Neurons Flashcards
What is the central nervous system?
-Acts as the integrating center
-Consists of the brain and spinal chord
What is the peripheral nervous system?
-Nerve tissue completely or partially outside the CNS: cranial nerves and branches, spinal nerves and branches, ganglia, plexuses and sensory receptors
What are the three types of neurons?
Afferent: Sensory (Input signal, carry info TOWARDS CNS)
-Carry information about temperature, pressure, light and other stimuli to the CNS
-Specialized receptor converts stimulus to electrical energy
Interneurons:
-Complex branching neurons that facilitate communication between neurons
Efferent: Somatic motor and autonomic (Output signal, carry info AWAY from CNS)
-Motor: control skeletal muscles
-Autonomic: influences many internal organs (sympathetic and parasympathetic)
-Usually have axon terminals or varicosities
What are the 3 subdivisions of the peripheral nervous system?
-Afferent: Sensory
-Efferent: Somatic and autonomic
-Enteric (digestive tract): can act autonomously or controlled by the CNS through the autonomic division of the PNS
Describe the parts of a neuron
Dendrites: Input signal
-Receive incoming signals from neighboring cells
Cell body (soma): Integration
-Considered the control center, with processes that extend outward
-Includes nuclues, other organelles and axon hillock
Axon: Output signal
-Initial segment stemming from axon hillock
-Covered in myelin sheath
-Carries outgoing signals from the integrating center to target cells through presynaptic terminals containing transmitting elements
What are the various structural categories of neurons?
Describe axonal transport
- Peptides are synthesized on rough ER and packaged by golgi apparatus
- Fast axonal transport walk vesicles and mitochondria along microtubule network (Down axon = Anterograde)
- Synaptic vesicle contents are released by exocytosis
- Synaptic vesicles are recycled
- Retrograde (back to soma) fast axonal transport
- Old membrane components digested in lysosomes
Fast axonal vs. Slow axonal transport
Fast:
-Membrane bound proteins and organelles (vesicles or mitochondria)
-Anterograde: Cell body to axon terminal, up to 400mm/day
-Retrograde: Axon terminal to cell body, 200mm/day
Slow:
-Cytoplasmic proteins (enzymes) and cytoskeleton proteins
-Anterograde, up to 8mm/day, some evidence for retro
-Not well characterized, may be slower due to frequent periods of pausing movements
What are the functions of motor proteins in axonal transport?
Kinesis: Anterograde transport toward positive, distal end of the axon
Dyneins: Retrograde transport towards negative, proximal soma
ATP hydrolysis drives movement of proteins to “walk” along filaments
Describe synapses
-Chemical (majority) and electrical
-Presynaptic cells axon terminals communicates with a target cells postsynaptic dendrites at the synaptic cleft
-This space contains extracellular matrix (proteins and carbohydrates) that hold cells in close proximity
What are glial cells and where are they found?
-Communicate with neurons and provide important biochemical support
CNS:
-Ependymal cells
-Astrocytes
-Microglia (modified immune cells)
-Oligodendrocytes
PNS:
-Schwann cells
-Satellite cells
What is myelin and the two glia that form it?
-A substance composed of mutliple concentric layers of phospholipid membrane wrapped around an axon
-Provide structural stability, acts as insulation around the axon to speed up electrical signals (saltatory conduction), supply trophic facters
CNS: Oligodendrocytes
-Wraps the axons of multiple neurons
PNS: Schwann cells
-Myelate a small segment of one axon
What is a sattelite glial cell?
-Exists within ganglia (bundles of cell bodies) in the PNS
-Form a supportive capsule around the cells bodies (somas) of neurons (sensory and autonomic)
-Supply nutrients
-Structural support, provide a protective cushion
What are astrocytes and their function?
-Highly branched glial cells in CNS believed to make up half of all cells in the brain
-Several subtypes, form a functional network
Functions:
-Take up and release chemicals at synapses
-Provide neurons with substrates for ATP production
-Help maintain homeostasis in the extracellular fluid (take up K+ and H2O)
-Surround vessels
-Part of blood brain barrier
-Influence vascular dynamics
What are microglia?
-Specialized immune cells that reside in the CNS
-Serve to protect and preserve neuronal cells from pathogens and facilitate recovery from metabolic insults
-If the signals that activate microglia pass a threshold with respect to intensity, or microglia remain activated past a certain time period, these cells start to display detrimental properties (Alzheimers, ALS, neuropathic pain)
What are ependymal cells?
-Line fluid filled cavities in the brain and spinal cord
-Help to circulate cerebrospinal fluid that fills these cavities and surrounds the brain and spinal cord (protection, chemical stability, clearing waste)
What is the difference between the Nernst equation and the Goldman-Hodgkin-Katz equation?
-Nernst equation calculates the equilibrium potential for a single ion whereas the GHK equation is determined by the combined contribution of each ion (concentration x permeability)
What are the 5 major types of gated channels that control ion permeability in neurons?
- Na+ channels
- K+ channel
- Ca2+ channel
- Cl- channel
(All allowed to flow through due to selectivity) - Monovalent cation channels (allow Na+ and K+ to pass)
Explain gated channels in neurons
Mechanically gated channels
-Open in response to physcial forces (pressure or stretch), found in sensory neurons
Chemically gated ion channels
-In neurons, respong to ligands including extracellular neurotransmitter and neuromodulators or intracellular signaling molecules
Voltage-gated channels
-Respond to changes in the cells membrane potential
Describe channelopathies
What is Ohm’s Law?
-Current flow (I) (flow of electrical charge carried by an ion) is directy proportional to the electrical potential difference (in volts, V) between two points and inversely proportional to the resistance (R)
-I=V/R
What are two sources of resistance in a cell?
-Membrane resistance (Rm): Resistance of phospholipid bilayer
-Internal resistance of the cytoplasm (Ri): cytoplasmic composition and size of the cell
-Resistance will determine how far current will flow in a cell before the energy is dissipated
What are graded potentials?
-Variable strength signals that travel over short distances and lose strength as they travel due to current leak and cytoplasmic resistance
-Generated by chemically (ligand) gated ion channels or closure of leak channels (CNS and efferent neurons)
-Can be depolarizing (Excitatory Postsynaptic Potential - EPSP) or Hyperpolarizing (Inhibitory Postsynaptic Potential - IPSP)
-If strong enough (excitatory), graded potentials reach the trigger zone and fire an action potential
Describe the trigger zone in the axon hillock
What are action potentials?
Very brief, large depolarizations that travel for long distances through a neuron without losing strength. Rapid signals over long distances
Describe the movements of Na+ and K+ across the membrane during action potentials
Rising phase (depolarization)
-Depolarizing stimuli open voltage gated Na+ channels (-55mV), allow Na+ to travel down electrochemical gradient
-At around +30mV Na+ channels inactivate
Falling phase (repolarization)
-Voltage gated K+ channels also open in response to depolarization, but do so more slowly than Na+ channels causing delayed efflux
After-hyperpolarization phase (undershoot)
-Voltage gated K+ do not immediately close when reaching -70mV causing membrane potential to dip below the resting membrane potential
-Leak channels bring membrane potential back to -70mV
-Na-K ATPase returns ions to original compartments (this does not need to happen before another AP can be triggered)
How do voltage gated Na+ channels suddenly close at the peak of an action potential?
What are refractory periods and their purpose?
-Ensure an AP travels in one direction
-Limits the rate at which signals can be transmitted down a neuron
Explain how action potentials are conducted (propogated)
What are the two phsical parameters that determine the velocity of action potentials?
The diameter of the axon
-a larger diameter axon will offer less internal resistance to current flow
-More ions will flow in a given time, bringing adjacent regions of the membrane to threshold faster
The resistance of the axon membrane to ion leakage
-Current will spread to adjacent sections more rapidly if it is not lost via leak channels (myelin)
Explain how myelinated axons rapidly increase conduction velocity
Explain how the concentration gradient for K+ is affected in normokalemia, hyperkalemia and hypokalemia
Explain the secretion of chemicals in neurons
Explain electrical synapses in cell-to-cell communication
Explain chemical synapse in cell to cell communication
Describe Ionotropic receptors
Describe metabotropic receptors (G-protein coupled receptors)
Describe phospholipase C signal transduction pathway when metabotropic receptors interact with a membrane bound enzyme
How do metabotropic receptors interact directly with ion channels
Describe Adenlyl cyclase signal transduction pathway when metabotropic receptros interact with a membrane bound enzyme
Describe the process of neurotransmitter release
Explain the process of neurotransmitter termination
How does a stronger stimuli effect neurotransmitters
What is convergence and divergence?
Convergence:
-many presynaptic neurons may converge on one or a small number of postsynaptic neurons
Divergence:
-neurons can have branching axons that contact many different postsynaptic neurons
What is spatial summation in synaptic signaling?
How can spatial summation create synaptic inhibition?
What is temporal summation in synaptic signaling?
What is presynaptic modulation?
Describe global presynaptic inhibition
Describe selective presynaptic inhibition
What is postsynaptic modulation
