Exam II Flashcards
Action potentials would never begin if it weren’t for ONE major difference between the response of voltage-gated sodium and potassium channels to depolarization – what is that difference? Note – name only ONE difference – no credit will be given for answers which list all of the differences!
K+ channels open with a delay. In excitable cells, such as neurons, the delayed counterflow of potassium ions shapes the action potential. The best-known flow of K+ is the outward current following depolarization of the membrane. This occurs through the delayed rectifier channel, which, activated by the influx of Na+, counteracts the effect of that cation by allowing the discharge of K+. By repolarizing the membrane in this way, the channel restricts the duration of the nerve impulse and participates in the regulation of repetitive firing of the neuron.
What electrical property of myelin allows it to speed up action potentials
The resistance of myelin. By acting as an electrical insulator, myelin greatly speeds up action potential conduction.
Why do action potentials generally not begin in the dendrites of a neuron, but rather in the cell body or axon hillock?
Voltage gated Na+ channels are not usually in the dendrites.
What did the discovery of miniature excitatory postsynaptic potentials tell researchers about the mechanism of neurotransmitter release?
This told researchers about how neurotransmitters are released in vesicles.
In what parts of a neuron are peptide neurotransmitters and small molecule amino acid neurotransmitters generally synthesized?
Small molecule neurotransmitters are generally synthesized in the synaptic terminal while peptide neurotransmitters are generally synthesized in the cell body.
Briefly describe an experiment that would map the receptive field of a somatosensory neuron in the cortex which displays an antagonistic center-surround form. What apparatus would you use, how would you use it, what results would you obtain and how would you interpret them?
(1) Record from a single cell in S1.
(2) Stimulate area of skin.
(3) Observe _ in action potential frequency in S1 cell at center of receptive field.
(4) Observe _ in action potential frequency at edge of receptive field.
In the spinal cord, neuronal circuits exist which might explain how the brain is able to suppress the sensation of pain. The neurotransmitter that mediates this suppression is enkephalin. On which cell does enkephalin act to suppress pain transmission?
The sensory receptor neuron (nocioceptor). It does so by presynaptic inhibition.
Fill in the blank: Phantom Limb Syndrome is an example of the brains use of
__________________ lines to perceive sensory information
Phantom Limb Syndrome is an example of the brains use of LABELLED lines to perceive sensory information.
Describe THREE differences between rod and cone photoreceptors
(1) Speed (rods have a slow response; cones have a fast response)
(2) Number of opsins expressed (color sensitivity)
(3) Size (rods are longer than cones)
(4) More cones in fovea and more rods in periphery
Briefly describe the response of horizontal cells when light hits the retina and how they communicate this response to other neurons in the retina.
The horizontal cells hyperpolarize. They inhibit neighboring photoreceptors:
(1) Illumination -> (2) Center photoreceptor hyperpolarization -> (3) Horizontal cell hyperpolarization -> (4) Surround photoreceptor depolarization
Describe TWO ways in which the receptive fields of retinal ganglion cells differ from the receptive fields of neurons in layers 1-3 and 5-6 of the primary visual cortex?
(1) V1 neurons are sensitive to bars, not spots
(2) V1 neurons can be binocular
(3) V1 neurons have orientation selectivity
Describe the loss of vision in each eye that would result from complete destruction of the fibers that travel from the LGN to the left visual cortex. Use diagrams if you wish
Loss of visual field seen by left eye (nasal side): right hand side of left eye visual field is lost. This is because contralateral side is affected. The LEFT visual cortex is for RIGHT hand vision. So the nasal side of the left eye is affected which is the right hand side of vision.
Loss of visual field seen by the right eye (temporal side): right hand side of right eye visual field would be lost. This is because the ipsilateral side is affected. So the temporal side of the right eye is affected which is the right hand side of vision.
Describe the difference(s) in function between neurons in the dorsal and ventral visual pathways of the cortex (i.e. what do the neurons in each pathway analyze?).
Dorsal Pathway (V1 -> V2 -> MT) detects motion–“where” things are.
Ventral Pathway (V1 -> V2 -> V4) detects form and color–“what” things are.
What is the function of the three bones of the middle ear?
To transmit vibration of tympanic membrane to the cochlea or oval window by acting as levers.
What is meant by a “tonotopic” organization of the properties of neurons in the auditory cortex?
Neurons are arranged in order of the frequencies that they are most sensitive to.
Describe how the displacement of the hair bundle of a receptor cell in the cochlea results in a change in its membrane potential
Tip link stretches when bundle is moved which opens channel and so K+ enters and the cell depolarizes.
What makes up a “motor unit”?
One motor neuron and all the muscle fibers that it innervates.
What two diseases indicate that damage to the basal ganglia results in abnormal motor planning and execution
Huntingtons Disease and Parkinsons Disease.
In what structure outside of the cerebral cortex do you find somatotopic maps of the body surface
Cerebellum.
You obtain the sequence of an unusual channel that is found in the postsynaptic membrane of a cell that you are studying. . Your colleague reviews the channel’s sequence and she makes two predictions…….
a) That this is an excitatory neurotransmitter-gated channel activated by glutamate.
b) That the channel will be blocked by intracellular calcium ions.
Describe how you would test both of your colleague’s predictions using patch clamp technology. Make sure that you address the following explicitly in your answer.
a) What configurations of the patch clamp would you use? (it may take more than one to provide a full test of both predictions)
b) What would be in the solutions that are inside or outside the patch for each test, and what Vm would you clamp at?
c) Sketch the recordings of current expected for each experiment – as well as an interpretation in terms of how the recordings might confirm the predictions. Be sure to label any graphs of the electrical recordings
(A) “Outside Out” Configuration and “Inside Out” Configuration. They both will be clamped at Vrest or Vthreshold. For the inside-out configuration, glutamate will be inside the pipette. For the outside-out configuration, glutamate will be outside the pipette.
(B) High Na+ and Low K+ in the extracellular fluid in bath (so outside of patch). Low Na+ and High K+ in the intracellular fluid in pipette (inside of patch). No calcium inside patch.
(C) When you add glutamate to bath, it opens the channels and you observe blips in the inward current. This shows excitation. When you add Ca2+ to the bath, the channels close and there are no blips in the inward current.
Action Potentials vs Graded Potentials
Graded potentials decay with distance because current leaks out of axon. Action potentials regenerate because of active and passive current flow.
Speed of action potentials in myelinated vs unmyelinated fibers
The speed of action potential conduction is much slower in unmyelinated vs myelinated axons because in advance of an action potential, passive current instantaneously spreads. Myelin insulates the axonal membrane, reducing leakage of passive current and reducing membrane capacitance. In unmyelinated axons, channels have to be open at every point of the axon.
Whole cell recording
The membrane patch within a pipette is disrupted by briefly applying strong suction so that the interior of the pipette is continuous with the cell cytoplasm to measure electrical potential and currents from the entire cell.
Inside-out Patch
The tip of the pipette is exposed to air so that the vesicle opens to yield a small patch of membrane with its former intracellular surface exposed.
ADVANTAGE: measurement of single channel currents with the ability to change medium of intracellular surface.
Outside-out Patch
The pipette is retracted while still in whole-cell configuration so that a membrane patch is produced that has the extracellular surface exposed.
ADVANTAGE: measurement of channel activity due to influence of extracellular chemical signals.
3 Major Structural Features of Channels
(1) Pore through which ions travel
(2) Selectivity filter in the pore which only allows certain ions to move through the channel
(3) Gate which opens or closes the pore in response to some stimulus
Electrical synapse
Gap junction proteins (connexons) connect the pre-synaptic and post-synaptic membranes. There is direct flow of electrical curent and small molecules occurs between the cells due to cytoplasmic connectivity. They are only excitatory and cannot be amplified. No neurotransmitters involved.
Chemical synapse
No contact occurs between pre-synaptic and post-synaptic membranes. No direct flow of electrical current occurs between the cells. Instead, transmission occurs via the release of neurotransmitters by the pre-synaptic cell to activate ion channels in the membrane of the post-synaptic cell. They can be excitatory, inhibitory, or amplified.
Excitatory Synapse
Sodium or calcium selective. Postsynaptic resting membrane potential rises beyond threshold.
Inhibitory Synapse
Potassium or chlorine selective. Postsynaptic resting membrane potential falls below threshold to suppress action potentials.
2 Types of Glutamate Receptors
(1) AMPA receptor is ionotropic so it binds glutamate and triggers conformational change to open ion channel so sodium can flow inside.
(2) mGluR receptor is metabotropic so it is a GPCR: upon ligand binding, G protein binds GTP after changing conformation and triggers alpha subunit to be released to bind to other protein to release signal.
Kinase
Kinase is an enzyme that mediates phosphorylated. By phosphorylating, a negative charge causes conformational change.
Hormones
Hormones are permeable and so their receptors are in cytosol. Upon ligand binding, hormone receptor goes to nucleus and binds to DNA to turn on gene expression
NMDA Receptor Opening
NMDA is a calcium channel. Glutamate binding isn’t enough to open it as a magnesium ion is blocking it. Once the membrane is depolarized, magnesium will come off to open the channel. NMDA triggers intracellular signaling through calcium.
First, Second and Third Order Neurons in the Somatosensory System
First Order Neurons: Mechanosensory receptor neuron with a cell body in the dorsal root ganglion of the spinal cord and whose axon travels up the spinal cord to make excitatory contact with a Second Order Neuron.
Second Order Neurons: Mechanosensory cell with dendrites and cell body in the medulla who sends an axon that crosses the spinal cord and terminates in the contralateral VPN (ventral posterior nuclear) Thalamus to make a connection with a Third Order Neuron
Third Order Neuron: Mechanosensory cell with dendrites and cell body in VPNA Thalamus whose axon terminates in the postcentral gyrus of the cerebral cortex.
Dynamic Range
Range of stimulus strength within which the action potential activity reflects stimulus strength
Importance of adaptation
Adaptation represents important part of processing sensory information as it tunes some receptors to rapidly changing high frequency stimuli and reduces impact of unchanging signals that are usually less informative.
Proprioceptive Sensory Neurons
Proprioceptors have their dendrites in sensory structures in muscles, joints, and tendons. They send axons via dorsal root ganglion to the spinal cord where they brain to synapse with neurons in the spinal cord as well as travelling up to the medulla, thalamus and somatosensory cortex. They inform the brain about position, direction and velocity of skeletal muscle movements. They also control activity of motor neurons in feedback reflexes.
Somatotopic Map
Sensory neurons within each dermatome of the skin surface project to a particular area in the somatosensory cortex and the sensory neurons with dendritic fields in neighboring areas of the skin surface project to neighboring areas of the cortical surface which creates a somatotopic map of the body surface to devote more cortical surface of the skin with greater receptor density.
Columnar Organization of Somatosensory Cortex
Within each little section of the cerebral cortex are columns of neurons with overlapping receptive fields arranged perpendicular to the cortical surface. Each column of neurons tends to respond to similar kind of sensory stimulus, grouped to create little columnar module that can process information from an individual part.
Nociceptive Pathway
Painful stimulus –> Dorsal Root Ganglion –> Dorsal Horn –> Brainstem (Pons & Medulla) –> Midbrain (Periaqueductal gray) –> Thalamus (VPN) –> Somatosensory Cortex, Limbic System & Cingulate Cortex.
Fast Pain
Fast pain travels via type Aδ fibers to terminate in the dorsal horn of the spinal cord. The axons of these neurons cross the midline and ascend contralaterally along the anterolateral system. These fibres terminate on the thalamus and synapse with the dendrites of the somatosensory cortex.
NOTE: Fast myelinated fibers
Slow Pain
Slow pain is transmitted via slower type C fibers. Impulses are then transmitted to nerve fibers that terminate in the dorsal horn, synapsing with neurons that join fibers from the fast pathway, crossing to the opposite side and traveling upwards through the anterolateral pathway. These neurons terminate throughout the brain stem, with fibres stopping in the thalamus and the rest stopping in the medulla, pons and periaqueductal grey of the midbrain tectum.
Contralateral side of hemisection of spinal cord
Reduced sensation of temperature and pain
Ipsilateral side of hemisection of spinal cord
Reduced sensation of discrimination, vibration, and proprioception
Modulating pain signals
Descending systems can modulate transmission of ascending pain signals via release of opiate peptides such as encephalin released by periaqueductal gray stimulated by hypothalamus, amygdala and somatosensory cortex.
Labelled Lines
Neurons in CNS pathways are “labelled” in terms of the perceptions they evoke. Labelled lines are response for phantom limb syndrome: continued spontaneous activity of central neurons that normally would have received information from that limb.
Why can the innermost segment be the photoreceptor cells in the retina?
This works because the layers of the retina are transparent and the visual systems ignores stationary shadows cast by blood vessels and nerves overlying the photoreceptors.
What happens to cGMP levels when it is dark and light in the outer segments of photoreceptors?
When it is dark, the outer segments brings in calcium and sodium so the cGMP levels are high to bind to the channels to keep them open. When it is light, the cGMP levels drop so that hyperpolarizaiton occurs.
Retinal Ganglion Cells Receptive Fields
Receptive fields are always circular which is the area of the retina within which light elicits a response from the cell.
On Center Ganglion Cells
They respond to illumination of the center of the receptive field with an increase in action potential frequency. They express metabotropic receptors which closes sodium/calcium channels when glutamate binds to it.
Off Center Ganglion Cells
They respond to illumination of the center of the receptive field with a decrease in action potential frequency. They express ionotropic receptors which opens when glutamate binds to it.
Significance of receptive fields in retinal ganglion cells
Retinal ganglion cells are maximally stimulated by edges and lines which allow more bandwidth to be allocated to edges and lines during transmission of information.
P Ganglion Cells
Display sustained responses that display color opponent center surround receptor fields. They detect color contrast between center and surround.
M Ganglion Cells
Display transient responses not dependent on color. Center and surround are driven by all cone types: they detect motion.
Fourier Analysis
Complex sounds can be thought of as being the sum of a large number of sine wave components, each having a different frequency amplitude and relative phase.
Bones of the middle ear
They act as impedance matching devices: a lever with high mechanical advantage to transfer vibrations in the atmosphere to fluid vibrations of the inner ear. Movements of the tympanic membrane due to airbone vibration are focused onto movements of the oval window and the cochlea.
Inner Hair cells vs Outer Hair cells
The inner hair cells send information toward the brain along afferent sensory axons. The outer hair cells receive information from the brain along efferent motor axons.
Basilar Membrane of the Cochlea
This contains the sensory receptor hair cells. Sound sets up vibration in oval window, setting up traveling waves in basilar membrane. Vibration moves down cochlea.
Frequency Tuning in Cochlea
Each part of the cochlea is tuned to a seperate frequency sound. The amplitude of vibration within the traveling wave is greatest in the region at a particular frequency .Regions closest to the stapes are most sensitive to higher frequency sounds. Regions farthest from stapes are most sensitive to lower frequency sounds.
Hyperpolarzation and depolarization of hair cells.
Hyperpolarization of hair cells occurs when hair cells bend away from the tallest hair cell. Depolarization occurs when the hair cells bend toward the tallest hair cells.
Phase-Locking
Below 3 kHz, the hair cell membrane potential can track the individual peaks of pressure during a sound wave so that the time intervals between action potentials produced by the auditory nerve axon can be used to determine frequency of the sound.
Parallel organization of multiple ascending pathways in the auditory system
(1) Auditory nerve enters the brainstem
(2) Branches innervate 3 subdivisions of the cochlear nucleus
(3) Cochlear nucleus neurons project axons to terminate in the ipsilateral and contralateral superior olive in the pons and the inferior colliculus in the midbrain.
(4) Inferior colliculus neurons project axons to terminate in the medial geniculate nucleus in the thalamus
(5) Medial Geniculate Neuron Thalamus neurons send axons to the primary auditory cortex in the temporal lobe.
Tonotopic organization of auditory system
Neurons are arrayed in brain nuclei and cortex in order of their most sensitive and characteristic frequencies.
Medial Superior Olive
Uses interaural time differences to calculate the direction from which sound is arriving: ipsilateral sound source takes longer to reach MSO.
Lateral Superior Olive
Uses interaural intensity differences so that if sound is intense on one side, the LSO inhibits the other side so that net excitation occurs from the side of the intense sound.
Inferior Colliculus
Information is processed here and neurons here are arrayed to create an auditory space map. Neighboring neurons are preferentially sensitive to sound coming from neighboring directions; a map based off of LSO and MSO neurons.
Medial Geniculate Nucleus Thalamus
Neurons here mediate the detection of specific and complex sound patterns
Primary Auditory Cortex
Creates a tonotopic map and cortical neurons can be binaurally stimulated so that strips of neurons are excited by one ear and not the other and these strips alternate with strips excited by both ears.
Population Coding in Olfaction
Brain learns to associate each pattern of active glomeruli with a particular odor.
Molecular mechanism of olfaction
Odorant molecule binds to receptor protein which activates G-protein to bind GTP which activates cAMP to bring in sodium and calcium to depolarize the cell.
Why can’t olfaction use the labelled line approach?
The brain cannot characterize odors using the labeled line approach. Instead, the brain must compare the output of an array of different odorant receptor neurons and label the specific pattern of activity to a given odor as indicating the presence of that odor.
Olfactory bulb targets
Olfactory bulb leads to pyriform cortex, amygdala, and such that leads to orbitofrontal cortex, thalamus, and hypothalamus as well as hippocampal formation.
Motor Cortex
Planning and initiating and directing voluntary movements.
Basal Ganglia
Act on motor cortex to properly initiate movement.
Brainstem Centers
Basic movements and postural control
Cerebellum
Acts on brainstem centers for sensory motor coordination and smooth movement.
Parkinson’s Disease
Loss of dopamingeric cells of substania nigra; causes resting tremor, shuffling gait and facial rigidity.
Huntington’s Disease
Degeneration of caudate region of basal ganglia associated with dominant autosomal mutation of gene coding for huntingtin.
How does nervous system control muscle contraction
Recruits right types of motor units for a task and varies action potential frequency within motor units to control rate of action potentials in motor neurons. This involves spinal reflexes.
Muscle Stretch
Stretch activates spindle sensory neurons which activate flexor muscle alpha motor neurons and inhibit extensor motor neurons.
