Exam 2: Neurophysiology Part 1 - Muscle Receptors and Central Control of Movement Flashcards
What is a reflex
Involuntary, quantitatively unvarying response of the nervous system to a stimulus
Components of a reflex arc (5)
- Receptor
- Sensory Neuron
- One or more synapses (interneurons) in the CNS (polysynaptic or monosynaptic reflexes)
- Motor Neuron
- Target Organ
What do sensory receptors collect and what do they turn them into
Environmental signals
Turn them into a cellular response (Action Potential)
What is a primary receptor
A neuron with a peripheral ending specialized for stimulus transduction
Ex - retinal photoreceptors
What is a secondary receptor
A non-neuronal cell designed for stimulus transduction that affects an adjacent neuron
Ex - hair cell in inner ear
3 different types of reflexes
Segmental
Intersegmental
Suprasegmental
Segmental reflex arc
Occurs in small segment of spinal cord
Info doesn’t travel to brain - spinal cord deals with info and sends signal to organ
Ex - knee jerk reflex, pupillary light reflex
Intersegmental reflex
Info travels to other parts of the CNS - upper spinal cord
Ex - cutaneous trunci reflex
Suprasegmental reflex
Requires connection with the brain
Ex - vestibulospinal reflex, proprioceptive positioning reflex
How does CNS control body movement (3)
- Assessing effect of gravity on body muscles
- Determining initial position of body parts to be moved
- Detecting any discrepancy between the intended movement and what actually occurred
How does CNS get info about movement
Muscle stretch receptors
Types of muscle stretch receptors
Muscle spindle
Golgi tendon organ
What type of fibers are in a muscle spindle receptor
Intrafusal fibers
How are fibers in the muscle spindle receptor arranged
Intrafusal fibers are arranged in parallel with the extrafusal muscle fibers
What does the muscle spindle receptor detect
Muscle stretching
How are fibers in the Golgi tendon organ arranged
Arranged in series with extrafusal muscle fibers located in the tendon of muscles
What does Golgi tendon organ detect
Muscle tension produced by contraction
What types of neurons innervate the muscle spindle receptor
Sensory - Ia and II afferent neurons
Motor - gamma motor neuron to intrafusal fibers and alpha motor neuron to same muscle and to an antagonistic muscle
3 effects of muscle spindle receptor activation in the spinal cord
- EPSP on a-motor neuron returning to same muscle –> muscle contraction
- IPSP in a-motor neuron of the antagonist muscle via synapsis with an inhibitory interneuron
- Concurrently: y-motor neurons located at the polar ends of intrafusal fibers cause shortening of the spindle’s polar regions –> the spindle receptor thus remains both morphologically and functionally intact
Activity of Ia and II muscle fibers during different muscle phases
- Static phase
Ia and II - evenly spaced AP - Dynamic phase - stretch
Ia - Very active, increased firing rate, lots of AP
II - Increased firing rate, but not as fast as Ia fibers - Static phase
Ia and II - Evenly spaced AP, but more than in first static phase - Dynamic phase - shortening
Ia - Does not produce any AP
II - Does produce AP but not a lot - Static phase
Ia - Have AP again, but not a lot in beginning of phase because needs time to get started again after not having any APs
II - still have AP and are more frequent than in the shortening phase
What happens to the golgi tendon organ during muscle contraction
Tension is applied to the golgi tendon organ causing the collagen fibrils to tighten
Mechanism of golgi tendon organ
APs are sent to the CNS via sensory neuron and induce IPSP in the a-motor neuron returning to the same muscle
Decreased extrafusal muscle fiber contraction
Lower motor neuron
a-motor neuron
Cell body and dendrites are located in CNS
Axons extend out through peripheral nerves to skeletal muscles
Upper motor neuron
Lies completely in the CNS
Controls the lower motor neuron
Recognizing disease of lower motor neurons (4)
Paralysis (no muscle contraction) or paresis (incomplete paralysis)
Atrophy (wasting of skeletal muscle mass distal to lower motor neuron lesion)
Loss of segmental and intersegmental reflexes
Changes in electrical activity of the muscle (observed by electromyogram)
Recognizing upper motor neuron disease (4)
Inappropriate movement (due to no control of lower motor neurons if upper motor neurons not functioning)
NO atrophy
Exaggerated segmental reflexes
Normal electromyogram
Define corticospinal
Starts in cortex and goes to spinal cord
Motor system functions (4)
Maintain posture
Support body against gravity
Provide stable platform for movement
Control voluntary movement and locomotion
What results in all movement in animals
Flexion and extension of muscles
Movement divided into 2 groups
Voluntary - learned, conscious, skilled
Involuntary - postural, subconscious, antigravity
What is voluntary movement dominated by (flexors or extensors)
Flexor muscles
Discrete contraction of muscles located distal to spinal column
What is involuntary movement dominated by (flexors or extensors)
Extensor muscles
Long-term contraction of large muscles located proximal to spinal column
What is needed for muscle fibers to contract
Stimulation by an a-motor neuron (lower motor neuron)
What is needed for an a-motor neuron to send an AP (2)
Signaled to do so by upper descending motor neuron (upper motor neuron)
Signaled to do so by an incoming sensory neuron (reflex arc)
Hierarchical organization of the motor system
Premotor and supplementary motor cortices - highest
Primary motor cortex
Brain stem
Spinal cord - lowest
Can the brainstem induce reflexes
Yes
The brain stem is similar to the spinal cord in that it receives sensory information from the head and neck via cranial nerves
What is the simplest level in the movement control hierarchy
Spinal cord
Motor neuron pool
Cigar shaped areas extending over few spinal cord segments
Contain the neuronal cell bodies of MN (motor neuron?) units of a given muscle
Motor neuron pools of distal muscles location and what kind of movement
Located in more lateral parts of ventral horn
Learned skilled movement
Motor neuron pools of axial and proximal musculature location and what are muscles for
Located more medially in the ventral horn
Posture muscles
Where are cell bodies and axons located in the spinal cord
Cell bodies are in the grey matter
Axons are in the white matter
Where are premotor neurons located and what do they do
Located in intermediate zone of ventral horn in spinal column
Activate the lower motor neurons that innervate muscles
4 major axon routes from brainstem
Vestibulospinal tract
Tectospinal tract
Reticulospinal tract
Rubrospinal tract
Which of the 4 major axon routes from brainstem control axial and proximal musculature - involuntary, maintenance of posture
Vestibulospinal tract
Tectospinal tract
Reticulospinal tract
Which of the 4 major axon routes from the brainstem control distal limb musculature - voluntary, skilled movement
Rubrospinal tract
Vestibulospinal tract: Origin Sensory input Information Function
Origin: Cell bodies in vestibular nuclear complex (medulla and pons)
Sensory input: VII cranial nerve (vestibulocochlear nerve) and cerebellum
Information: Position of head and acceleration
Function: Detects disturbances of balance and activates antigravity musculature to counteract
Righting relfex
Combination of vestibular, ocular, and muscle spindle organs in neck musculature
These together with the vestibulospinal tract aim to restore normal posture in a falling animal
Tectospinal tract:
Origin
Sensory input and information
Function
Origin: Superior colliculus (midbrain)
Sensory input and information: Visual, auditory, and somatosensory information about the relative position of stimuli in the environment with respect to the organism
Function: Controls muscles that move the head, reflex orientation of head toward environmental stimuli
Reticulospinal tract:
Origin
Sensory input and information
Function
Origin: Reticular formation (pons and medulla)
Sensory input and information: Sensory information important for pain perception, respiration, circulation
Function: Antigravity regulation
Rubrospinal tract:
Origin
Sensory input and information
Function
Origin: Cells of the red nucleus (n. ruber) of midbrain
Sensory input and information: Cerebral cortex and cerebellum
Function: Voluntary control of musculature that participates in skilled movement of the extremities (distal limb flexor musculature)
Corticospinal tract
Pyramidal tract/motor system
Direct projection from cerebral cortex to spinal cord
Most complex level of motor control
Bypasses brain stem
Pyramidal tract (corticospinal tract):
Origin
Function
Characteristic
Origin: Cerebral cortex, adjacent to frontal lobe
Function: Responsible for the most elaborate and dexterous voluntary movement
Characteristic: Control over small number of a-motor neurons (fractionation)
Lateral corticospinal tract
Pyramidal cell axons that cross midline at pyramidal decussation - 75-90%
Skilled, manipulative, voluntary movements
Signs of damage to lateral corticospinal tract
Signs of damage to this tract rostral to the pyramidal decussation causes weakness of the contralateral side of the body (hemiparesis)
What is the pyramidal decussation
Medulla - spinal cord border
Ventral corticospinal tract
Pyramidal cell axons that do not cross midline at pyramidal decussation and remain on the same side of the body - 10-25%
Voluntary control of antigravity muscles
Several axons of the ventral corticospinal tract cross the midline locally in the spinal cord
What is the highest level of motor control hierarcy
Motor cortices
Primary motor cortex (MI)
Located rostral to the central sulcus, near the cruciate sulcus
Somatotropic map
What is a somatotropic map
Shows what part of the brain is important in different body parts
What does dorsomedial MI activate
Muscles in the caudal part of the body
What does the ventrolateral MI activate
Muscles of the face
What happens after stroke in the Primary motor cortex
Humans - hemiparesis
Animals - Proprioceptive positioning reaction is slowed with damaged corticospinal tract neurons
Supplementary motor cortex and premotor cortex location and where do axons go
Located in frontal lobe
Give rise to axons that project to nuclei of descending brainstem motor pathways
Function of supplementary motor cortex
Simultaneous movement of limbs in order to accomplish a task
Function of premotor cortex
Preparatory orientation of the body for the execution of a motor task
Example of motor cortices in playing the piano
Prefrontal motor cortex - orientation of arms and hands
Supplementary motor cortex - planning and organizing the sequence of finger movements
Primary motor cortex - muscle activation necessary to press a single key
Basal ganglia (4)
Caudate nucleus and putamen (aka striatum)
Globus pallidus
Substantia nigra
Subthalamic nucleus
What do basal ganglia do
Receive input from cerebral cortices and project back to premotor and supplementary cortices
Help select appropriate movement pattern while suppressing less appropriate competing patterns
Basal ganglia and Parkinson’s
Striatum and substantia nigra involved in Parkinson’s
These are needed to pick right movement patterns
Accumulation of a-synuclein –> degradation of striatum and substantia nigra basal ganglia –> loss of dopamine in basal ganglia that control muscle tone and movement
Signs: tremors, rigidity, gait disturbance
