Unit 2 Week 3 Flashcards
What is the retinofugal projection?
the visual pathways from the eye to the brain
Retina to optic nerve to optic chiasm to optic tract ot LGN to optic radiation to primary visual cortex (V1)
What is Decussation?
The crossing of a fiber bundle from one side to the other side
Where do ganglion cell axons from the nasal retina cross?
optic chiasm
What is the visual pathway that mediates conscious visual perception?
Retina to LGN to Primary Visual Cortex (V1)
left hemifield projects to right side of brain
right hemifield projects to left side of brain
What is Retinotopy?
position on retina projects to position along LGN and primary visual cortex
the topographic organization of visual pathway in which neighboring cells in the retinal send information to neighboring cells in a target brain structure
What is the purpose of a sensory map for visual pathway?
provide spatial information of the visual field
How can we tell where neurons send their information to?
anterograde tracing
What is anterograde tracing?
identify neural connections from their source (cell body) to their point of termination (the synapse)
How can we tell where neurons receive their information from?
retrograde tracing
What is retrograde tracing?
to identify neural connections from their point of termination (the synapse) to their source (the cell body)
Where do RGC neurons from both eyes send their projections in the brain?
all layers of the LGN, the neurons are monocular
What does it mean if a neurons is monocular?
each LGN neuron only responds to light in one eye
What are the cell types of retinal ganglion cells (RGCs)?
M-type RGCs (larger)
P-type TGCs (smaller)
nonM-nonP RGCs
How are the different layers within the LGN organized?
inputs segregated by eye and ganglion cell type
Describe the receptive fields of Receptive Field LGN neurons
Receptive fields of LGN neurons: almost identical to the ganglion cells that feed them
-ON-center/surround
-OFF-center/ON-surround
Describe the receptive fields of Magnocellular LGN neurons
Magnocellular LGN neurons: large center-surround receptive field with transient response
Describe the receptive fields of Parvocellular LGN neurons
small center-surround receptive fields with sustained response
What is another name for the primary visual cortex?
striate cortex
Where do RGC neurons from both eyes send their projections in the brain?
studied with transneuronal autoradiography from ratina, to LGN, to striate cortex, using anterograde tracing
Where do most inputs from LGNs go in the striate cortex? Give examples
layer 4
Magnocellular LGN neurons project primarily to layer IVC(alpha)
Parvocellular LGN neurons project to layer IVC(beta)
Where do few LGN neurons go in the striate cortex?
Give examples.
layer 2/3
Koniocellular LGN axons make synapses primarily in layers 2 and 3
What are ocular dominance columns?
eye-specific LGN projections into layer 4 of striate cortex- not in layers like in LGN, but in cortex along surface
Where are most binocular neurons found?
layer III of striate cortex
Describe the physiology of monocular receptive fields of the striate cortex
Layer IVC: similar to LGN cells
One receptive field- respond to light only in one eye (receive inputs from only one eye)
Describe the physiology of binocular receptive fields of the striate cortex
Most neurons in layers superficial to IVC are binocular
Two receptive fields- respond to light in both eyes (receive inputs from both eyes)
Describe the orientation selectivity of many neurons outside of layer IVC in striate cortx
neuron fires action potentials depending on the orientation of the bar of light in the visual field
important for the analysis of object shape
Describe the direction selectivity of many neurons outside of layer IVC in striate cortex
neuron fires action potentials in direction-dependent response to moving bar of light
important for the analysis of object motion
What are simple cells?
bionocular, orientation-selective, elongated ON or OFF area flanked with antagonistic surround
Describe simple cell receptive fields
receptive field may be composed of three lGN inputs from cells with aligned center-surround receptive fields
What are complex cells? Describe their receptive fields
binocular, orientation-selective, ON and OFF responses to the bar of light but unlike simple cells, no distinct ON and OFF regions
What is Cytochrome Oxidate?
mitochondrial enzyme used for cell metabolism
What are blobs?
cytochrome oxidate- stained pillar in striate cortex
Descrive cytochrome oxidate blobs
each blob centered on an ocular dominance column in layer IV
receive koniocellular inputs from LGN
Describe blob receptive fiels
monocular
no direction selectivity
likely orientation selectivity
specialized for the analysis of object color
Describe the cortical module of the striate cortex
each module capable of analyzing every aspect of a potion of the visual field
What are the two functional ends of the nervous sytem?
sensory system and motor system
What is the relationship between a flexor and an extensor?
antagonist
What is the relationship between two flexors?
synergist
describe the hierarchical organization of movement control
-motor systems are complex
-we only discuss the motor control from the spinal cord
-addition controls from other parts of motor system (complex coordinated movements)
Where do the lower motor neurons lie?
ventral horn
What is a Neuromuscular Junction (NMJ)?
chemical synapses between motor neurons and muscles
Describe the relationship between muscles, muscle fibers, and myofibrils
myofibrils make up muscle fibers which make up muscles
What makes up a myofibril?
Z-line, thing filaments, thick filaments, and sarcomere
What is the molecular basis of muscle contraction?
Ca2+ binding to troponin allows myosin heads to bring to actin - myosin heads then pivot, causing filaments to slide
Describe the structure of skeletal muscle
-muscle is a bundle of muscle fibers
-a muscle fiber is a bundle of myofibrils
-myofibrils are composed of segments called sarcomeres
-sarcomeres are composed of thin (actin) and thick (myosin) filaments
Describe the steps in excitation-contraction coupling
Excitation
-motor neurons are excited and release ACh
-ACh produced large EPSPs in muscle fibers
-EPSP evokes muscle action potential
-Action potential triggers Ca2+ release from sarcoplasmic reticulum in muscle
Contraction
-Ca2+ bind to troponin, myosin filaments slide along actin filaments
Relaxation
-EPSPs ends, the sarcolemma return to resting potential
-Ca2+ reuptake by sarcoplasmic reticulum
What is a motor unit?
one motor neuron and all muscle fibers it innervates (3-1000 muscle fibers per motor unit)
What are the three types of motor units?
Fast fatigable
Fast fatigue resistant
Slow
What is a motor neuron pool?
all the alpha motor neurons that innervate a single muscle
Describe graded control of muscle contraction by alpha motor neurons
-single action potential in the alpha motor neuron generate a muscle twitch
-increased firing rate of alpha motor neurons generates sustained muscle contraction
How is muscle force controlled?
-firing rate of motor units
-recruitment of motor units
-size principle
smallest motor units are recruited first
Describe the two types of lower motor neurons
Alpha motor neurons
-directly trigger muscle contraction to generate force
-synapse to normal muscle fibers
Gamma motor neurons
-indirectly modulate force generation
-synapse to modified muscle fibers (intrafusal fibers)
What are the inputs to alpha motor neurons?
-input from spinal interneurons
-sensory input from muscle spindles
-input from upper motor neurons in the brain
Describe the sensory feedback of muscle
-when muscle is pulled on, it tends to pull back, which requires sensory feedback
-proprioception
-muscles use muscle spindles and la axons to detect the change of muscle length
What is proprioception?
“body sense”, which informs us about how our body is positioned and moving in the space
What are muscle spindles?
special fibers innervated by la axons
also called stretch receptors
provide sensory feedback to muscle
an example of proprioceptors
What does the firing rate of la axons depend on?
muscle length
Name two examples of the stretch reflex
antigravity feedback loop
the knee-jerk reflex
Describe Gamma motor neurons
-innervate the two poles of intrafusal fibers inside muscle spindle
-additional control of alpha motor neurons and muscle contraction
What is the gamma loop?
the circuit
gamma motor neuron to intrafusal muscle fiber to la afferent axon to alpha motor neurons to extrafusal muscle fiber
What does the activation of alpha motor neuron lead to?
decrease la activity
What does the activation of gamma motor neuron lead to?
increase la activity
What are golgi tendon organs?
additional proprioceptive input - acts like strain guage - monitors muscle tension
muscle tension activates lb sensory axons
What is the circuit for the golgi tendon organ?
-innervated by lb sensory axons
-excite an inhibitory neuron to inhibit the alpha motor neuron of the same muscle
-Function: to regulate the muscle tension within optimal range
What is the sensory feedback on tension?
muscle contraction, increased tension of golgi tendon organ, firing of the lb axons
Describe how anatomical arrangement is correlated with function
muscle spindles in parallel with fibers control muscle length
golgi tendon organs in series with fibers control muscle tension
What is reciprical inhibition?
contraction of one muscle set accompanied by relaxation of antagonist muscle
Describe the circuitry of the flexor withdrawal reflex
-complex reflex arc used to withdraw limb from aversive stimulus
-poly-synaptic
-excitatory input from interneurons
-activate flexor muscles (and inhibit extensor muscles, not shown here) on same leg
Describe the circuitry of the crossed-extensor reflex
-one limb flexes, while the other limb extends
-activation of extensor muscles and inhibition of flexors on opposite side
Describe the spinal control of motor units
Central pattern generators
-circuits that give rise to rhythmic motor activity
Example: spinal motor programs for walking
-circuitry for walking resides in spinal cord
-rhythmic activity in spinal interneurons
-coordination depends on multiple mechanisms
Describe the cycle of intrinsic activity in a spinal interneuron
Glu binds to NMDA receptor, depolarization, Ca2+ - activated K+ channel, Hyperpolarization, Mg2+ clogs the NMDA, Ca2+ falls and K+ channels close, reset the membrane potential, repeat
What causes rhythmic pattern?
reciprocal inhibition within a circuit
Describe the hierarchical organization of movement control
-motor systems are complex
-we only discuss the motor control from the spinal cord
-addition controls from other parts of the nervous system result in complex coordinated movements
