Neurophysiology Part 17 - Central Control of Movement Summer 2024 PDF

Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: List the 3 main functions of the motor system List the CNS areas involved in voluntary movement Describe the major rol...

Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: List the 3 main functions of the motor system List the CNS areas involved in voluntary movement Describe the major role of the Corticospinal Tracts Describe the major role of the Lateral and Medial Brainstem UMN Pathways Describe the role of the spinal cord in motor function Describe how the motor system maintains posture (reflexes/gamma loop/cortex modulation) Understand the crossed extensor reflex Describe the role of the cerebellum in controlling posture and movement  FYI Argento w met https://www.youtube.com/watch?v=9wHQd2Dc-gk  SENSORY COMPRISES CEREBRAL CORTEX AND SYSTEM DIVERSE CONNECTIONS AND FEEDBACK INTERNEURONS LOOPS THROUGHOUT THE BRAIN INCLUDING THE CEREBELLUM AND BASAL MOTOR SYSTEM NUCLEI COMPRISES THE UPPER MOTOR NEURONS THAT ORIGINATE IN THE BRAINSTEM AND MOTOR CORTEX UMNs MAKE CONNECTIONS WITH LOWER MOTOR NEURONS. LMNs ALSO RECEIVE INPUT FROM LOCAL INTERNEURONAL CIRCUITRY (REFELX ARC) THE WORKERS COMPRISE THE LOWER MOTOR NEURONS WHICH CONNECT DIRECTLY WITH THE MUSCLE  The motor system has 3 main functions 1. Support of the body against gravity, maintaining posture and providing a stable platform for movement 2. Control of voluntary movement and locomotion 3. Control of visceral motor function (ANS)  All movement ultimately results the through excitation or relaxation of muscles Voluntary movements are usually initiated consciously but are coordinated or controlled at a more subcortical level of the brain It would be inefficient for an animal to have to actually “think” about the infinite dynamic positions of the body during something as simple as walking or running  SMS has a hierarchical organization: Motor cortexes Basal nuclei GIVE DIRECTIONS THAT Thalamus CONTROL THE SEQUENTIAL CORD ACTIVITY Cerebellum Brainstem Spinal cord CONTAIN THE CIRCUITS FOR Peripheral nerves THE MOVEMENT Voluntary movement is directed by the primary motor cortex Most voluntary movements initiated by the cerebral cortex are achieved when the cortex activates “patterns” of function stored in lower CNS areas  COMPLEX AND SKILLED PATTERNS Organized by progressively more rostral regions The higher-order areas can concern themselves with more global tasks regarding action, such as deciding when to act, planning an appropriate sequence of actions, and coordinating the activity of many limbs. SIMPLE MOVEMENTS OR MOVEMENT PATTERNS Organized by more caudal regions of the CNS Higher-order areas do not have to program the exact force and velocity of individual muscles, or coordinate movements with changes in posture; these low-level tasks are performed by the lower levels of the hierarchy. TO CONTROL BODY MOVEMENT, THE CNS ALSO MUST: Assess the effect of gravity on the many muscles of the body Determine the initial position of the body parts to be moved Detect any discrepancy between intended and actual movement Proprioceptive information provided by: MUSCLE SPINDLES and GOLGI TENDON ORGANS  The LMNs are stimulated or inhibited by both reflex connections and the input from the UMN system UMN tracts INITIATE, MODIFY, and TERMINATE muscle activity of the limbs and body There are 2 major descending motor system pathways from the brain to the spinal cord CORTICOSPINAL TRACTS (PYRAMIDAL TRACTS) Direct connection between motor cortex and spinal cord Neuronal bodies located in the motor area of the cerebral cortex Very important for voluntary motor control in primates/humans BRAINSTEM UMN PATHWAYS (EXTRAPYRAMIDAL TRACTS) Neuronal bodies located in brainstem nuclei Most important for gait and movement in quadrupeds MOTOR TRACTS IN THE SPINAL CORD PYRAMIDAL 0 EXTRAPYRAMIDAL  Pyand Exhi In quadrupeds the extrapyramidal tract is most important for gait and movement The pyramidal tract influences voluntary skilled movement but has minimal influence on gait  Direct projection from cerebral cortex to spinal cord Responsible for most skilled voluntary movements of mammals Specially the movements involving the extremities Also participates in less elaborated voluntary movements of the distal muscles The ability to control the skilled movements derives from synaptic termination pattern of several axons Axons bypass not only the brainstem motor pathways to the cord, but also can bypass the premotor neurons of the spinal cord, contacting alfa motor neuron directly! A given corticospinal neuron control smaller numbers of alfa neurons  Increased independence of the actions of different muscles (i.e., primates can move individual fingers instead of all together) Minimal influence on gait Decussation of axons occurs at the ventral surface of the medulla oblongata (pyramids) Originate the lateral corticospinal tract 75% of fibers decussate on dogs 90% of fibers decussate on humans Lesions to the motor cortexes on one side of the body have devastating effects on voluntary movement of the distal flexor musculature on the contralateral side of the body  Direct projection from cerebral cortex to spinal cord Responsible for most skilled voluntary movements of mammals Specially the movements involving the extremities Also participates in less elaborated voluntary movements of the distal muscles The ability to control the skilled movements derives from synaptic termination pattern of several axons Axons bypass not only the brainstem motor pathways to the cord, but also can bypass the premotor neurons of the spinal cord, contacting alfa motor neuron directly! A given corticospinal neuron control smaller numbers of alfa neurons  Increased independence of the actions of different muscles (i.e., primates can move individual fingers instead of all together) Minimal influence on gait Decussation of axons occurs at the ventral surface of the medulla oblongata (pyramids) Originate the lateral corticospinal tract 75% of fibers decussate on dogs WHY THERE IS A PREMOTOR NEURON HERE??? 90% of fibers decussate on humans Lesions to the motor cortexes on one side of the body have devastating effects on voluntary movement of the distal flexor musculature on the contralateral side of the body  Mostinpttgug.at rubrospinal tract medullary reticulospinal tract medial vestibulospinal tract tectospinal tract pontine reticulospinal tract lateral vestibulospinal tract  Four major axon tracts originate in the brainstem and descend to the spinal cord to influence spinal LMNs EXTRAPYRAMIDAL TRACTS 1. VESTIBULOSPINAL TRACT 2. RETICULOSPINAL TRACT MEDIAL (ventral column) 3. TECTOSPINAL TRACT Axons travel in medial regions of the white matter and synapse within medial regions of the grey matter 4. RUBROSPINAL TRACT LATERAL (lateral column) Axons run in a more lateral region of the white matter and synapse in the lateral spinal grey matter  Subconscious control of the postural musculature is also an integral part of the ability to execute skilled voluntary movement; Voluntary movement requires a stable platform on which it can proceed. The major role of the medial pathways is to maintain the body subconsciously in an upright position against the pull of gravity Control of the axial and proximal extensor musculature Bilateral control Prevents animal to falling to the ground 1. VESTIBULOSPINAL TRACT Receives sensory information from the VESTIBULAR SYSTEM Information about the position of the head with respect to gravity and acceleration of the head Regulate antigravity muscle tone Activating the antigravity muscles in response to destabilization of the body 2. RETICULOSPINAL TRACT Regulate the steady-state contraction level (tone) of antigravity muscles, together with vestibulospinal tract Just before the execution of a voluntary movement, subconsciously activates axial and proximal musculature that will compensate for the postural destabilization Under the control of descending cortical projections helps in the voluntary execution of crude (nonskilled) movements of the proximal limb musculature, such as pointing or locomotion Initiating and controlling the speed of CENTRAL PATTERN GENERATORS in the spinal cord 3. TECTOSPINAL TRACT Axons only project as far as the upper cervical regions of the spinal cord Muscles that moves the head Involved in reflex orientation of the head toward environmental stimuli The cells of origin of the tectospinal tract also controls rapid reflex movement of the eyes Coordinate head and eyes so the animal’s gaze is fixated directly on the stimulus  The RUBROSPINAL TRACT is a lateral brainstem UMN pathway that controls distal limb musculature associated with movement Exerts unilateral control over a limited number of flexor muscles of the distal limb Skilled movements of the extremities Initiation of protraction (forward movement) of the limbs in gait Very important for GAIT GENERATION in quadrupeds Interact with CENTRAL PATTERN GENERATORS in the spinal cord Receives a lot of input from higher levels of the motor system Provides a means for the motor cortices to influence indirectly the spinal LMNs of the distal limb flexor musculature Receives also significant input from the CEREBELLUM Synchronizes muscle activity by fine-tuning movement initiated by the corticospinal tract  Movement can be divided into 2 general forms: DOMINATED BY FLEXOR MUSCLES Largely learned, voluntary, conscious and skilled Fairly discrete contraction of a few muscle groups Many are distal to the spinal cord DOMINATED BY EXTENSOR MUSCLES Postural, antigravity muscle activity, generally subconscious and involuntary Long-term contraction of larger groups of muscles Many are located closer to the spinal cord  UMN tracts, mainly facilitatory to flexor muscles and inhibitory to The control of those 2 types of extensors, are located in the lateral funiculus: corticospinal, rubrospinal, and medullary reticulospinal movements is made by different neurons and tracts in the nervous system Skilled voluntary movement of distal musculature (FLEXORS) is primarily controlled by a lateral system (lateral funiculus) of lower and upper motor neurons’ spinal tracts Postural and antigravity activity of the proximal and axial musculature (EXTENSORS) is controlled by a more medial system (ventral funiculus) of such neurons and tracts Those facilitatory to limb extensors and inhibitory to flexors are located in the ventral funiculus of the cord: pontine reticulospinal and vestibulospinal  The spinal cord is the most caudal and simplest level of movement control Contain the UMN tracts Contain the alfa lower motor neurons innervating skeletal muscle fibers Contain interneurons and complex neural circuits for motor control Spinal Reflexes CENTRAL PATTERN GENERATORS Execute the low-level commands that generate proper forces on individual muscles groups to enable adaptive movements LMNs can recruit more motor units or increase frequency of APs to control the amount of force that is exerted by muscle fibers The number of muscle fibers innervated by a single LMN decreases as the need for fine control of a muscle increases.  The STRETCH REFLEX counteracts gravity! Every time we stand up, we are fighting against gravity Gravity causes bending/flexion stretching of the muscle, and activation of muscle spindles. To counteract gravity, we contract our extensors! Recall how muscles are arranged in opposing groups The stretch reflex helps to maintain equilibrium and posture  The quadriceps, hock extensor muscles and the epaxial and hypaxial muscles will be stretched by gravity, resulting in reflexively stimulated contraction and postural support.  Posture is maintained by the motor system by: 1. Providing a tonic excitatory bias to motor circuits that excite extensor muscles (medial system) 2. Modulating stretch reflex circuits The gamma loop Extensor muscles are antigravity muscles! The stretch reflex induces contraction of extensor muscles whenever they are stretched by postural changes + The CNS modulates stretch reflexes and muscle tone by modulating the gamma loop Higher motor centers via reticulospinal tract activate the gamma motor neuron innervating the muscle spindle. This causes increasing in firing of the sensory Ia fiber and excitation of the alpha motor neuron innervating the extensor muscle. The extensor muscle will then contract.  Walking can be defined as a method of locomotion using the limbs alternately, to provide both support and propulsion. At least one foot being in contact with the ground at all times Each step cycle of locomotion consists of 2 phases: 1. Swing phase – the foot is off the ground and swinging forward Mediated by flexors 2. Stance phase – the foot is planted on the ground and the leg is moving backward Mediated by extensors  In order to move a limb toward a particular location, it is imperative to know the initial starting position, and any force applied to the limb Proprioceptive information is vital for movement! Muscle spindles and Golgi tendon organs help to provide this information Together with other receptors These receptors are also component of some spinal reflexes – important for a basic understanding of the motor control Walking depends on alternation between extension and flexion which is largely generated by spinal cord reflexes STRETCH REFLEX (musle spindle ) – contracts the muscle being stretched INVERSE STRETCH REFLEX (golgi tendon organ) – relaxes the muscle being tensioned WITHDRAWAL REFLEX or FLEXOR REFLEX (propioceptive and nonproprioceptive receptors) – causes flexion of the limb being stimulated Recall the flexor reflex protects the body against injury – but can be modulated by the CORTEX!!!  About 0.2 to 0.5 second after a stimulus elicits a withdrawal reflex in one limb, the opposite limb begins to extend The crossed-extensor reflex: the flexion of one limb can induce reflex extension in the other limbs, both of the same limb girdle and the other limb girdle This reflex works in coordination with the withdrawal reflex to avoid loss of balance when the other limb was flexed Allows balance and body posture to be maintained any time a limb is reflexively flexed after an injury or stumble This reflex is physiological while walking but pathological during neurological examination; it indicates UMN lesion  sometings aetite Positive supportive reaction Stunkkefir pressure in the footpad causes the limb to extend against the pressure applied to the foot. this reflex involves a complex circuit in the interneurons like the circuits responsible for the flexor and crossed extensor reflexes this reaction helps keep an animal from falling to that side  All stepping and oscillation between extension and flexion of the limbs and body, weight bearing and non-weight bearing, is based primarily on reflex circuits of spinal interneurons that control LMNs in a repetitive, oscillating manner  CENTRAL PATTERN GENERATORS (CPGs) The CNS contains many CPGs that produce oscillatory outputs used for controlling rhythmical motor activity such as: Locomotion, scratching, chewing, breathing They contain excitatory and inhibitory neurons CPGs associated with locomotion and scratching are in the spinal cord intumescences (spinal cord reflexes) Control centers that initiate and terminate the rhythmical activity are in the brainstem  Sensory input for the reflexes comes from muscle spindles, Golgi tendon organs, joints and tactile receptors Integration of that input in the spinal cord causes inhibition or excitation of LMNs, as appropriate In the same limb In the opposite limb Limbs of other girdle Activity of the muscles of the trunk, neck and tail are interlinked The cerebellum coordinates agonistic/antagonistic muscle activity to permit posture and to create movement that occurs at the correct rate, range and force POSTURE The cerebellum coordinates overall posture by coordinating contraction-relaxation of all muscles in the body used for maintaining posture Both at rest and during movement Failure to stablish a postural platform would prevent normal, coordinated movement. DURING MOVEMENT The cerebellum coordinates initiation of movement, the actual movement itself and the termination of movement, but CANNOT INITIATE MOVEMENT PER SE throughout movement, proprioceptive input cerebellum compares the it determines when the from the head, body and limbs, continually achieved movement with correct range of movement informs the cerebellum how much movement the planning information has been achieved, and thus has occurred, how fast it is occurring and how received about that when the action should be forceful the movement is movement terminated 1. For voluntary, learned movement (e.g capturing prey or pawing) the planning occurs in the executive motor planning areas of the brain Integration/interpretation areas associated with a variety of sensory receiving areas Visual cortex Somatosensory cortex Memory and behavior centers 2. A copy of the planned movement is sent to the cerebellum which then stablishes the appropriate postural platforms The cerebellum feeds back to the motor planning centers informing them that the posture has been established 3. The executive centers then direct the Pyramidal and Extrapyramidal tracts Movement is initiated  FYI SPINAL WALKING The acquisition of an involuntary motor function in paraplegic dogs and cats without pain perception after a severe thoracolumbar injury. It can take up to a year or more to develop. Mediated by reflex activation of CPGs in the lumbar intumescence  no brain control. Additional information: https://www.frontiersin.org/articles/10.3389/fvets.2020.00560/full https://www.researchgate.net/publication/314069608_Acquisition_of_ Involuntary_Spinal_Locomotion_Spinal_Walking_in_Dogs_with_Irrevers ible_Thoracolumbar_Spinal_Cord_Lesion_81_Dogs https://www.youtube.com/watch?v=22UFCdqBzfA Lizard severed tail spinal cord CPGs