Kapitel 8: Det sensomotoriske system

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Chapter Overview and Learning Objectives

• The sensorimotor system is hierarchically organized
• Sensory input is crucial for motor output
• Learning modifies sensorimotor control
• A general model of sensorimotor function exists
• Explain the role of areas of secondary motor cortex
• Explain the major features of mirror neurons
• Give the conventional view of primary motor cortex function
• Expand on the current view of primary motor cortex function
• Describe the structure and connectivity of the cerebellum and explain cerebellar function
• Describe the anatomy of the basal ganglia and explain the current view of their function
• Compare the two dorsolateral motor pathways and the two ventromedial motor pathways.
• Describe the components of a motor unit and distinguish between the different types of muscles.
• Describe the receptor origins of tendons and muscles
• Explain the stretch reflex and explain its mechanism
• Explain the withdrawal reflex and explain its mechanism
• Explain recurrent collateral inhibition
• Explain the phenomenon of walking
• Explain central sensorimotor programs
• Describe characteristics of central senorimotor programs
• Explain Jenkins study
• Describe neuroplasticity

Three Principles of Sensorimotor Function

• The sensorimotor system is hierarchically organized, similar to a large efficient company, with commands flowing from high to low levels.
• Motor output is guided by sensory input, allowing for continuous adjustments and fine-tuning of actions.
• Learning alters sensorimotor control, transferring control from conscious to subconscious levels over time.
• These principles are best appreciated when recognised as governing any system be it companies or the sensorimotor system

Motor Output is Guided by Sensory Input

• Efficient systems continuously monitor and adjust their own actions, using sensory feedback to fine-tune their performance.
• Sensory feedback in the sensorimotor system influences responses in most cases; ballistic movements are exceptions.
• Sensorimotor feedback, for example from the somatosensory system, is crucial in adjusting ongoing responses.
• The study of G.O, the darts champion, demonstrates that somatosensory nerves are crucial to motor output.
• Without sensory feedback, motor output that requires a constant activity must be continually visually adjusted.
• Adjustments in motor output that occur in response to sensory feedback are controlled unconsciously by the bodies lower levels

Learning Changes the Nature and Locus of Sensorimotor Control

• Initial tasks are carried out via conscious control and attention, involving the superior parts of the sensorimotor system.
• With practice, actions are coordinated into continuous sequences, transferring control to lower levels of the hierarchy.
• This relates to motor output for tasks such as typing, swimming, knitting, or playing basketball
• Development of the company sees people at lower levels carrying out lower levels of the hierarchy

General Model of Sensorimotor System Function

• The sensorimotor system has a hierarchical structure (Figure 8.1)
• Functional segregation is apparent; each part performs a specific function.
• Parallel pathways allow for multiple ways of controlling lower levels
• Feedback loops are crucial in the hierarchy
• This chapter focuses on the neutral function that plays a role in voluntary movement
• The model shows descending pathways, and circuits and feedback loops

Posterior Parietal Association Cortex

• The posterior parietal cortex integrates sensory information (visual, auditory, somatosensory) regarding location and position for directing behavior.
• Damage to the posterior parietal cortex can cause apraxia or contralateral neglect.
• Apraxia involves difficulty in carrying out planned movements, even if those movements are not physically impossible.
• Contralateral neglect occurs when one side of the body or world is essentially ignored.
• The nervous system must know the current position before an effective movement can be initiated
• The posterior parietal cortex plays an important role in directing behaviours by providing the brains special information

Dorsolateral Prefrontal Association Cortex

• The dorsolateral prefrontal cortex actively anticipates and plans movements.
• Neurons in this area exhibit response properties related to intended target locations, rather than direction for movement.
• Damage to this region can cause issues related to motor planning and voluntary movements.
• Studies show several studies have characterised monkey dorsolateral prefrontal neurons
• Decisions to initial voluntary movements may be made in this aread of cortex, but depends of critical intersections with parietal cortex.

Secondary Motor Cortex

• It receives input from association cortex
• It sends output to primary motor cortex
• It is involved in planning and executing sequences of movements, also in species-typical behaviour.
• There is evidence to show the human secondary motor cortex has comparable organisation,.
• Area must be appropriately connected with association and secondary motor areas
• Neurons becomes more active prior to the initiation of a voluntary movement and stays active during said movement
• They are thought to be involved in the programming of pacific patterns

Mirror Neurons

• Mirror neurons fire both when performing an action and when observing another performing the same action.
• Mirrors neurons are found in ventral premotor area and parietal cortex in monkeys.
• Their role likely involves social cognition.
• Discoveries are related to determining the understanding of action, not to superficial characteristics of the action itself
• Scientists still not sure how mirror neurons are responsible for these in human
• Many studies have found that the human motor cortex is active when a person performs watches or imagines an action

Primary Motor Cortex

• It is a crucial area for coordinating and initiating movements.
• The primary motor cortex has a somatotopic organization, or a map of the body.
• Current studies suggest a broader function, responding to intended movement targets rather than just movement direction.
• Patients were operated on by brief, low-intensity electrical stimulation to various parts of the cortical surface whilst noting the responses
• They found the that cortex organised somatotopically.
• Stereognosis can be adapted through the help of this

Cerebellum

• The cerebellum plays a role in refining and coordinating movements.
• It receives input from various parts of the nervous system and compares intended movements with actual movements to produce adjustments.
• The effects of diffuse damage are devastating
• They contribute few fibers to descending motor pathways

Basal Ganglia

• The basal ganglia are involved in motor control and also related to motivation and cognition.
• Its anatomy is more complex than the cerebellum, comprising a collection of interconnected nuclei. - It influences movement vigor and the suppression of unwanted actions.
• Movement vigor includes motivation and speed or amplitude

Descending Motor Pathways

• Neural signals travel from the primary motor cortex to spinal motor neurons via the dorsolateral and ventromedial pathways.
• The pathways have differing functions - dorsolateral for distal muscles, and ventromedial for proximal muscles, posture, and locomotion.
• These pathways are crucial for coordinating and controlling voluntary movements.
• They descending in the dorsolateral region of the spinal cord

Sensorimotor Spinal Circuits

• The spinal cord circuits are complex, and capable of controlling movement independent of cortical input.
• They include a hierarchy of motor neurons associated with posture, movement patterns, reflexes, and other voluntary coordination functions.

Muscles

• Motor units are the functional units of motor activity (motor neuron and associated muscle fibers)
• Muscles differ in their properties, reflecting their roles in various tasks.
• Fast muscle fibers support quick movements, while slow-twitch fibers allow for prolonged contractions.
• Activating a muscle can increase tension without shortening of the muscle, and this is then called isometric contraction

Receptor Organs of Tendons and Muscles

• Golgi tendon organs detect changes in muscle tension
• Muscle spindles monitor changes in muscle length
• Under Nornal conditions tendone response to increases in muscle tension

Stretch Reflex

• The stretch reflex rapidly adjusts for unexpected stretches, typically in response to sudden external force.
• Feedback from muscle spindle and other receptors is essential in the reflex's rapid actions.
• When the word reflex it often thinked of yourself
• Can be referred to The patellae tendon reflex

Reciprocal Innervation

• Reciprocal innervation coordinates antagonistic muscle action by inhibiting the opposing muscle (e.g., biceps during extension)
• This ensures a smooth and controlled motor activity.

Withdrawal Reflex

• The withdrawal reflex triggers a rapid withdrawal of a limb from a noxious stimulus.
• It integrates sensory information and motor output to protect from harm.

Recurrent Collateral Inhibition

• Recurrent collateral inhibition ensures that each motor neuron is temporarily inhibited after firing, ensuring other motor neurons take over control, and allowing for rest.
• This regulation ensures efficient use of motor neurons in a coordinated manner.

Walking: A complex Sensorimotor Reflex

• Walking is a complex coordinated sequence of movements modulated by various sensory input and spinal circuits.
• Control of posture and changes in terrain are examples of how the system adapts to complex and changing environments like walking and climbing.

Central Sensorimotor Programs and Learning

• Sensorimotor programs are organised in hierarchies and encompass various levels in the hierarchy, from association cortex to spinal motoneurons.
• The sensorimotor system demonstrates plasticity, adjusting itself by improving motor sequences from conscious to more automatic action plans and co-ordination.
• Sensory feedback affects behaviour, which is important for smooth movement and learning.
• Company president and others pass commands so to be efficient without having to control all movement.
• System is capable of movement as it is not always carried the same way
• That specific senorimotor problems are stored not in neural circuits but more higher in the hierachy.

Functional Brain Imaging of Sensorimotor Learning

• The Jenkins study of motor learning used PET scans to observe brain activity during different motor tasks and found that association areas and cerebellum involvement decreased with practice.

Neuroplasticity Associated with Sensorimotor Learning

• Sensorimotor learning leads to both cortical and subcortical changes.

• The number of dendritic spines in motor cortex increase, suggesting enhanced synaptic strength with learning

• Increased oligodendrocytes in subcortical pathways support myelination of new and existing axons, increasing their efficiency. Themes Revisited

• Clinical implications, evolutionary perspective and also metaphors can be discussed.

• Sensormotor functions in non humans are similar but not identical

• How ever the general commends move issues are delivered by cortical circuits </themes_revisited>

Denne quiz giver en overordnet forståelse af sensorimotoriske funktioner og deres hierarkiske organisation. Du vil lære om, hvordan sensorisk input er afgørende for motorisk output, og hvordan læring kan ændre kontrol over disse systemer. Få indsigt i de tre grundlæggende principper for sensorimotorisk funktion og deres betydning.