Flashcards in Medical Physiology Block 2 Week 4 Deck (18):
Understand and be able to describe the organization of local circuits, micro-circuits, and large scale neural networks in the central nervous system.
local circuit: within a local brain region, the arrangement of neurons and their synaptic connections (sensory component, interneuron, and projection motor neuron); micro-circuits: may be repeated numerous times within a local circuit (distinguishable interconnected synapses?); combinations of local circuits define brain regions
Describe the myotatic reflex.
Stretching of the tendon pulls on the muscle spindle, exciting the primary sensory afferents, which convey their information through group Ia axons. These axons make monosynaptic connections to the α motor neurons that innervate the quadriceps, resulting in the contraction of this muscle. The Ia axons also excite inhibitory interneurons that reciprocally innervate the motor neurons of the antagonist muscle of the quadriceps (the flexor), resulting in relaxation of the semitendinosus muscle. Thus, the reflex relaxation of the antagonistic muscle is polysynaptic.
Describe the golgi tendon organ reflex
Active contraction of the quadriceps muscle elicits a reflexive relaxation of this muscle and contraction of the antagonistic semitendinosus muscle: the inverse mytotatic reflex. Contraction of the muscle pulls on the tendon, which squeezes and excites the sensory endings of the Golgi tendon organ, which convey their information through group Ib axons. These axons synapse on both inhibitory and excitatory interneurons in the spinal cord. The inhibitory interneurons innervate α motor neurons to the quadriceps, relaxing this muscle. The excitatory interneurons innervate α motor neurons to the antagonistic semitendinosus muscle, contracting it. Thus, both limbs of the reflex are polysynaptic.
Understand and describe the concept of a "central pattern generator."
motor programs that operate rhythmic motor activities (walking and breathing); sensory feedback unneccessary; interconnected neuronal network coordinate timing of movements
Describe the half-center model of alternating rhythm generation in the locomotion pattern generator. Be able to describe state-dependent changes in locomotion.
Stimulation of an excitatory interneuron has two effects. First, the stimulated excitatory interneuron excites the motor neuron to the flexor (or extensor) muscle. Second, the stimulated excitatory interneuron excites an inhibitory interneuron, which inhibits the extensor (or flexor).
Be able to describe the concept of a "map" in the central nervous system.
model of a part of the world; left side of the body is represented on the right side of the brain; generally, upper somatosensory receptors are represented on the bottom of the brain map (some exceptions); b. the relative size of cortex that is devoted to each body part is correlated with the density of sensory input received from that part
Understand and be able to describe the concept of plasticity in neural maps.
Severing of a peripheral nerve causes the part of the map that normally relates to the body part served by this severed nerve to become remapped to another body part
Understand and be able to describe how local networks are modulated by ascending and descending inputs.
in the example of a sea lamprey, central pattern generators are interconnected with the spinal segment above and below the site of the local circuit (increases the efficiency of the system); communication of central pattern generators (or local circuits) with the brainstem (afferent and efferent signals)
Describe a local circuit in the brain.
consists of inputs (e.g., afferent axons from the thalamus), excitatory and inhibitory interneurons, and output neurons (e.g., pyramidal cells)
Describe the flexor reflex
A painful stimulus to the right foot elicits a reflexive flexion of the right knee and an extension of the left knee: the flexor reflex. The noxious stimulus activates nociceptor afferents, which convey their information through group Aδ axons. These axons synapse on both inhibitory and excitatory interneurons. The inhibitory interneurons that project to the right side of the spinal cord innervate α motor neurons to the quadriceps and relax this muscle. The excitatory interneurons that project to the right side of the spinal cord innervate α motor neurons to the antagonistic semitendinosus muscle and contract it. The net effect is a coordinated flexion of the right knee. Similarly, the inhibitory interneurons that project to the left side of the spinal cord innervate α motor neurons to the left semitendinosus muscle and relax this muscle. The excitatory interneurons that project to the left side of the spinal cord innervate α motor neurons to the left quadriceps and contract it. The net effect is a coordinated extension of the left knee.
How can the CNS control spinal reflexes? Give two examples of why this is beneficial to the human body
Axons descend from numerous centers within the brainstem and the cerebral cortex and terminate primarily onto the spinal interneurons; if you anticipate catching a falling ball, for a short period around the time of impact (about ±60 ms), both your stretched muscles and the antagonist muscles contract! This maneuver stiffens your arm just when you need to squeeze that ball to not drop it; With mental effort, painful stimuli can be tolerated and withdrawal reflexes suppressed.
What is a motor program?
a set of structured muscle commands that are determined by the nervous system before a movement begins and that can be sent to the muscles with the appropriate timing so that a sequence of movements occurs without any need for sensory feedback.
Can central pattern generators be influenced by sensory feedback?
yes; stretch receptors causing contraction/relaxation in sea lampreys (in this case, there are a pair of central pattern generators; one on each side of the fish)
Name a region of the CNS that has a map for vision, hearing, sensation, and motor control.
Describe the organization of the primary visual cortex (layer IV; II & III)
ocular dominance columns (layer IV) (zebra-like stripes; projections from each eye are adjacent); layers II and III have structures called cytochrome oxidase blobs (polka-dot) (some blobs are sensitive to color; others, motion)
Describe the homonculus that represents somatosensory and motor mapping
The legs are hooked over the top of the postcentral gyrus and dangling into the medial cortex between the hemispheres, and the trunk, upper limbs, and head are draped over the lateral aspect of the postcentral gyrus; important to remember that the representation of the hand separates those of the head and face; mouth, tongue and fingers have very large representation
Describe sound detection
The brain localizes the source of the sound in the vertical plane by detecting differences in the combined sounds from the direct and reflected pathways (vertical plane); For frequencies between 2 kHz and 20 kHz, the CNS detects the ear-to-ear intensity (loudness) difference; For frequencies below 2 kHz, the CNS detects the ear-to-ear delay (Only when action potentials from the right and left sides arrive at the MSO neuron simultaneously does the neuron fire an action potential; Each MSO neuron is tuned to a different interaural delay and a different sound locale along the horizontal axis)