits that control reflexive behavior. Sensory neurons connect with interneurons in the spinal cord and send signals to motor neurons. Interneurons also transmit signals to brain after activation of reflex arc. Can be Monosynaptic Reflex Arcs (Single synapse between sensory neuron and motor neuron; Knee-Jerk Reflex) or Polysynaptic Reflex Arcs (at least one interneuron exist between sensory neuron and motor neuron; Withdrawal Reflex).
