Exam 3 Neuro IV Flashcards
Much of overt behavior is organized at the ___ level, with the ____ structures sending command signals to the ___, where much of the behavior is worked out.
Sensory signals enter via ___ & branch; one branch goes into ____ to evoke local reflexes while the other ascends as a ___.
Describe motor neurons and their fibers.
Much of overt behavior is organized at the cord level, with the upper cortical structures sending command signals to the cord and much of the behavior is worked out at the cord.
Organization at cord level: sensory signals enter via dorsal root (posterior root) & branch; one branch goes into gray matter to evoke local reflexes while the other ascends as a sensory tract.
Motor neurons are alpha or gamma MN. Alpha = large type Aα fibers (often branch several times before innervating a large muscle). The alpha MN + the muscles it innervates = the “motor unit”.
Gamma MN = smaller Aγ fibers, on their way to a muscle spindle. spindle and golgi tendon organs provide sensory information about the position of/tension on the muscle.
Muscle Spindle: Built around _____ imbedded into an ___ muscle fiber. These are small fibers with no actin/myosin filaments in the middle so only the ends can contract. Contractile ends innervated by ____, originating from type ___ motor neurons. Extrafusal fiber innervated by ____.
Stretch Receptor; in middle of fiber, excited by stretch of middle, either by stretch of whole muscle or by contraction of intrafusal fiber ends. Explain its 2 types of intrafusal fibers. Explain its 2 types of receptor endings.
Maintaining control of muscles requires receiving info about their position and condition & local reflexive circuits which co-ordinate and protect muscles. Most infor comes from muscle spindles and golgi tendon organs.
Muscle Spindle: Built around intrafusal muscle fibers imbedded into an extrafusal muscle fiber (no actin/myosin filaments in the middle so only the ends can contract). Contractile ends innervated by gamma nerve fibers, originating from type Aγ motor neurons. Extrafusal fiber innervated by Aα motor neuron.
Stretch Receptor; in middle of fiber, excited by stretch of middle, either by stretch of whole muscle or by contraction of intrafusal fiber ends.
2 types of receptor endings:
-Primary ending (annulospiral ending): type Ia fiber, very fast.
-Secondary ending: smaller type II fiber, on one or both sides of primary ending.
2 types of intrafusal fibers:
-Nuclear bag muscle fibers; nuclei cluster to form ‘bag’- receives primary and secondary afferents.
-Nuclear chain fibers; smaller, nuclei align in chain- receives only secondary afferents.
Slow stretch vs fast or sudden stretch responses?
Slow stretch excites both primary and secondary “static response”, γ static (γs) excites nuclear chain fibers.
fast or sudden stretch excites primary afferents, “dynamic response”, γ dynamics (γd)-excites nuclear bag fibers.
The Role of γ Motor Neuron in Regulation of Muscle Spindles: Sensitivity or “gain” can be adjusted by ?
A. What happens if α motor neuron is stimulated without activating γ motor neuron?
B. If both α and γ are activated together?
The Role of γ Motor Neuron in Regulation of Muscle Spindles: Sensitivity or “gain” can be adjusted by contracting or relaxing the intrafusal muscle fiber.
A. If α motor neuron is stimulated without activating γ motor neuron, the spindle becomes slack and Ia output is reduced.
B. If both α and γ are activated, there is no decrease in Ia firing. γ motor neuron controls ‘gain’ of muscle spindle.
What/where is the GTO? By what is it innervated? These are ‘in series’ with muscle, as versus spindles which are in parallel with extrafusal muscle fibers. GTO sensitive to?
Spindles detect muscle ____, while GTO detects ____. Has both dynamic and static response.
GTO is another sensory receptor important in regulation of reflexes. GTO is ‘encapsulated’ nerve endings at junction of muscle and tendon. Each is innervated by Ib sensory axon. These are ‘in series’ with muscle, as versus spindles which are in parallel with extrafusal muscle fibers. GTO sensitive to active contraction of muscle, which puts stress on tendon.
Spindles detect muscle length and changes of length, GTO muscle tension. Has both dynamic and static response.
Explain GTO output and its feedback mechanism.
GTO output is through type Ib fibers, very fast, synapse on local circuits at level of cord, also synapse on fibers going to cerebrum and cerebellum.
Negative feedback: GTO sensing tension causes negative feedback reflex- will reduce contraction in muscle experiencing too much tension, will contract opposing muscle. Protective mechanism.
explain the 3 sections of the motor cortex.
The motor cortex is divided into the 1) primary motor cortex, 2) premotor area, and 3) supplementary motor area.
Primary motor cortex: generates discrete movement patterns, topographical representations of different muscle areas of the body are marked on the different areas of the motor cortex. As with the sensory systems, the finer the control give area requires, the more cortical real estate is dedicated to that area.
Premotor area: anterior to the primary motor cortex. organized in topographical manner similar to primary motor cortex. Nerve signals here generate “patterns” of movements “motor image”- while the primary motor cortex’s discrete movement patterns are generated. Info from premotor area may go through basal ganglia, thalamus, and primary motor cortex.
Supplementary motor area: Also topographically organized, lies along longitudinal fissure. Stimulation here may cause bilateral movements involved in ‘body-wide’ attitudinal movements.
Specialized areas: motor regions involved in head rotation, eye movements, speech.
Motor signals are transmitted from where to where?
_____ is the most important pathway for motor commands. 30% starts in the____, 30% in the ____, and 40% in the ____.
Motor signals are transmitted from the cortex to the spinal cord via corticospinal tract, and indirectly through the multiple accessory pathways via the basal ganglia, cerebellum, and brain stem nuclei.
Corticospinal (pyramidal) tract is the most important pathway for motor commands. 30% starts in the primary motor area, 30% in the premotor & supplementary motor area, and 40% from somatosensory areas posterior to the central sulcus.
contrast the lateral corticospinal tract with the ventral corticospinal tract
Lateral corticospinal tract:
1) 3/4 of axons.
2) cross in the medulla.
3) originate in the cortex that represents LIMBS.
4) terminate on the lateral a-MN.
Ventral Corticospinal Tract:
1) 1/4 of axons.
2) axons do not cross.
3) originate in the cortex that represents neck, shoulders, trunk.
4) terminate on the medial a-MN.
What did Hitzig, Fritsch, Hughlings-Jackson, Sherrington, and Penfield each contribute to neuroscience?
Hitzig - eye movemnts evoked by hitting back of brain of wounded soldiers.
Fritsch - stim. live dogs brains, stim. parts of the cortex caused contralateral movements.
Hughlings-Jackson - wife w/ epilepsy, noticed motor cortex had a “map” of the body, observed epileptic seizures progressed systematically around the body.
Sherrington - used electronic stim. to map the body representation of the motor cortex in apes.
Penfield - mapped motor and sensory cortexes, some areas evoked vidi memories when stim. in epilepsy patients.
Brain ste parts? (3) Functions? (6).
What two groups make up the reticular nucleus? Explain them.
What role do the vestibular nuclei play? (3) Which tract does it use to send its signals?
Brain stem: medulla, pons and mesencephalon. Thought of as an extension of the spinal cord into the cranial cavity, or also ‘the old brain’. Functions: 1) respiration, 2) cardiovascular control, 3) partial control of GI functions, 4) some stereotypic movements, 5) equilibrium, 6) eye movements
Reticular N - 2 groups: PONTINE reticular nuclei and MEDULLARY reticular nuclei. Pontine excites antigravity muscles, medulary relaxes these muscles. Excitatory signals are sent down pontine reticulospinal tract (in column of spinal cord) and terminate on medial motor neurons that excite axial muscles (vertibral column and extensor muscles).
Medullary reticular N send inhibitory signals to same muscles via medullary reticulaspinal tract.
Vestibular N: work with reticular system to control antigravity muscles, maintain balance. Send excitatory signals via medial and lateral vestibulospinal tracts.
What is decerebrate rigidity?
What will happen if an animal’s brain stem is sectioned below mesenchephalon, but leaves the reticular and vestibular system intact? why?
How is decerebrate posture characterized?
In severe cases, the back is acutely arched (opisthotonos), this sign indicates what?
Decerebrate rigidity: If an animal’s brain stem is sectioned below mesenchephalon, but leaves the reticular and vestibular system nuclei intact, the animal develops a rigidity in antigravity muscles due to the loss of signals to medullary reticular nuclei. These N become nonfunctional and pontine N become uninhibited and overactive.
Decerebrate posture is characterized by adduction (internal rotation), extension of arms w/ the wrists pronated, fingers flexed, legs stiffly extended, with plantar flexion of the feet. In severe cases, the back is acutely arched (opisthotonos).
This sign indicates upper brain stem damage, may result from primary lesions eg infarction, hemorrhage, tumor; metabolic encephalopathy; head injury; or brain stem compression/increased intracranial pressure (ICP).
The vestibular apparatus is located where and comprised of what (4)? What do they do?
The vestibular apparatus located in the “bony labyrinth” of temporal bone. Made of cochlea, three semicircular canals, and the utricle and saccule.
The cochlea is responsible for sound sensitivity.
Inside the utricle and saccule hair cells project into statoconia (gelatinous material with calcium carbonate crystals).
Explain how the vestibular nucleui control antigravity muscles and maintain balance.
Similar to cochlear hair cells, movement of stereocilia TOWARD the tall kinocilium OPENS mechanically gated K+ channels in stereocilia to depolarize the hair cell, opening voltage-gated Ca2+ channels, causing transmitter release onto vestibular nerve fibers.
Movement of stereocilia AWAY from kinocilium CLOSES apical K+ channels, but OPENS K+ in basolateral hair cell, causing a hyperpolarizing of the hair cell.
Contrast how the hair cell bundle have specific orientation in the ampulla, hair cells vs hair cells in the utricle and saccule.
Hair cell bundles have specific orientation allowing response to movement in all directions. In ampulla, hair cells are polarized in same direction. In utricle and saccule, the striola divides hair cells into two populations with opposing polarities.
