lecture 14: reflexes motor cortex and descending control

3 things that mediate the different sensory inputs for the sensorimotor reflex pathways

1. muscle tone/length reflexes- muscle spindles

2. muscle contraction force reflex- golgi tendon organ (GTO)

3. flexion reflex- pain receptors

sensorimotor reflexes- muscle (spindles, GTO)

sensory neuron afferents project to interneurons and motor neurons

sensorimotor reflexes- pain

sensory neuron afferents only project to interneurons not motor neurons

proprioceptor types and function

provide sensory input from the muscles to the spinal cord

types: muscle spindle and golgi tendon organ

muscle spindle

integrated within the muscle

provides information about muscle length

golgi tendon organ

found in the tendon

provides information about the force of muscle contraction

muscle spindles vs golgi tendon organs when the muscle contracts

spindles decrease AP firing rates (length is getting shorter)

golgi tendon organs increase firing rates (force is getting stronger)

reciprocal innervation in the stretch reflex circuitry definition

causes contraction of the stretched muscle and relaxation of the antagonistic muscle

reciprocal innervation in the stretch reflex circuitry explanation

sensory afferents split in the spinal cord

one connection excites the lower A motor neurons of the same muscle as the activated spindle causing contraction

other connection excites inhibitory neurons that decrease the firing rates of the A motor neurons of the antagonistic muscle causing relaxation

steps for i dont want to spill my drink reflex

1. added load of soda stretches intrafusal muscle fibers of bicep

2. increase in spindle AP rate signal sent to spinal cord

3. spindle input activates bicep motor neurons

4. spindle input inhibits triceps motor neurons

golgi tendon organ reflex circuit outcome

causes relaxation of the activating muscle

golgi tendon organ reflex steps

protective circuit that causes the muscle to relax if it is overloaded

GTO inputs activate local neurons that inhibit motor neurons of the same muscle causing muscle relaxation

sensorimotor reflexes- pain

sensory neuron afferents only project to interneurons not motor neurons

example is the withdrawal reflex

withdrawal reflex

type of polysynaptic reflex

example is a flexor reflex that affects muscles of a limb

occurs when you grab a hot pan

1. grabbing a hot pan stimulates pain receptors

2. sensory neurons activate interneurons in spinal cord

3. stimulate motor neurons in anterior gray horns

4. result= contraction of flexor muscles that yank hand away from stove

inhibitory signals used to relax extensor muscles

descending pathways from cortex to spinal cord

axons of upper motor neurons in the corticospinal tracts synapse with interneurons and lower motor neurons in the ventral horn of the spinal cord

descending inputs to the spinal cord- brainstem

project to local circuit neurons in the medial part of the spinal cord (body, trunk)

bilaterally coordinate reflexive postural and other body alignments

descending control by brainstem nuclei (3 parts)

superior colliculus, vestibular nuclei, reticular formation

superior colliculus

colliculospinal tract

movements that orient eyes, head and body toward sensory stimuli

vestibular nuclei

vestibulospinal tract

reflexive changes in posture and reflexive eye movements

reticular formation

reticulospinal tract

anticipatory changes in posture

the vestibulospinal, reticulospinal and colliculospinal tracts

make up the spinal cord medial white matter

specializations for coordination of multiple muscle groups:

1. run the fill length of the spinal cord

2. project mainly to interneurons rather than directly to lower motor neurons

descending inputs to the spinal cord- motor cortex

project to both local circuit and lower motor neurons

coordinate both gross and fine motor movements

spinal cord circuits- medial to lateral maps

the A motor neurons are organized from medial to lateral according to the muscle groups innervated

muscles of the trunk and proximal limbs (i.e. the shoulder) are represented medially while muscles of distal limbs (arm, hands, fingers) are represented laterally

spinal cord circuits-longitudinal maps

motor neurons are also grouped along the length of the spinal cord

the somas of neurons that innervate a given muscle (bicep) can be found in many sequential spinal cord segments

this can lead to enlargements of the spinal cord at regions where a lot of muscles are represented such as the arms and hands

spinal cord circuits- enlargements

cervical enlargement: motor neuron pools for arms, hand

lumbar enlargement: motor neuron pools for legs, feet

spinal cord circuits- local circuit neurons (2 types)

local circuit neurons also interact with motor neurons in different segments of the spinal cord

2 types: medial and lateral

medial local circuit neurons

medial local circuit neurons project over many spinal cord segments as well as bilaterally to coordinate left/right and upper/lower body movement as well as posture

later local circuit neurons

lateral local circuit neurons project to fewer segments and unilaterally to coordinate fine, independent muscle movements (i.e. finger movements on one hand)

primary motor cortex

located in precentral gyrus

stimulation directly evokes movement

lowest threshold for initiation of movement

contains a map for the musculature of the body

contains a map for movements

L5 betz cells

defining feature of motor cortex

upper motor neurons

large neurons somas found in L5

have the longest axons

project to the spinal cord interneurons and lower motor neurons for the hand

the penfield maps- motor humunculus

mapped by wilder penfield

electrical stimulation of the surface of the brain to map locations that elicit specific muscle contractions

body regions that require find motor control (hand/face) have a lot of cortical representation

similar to the homunculus in somatosensory cortex

fine organization of motor cortex shows a map of organized behaviors

microstimulation of a neuron in primary motor cortex can elicit a response in several muscles leading to an organized behavior

also, one particular movement can be elicited by stimulation of separate sites

suggests that upper motor neurons are linked by circuits in the cortex to organize and evoke a specific movement

purposeful movements of the contralateral arm and hand in macaque monkey

stimulation of precentral gyrus (primary motor cortex) results in purposeful movements that are sequentially distributed across multiple joints/muscles

suggests a mapping of purposeful movements in motor cortex (movements that are often repeated and that are important to the animal)

premotor cortex and supplementary motor area (SMA)

receive input from sensory systems

planning and organization of complex movements

project mainly to motor cortex

premotor cortex: motivation and movement intention, can project to spinal cord

premotor neurons increase action potential firing rates seconds before the movement

how does the motor cortex take part in the control of movement?

the motor cortex takes part in planning movement, executing movement, and adjusting the force and duration of a movement

blood flood studies

more complex motor tasks activate high levels of motor cortex

lesion of premotor cortex causing what

damage to premotor cortex can lead to failure to coordinate complex motor actions

motor cortex damage and plasticity

motor maps can change as a result of leading and in response to damage

population coding of movement direction does what

looking at the firing rates of a lot of motor cortex neurons can allow us to predict the direction of the movement