Control of Body Movement

Lower Motor Neurons

Motor neurons whose cell bodies are located in the lower parts of the CNS and whose axons directly innervate skeletal muscles.

Lower Motor Neurons: Control

Excitatory and inhibitory signals from neural circuits throughout the brain and spinal cord ultimately converge on lower motor neurons to control movement.

Lower Motor Neurons: Brainstem

Axons of LMNs extend from the brainstem through cranial nerves to innervate skeletal muscles of the face and head

Lower Motor Neurons: Spinal Cord

Axons of LMNs extend from the spinal cord through spinal nerves to innervate skeletal muscles of the limbs and trunk

Lower Motor Neurons (LMNs): Final Common Pathway

Lower motor neurons are called the final common pathway because they are the only neurons that provide motor output from the CNS directly to skeletal muscle fibers.

Neural Circuit

The neural circuits controlling movement are organized into four distinct subsystems that provide input to lower motor neurons

Neural Circuit: Local Circuit Neurons

Local circuit neurons receive impulses from somatic sensory receptors (e.g., nociceptors and muscle spindles) and higher brain centers, and integrate them into lower motor neurons to coordinate rhythmic muscle movements.

Neural Circuit: Local Circuit Neurons: Example

Local circuit neurons help coordinate rhythmic activity in specific muscle groups, such as alternating flexion and extension of the lower limbs during walking.

Neural Circuit: Upper Motor Neurons

Upper motor neurons have cell bodies in the upper CNS that either directly or indirectly

synapse with lower motor neurons to initiate and control voluntary movement.

Neural Circuit: Upper Motor Neurons: Indirect Pathway

Most upper motor neurons synapse with local circuit neurons, which in turn synapse with lower motor neurons.

Neural Circuit: Upper Motor Neurons: Direct Pathway

Few upper motor neurons synapse directly with lower motor neurons

Neural Circuit: Upper Motor Neurons: Primary Motor Cortex

Upper motor neurons from the primary motor cortex of the cerebrum plan and execute voluntary movements

Neural Circuit: Upper Motor Neurons: Brainstem

Upper motor neurons from motor centers of the brainstem help regulate posture, balance, muscle tone, and reflexive movements of the head and trunk.

Neural Circuit: Upper Motor Neurons: Brainstem: Example

Examples of upper motor neurons originating from motor centers of the brainstem are the following: the vestibular nuclei, reticular formation, superior colliculus, and red nucleus

Neural Circuit: Corpus Striatum Neurons

Corpus striatum neurons provide input to upper motor neurons to assist movement.

Neural Circuit: Corpus Striatum Neurons: Connection

Neural circuits connect the corpus striatum nuclei to motor areas of the cerebral cortex (via the thalamus) and the brainstem to help initiate and terminate movements, suppress unwanted movements, and establish muscle tone.

Neural Circuit: Cerebellar Neurons

Cerebellar neurons aid movement by controlling the activity of upper motor neurons

Neural Circuit: Cerebellar Neurons: Connection

Neural circuits connect the cerebellum to the motor areas of the cerebral cortex (via the thalamus) and the brainstem to enable upper motor neurons to fix errors from the differences between intended and actual movements

Neural Circuit: Cerebellar Neurons: Function

Connections between the cerebellum to the cerebral cortex and brainstem help coordinate body movement and maintain posture and balance.

Motor Areas of the Cerebral Cortex

Control of body movements involves motor pathways that begin in motor areas of the cerebral cortex: premotor cortex and the primary motor cortex

Motor Areas: Premotor Cortex

The desire to move a part of the body is processed in the premotor cortex, which forms a motor plan for which muscles should contract, how much they contract, and in what order.

Motor Areas: Premotor Cortex: Pathway

Cortical areas (such as prefrontal and association areas) create the desire to move; the corpus striatum processes it; the premotor cortex plans the movement via the thalamus; and the primary motor cortex executes it.

Motor Areas: Premotor Cortex: Additional Function

The premotor cortex also stores information about learned motor activities and creates a specific sequence for which muscles to contract.

Motor Areas: Primary Motor Cortex

The main brain region that executes voluntary movements by sending signals through descending pathways to the spinal cord and brainstem, controlling specific muscles on the opposite side of the body.

Motor Areas of the Cerebral Cortex: Primary Motor Cortex: Motor Homunculus

A visual map of the human body laid across the primary motor cortex, showing how much cortical area is devoted to those muscles in skilled, complex, or delicate movements

Motor Pathways

The axons of upper motor neurons extend from the brain to lower motor neurons via two types of pathways—direct and indirect

Motor Pathways: Direct

The direct motor pathway allows nerve impulses for voluntary movements to propagate from the primary motor cortex to lower motor neurons; also known as the pyramidal pathways

Motor Pathways: Direct: Components

The direct motor pathways consist of axons that descend from pyramidal cells of the primary motor cortex and premotor cortex

Motor Pathways: Direct: Pyramidal Cells

Upper motor neurons that have pyramid-shaped cell bodies, which are also the main output cells of the cerebral cortex

Motor Pathways: Direct: Pathways

The direct motor pathways consist of corticospinal pathways and the corticobulbar pathway

Direct Pathways: Corticospinal Pathways

The corticospinal pathway carries nerve impulses from upper motor neurons in the primary motor cortex, which form corticospinal tracts that descend through the internal capsule and cerebral peduncle to control muscles of the limbs and trunk.

Direct Pathways: Corticospinal Pathways: Medullary Pyramids

The descending axon bundles of the corticospinal tracts form two prominent ventral bulges on the medulla oblongata, known as the medullary pyramids

Direct Pathways: Corticospinal Pathways: Lateral Corticospinal

About 90% of corticospinal axons cross to the opposite side of the medulla oblongata, forming the lateral corticospinal tract in the lateral white funiculus, then synapse with local circuit neurons or lower motor neurons in the anterior gray horn of the spinal cord.

Direct Pathways: Corticospinal Pathways: Lateral Corticospinal: Output

Lower motor neurons in the spinal cord's anterior horn innervate and terminate in distal limb muscles to execute precise, agile, skilled movements of the hands and feet.

Direct Pathways: Corticospinal Pathways: Anterior Corticospinal

About 10% of corticospinal axons remain on the same side of the medulla oblongata, forming the anterior corticospinal tract in the anterior white funiculus

Direct Pathways: Corticospinal Pathways: Anterior Corticospinal: Output

Some corticospinal axons cross over through the anterior white commissure at each spinal cord level, then synapse with local circuit neurons or lower motor neurons that innervate and terminate in the trunk and proximal parts of the limbs

Direct Pathways: Corticobulbar Pathways

The corticobulbar pathway carries nerve impulses from upper motor neurons in the primary motor cortex, which form corticobulbar tracts that descend through the internal capsule and cerebral peduncle to control muscles of the head

Direct Pathways: Corticobulbar Pathways: Corticobulbar Axons

Corticobulbar axons may either cross to the opposite side and terminate in the motor nuclei of cranial nerves III, IV, V, VI, VII, IX, X, XI, and XII

Direct Pathways: Corticobulbar Pathways: Lower Motor Neurons

The lower motor neurons of the cranial nerves control precise, voluntary movements of the eyes, tongue, and neck (e.g., chewing, facial expression, speech, and swallowing).

Control of Movement by the Brainstem

Control of body movements involves four major motor centers of the brainstem that help regulate body movements: vestibular nuclei, reticular formation, superior colliculus, and red nucleus

Motor Pathways: Indirect

The indirect motor pathways allow nerve impulses for involuntary movements to propagate from the brainstem motor centers to lower motor neurons, also known as extrapyramidal pathways

Motor Pathways: Indirect: Types

Upper motor neurons descend from the brainstem motor centers into five major tracts of the spinal cord that terminate on local circuit neurons or lower motor neurons: rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, and medial reticulospinal tracts

Motor Pathways: Indirect: Function

The indirect motor pathways regulate posture, balance, muscle tone, and head and trunk reflexes; however, the rubrospinal tract also helps control voluntary upper-limb movements.

Indirect Pathways: Postural Reflex

These reflexes, controlled by upper motor neurons in the brainstem, involuntarily keep the postural muscles of the trunk and limbs in an upright and balanced position=

Indirect Pathways: Postural Reflex: Input

Postural reflexes receive input from the eyes (body position), vestibular apparatus (head position), and proprioceptors in muscles and joints (limb position) to help maintain posture and balance.

Indirect Pathways: Postural Reflex: Output

In response to this sensory input, upper motor neurons in the brainstem activate lower motor neurons, which in turn cause the appropriate postural muscles to contract in order keep the body properly oriented in space.

Indirect Pathways: Vestibular Nuclei

The vestibular nuclei receive equilibrium information from the vestibulocochlear (VIII) nerve and cerebellum, which send impulses through the vestibulospinal tract to contract muscles of the trunk and proximal parts of the limbs to maintain posture.

Indirect Pathways: Reticular Formation

The reticular formation receives information about the body’s position and movement, then uses discrete nuclei to send impulses through the medial and lateral reticulospinal tracts to maintain posture and appropriate muscle tone during movement.

Indirect Pathways: Reticular Formation: Source

The reticular formation receives input from several sources, including the eyes, ear, cerebellum, and corpus striatum

Indirect Pathways: Reticular Formation: Medial Reticulospinal Tract

The medial reticulospinal tract excites the trunk and proximal limb extensor muscles and works with the lateral reticulospinal tract to maintain posture and regulate muscle tone during movement.

Indirect Pathways: Reticular Formation: Lateral Reticulospinal Tract

The lateral reticulospinal tract inhibits the trunk and proximal limb extensor muscles and works with the medial reticulospinal tract to maintain posture and regulate muscle tone during movement.

Indirect Pathways: Superior Colliculus

The superior colliculus receives visual and auditory information (from the eyes and ears via the inferior colliculus) that signals through the tectospinal tract to muscles of the head and trunk, allowing the body to respond to sudden stimuli.

Indirect Pathways: Superior Colliculus: Function

Because superior colliculus protect you from potentially dangerous stimuli, this means the input must be sudden and unexpected

Indirect Pathways: Superior Colliculus: Saccades

The superior colliculus is also an integrating center for saccades, rapid, simultaneous eye movements that quickly shift from one point of focus to another (e.g., reading sentences)

Indirect Pathways: Superior Colliculus: Gaze Centers

The superior colliculus contains upper motor neurons that activate gaze centers in the reticular formation of the midbrain and pons, which then controls cranial nerves III, IV, and VI to coordinate horizontal or vertical saccades (rapid eye movements).

Indirect Pathways: Red Nucleus

The red nucleus receives input from the cerebral cortex and the cerebellum, which signals through the rubrospinal tract to the distal parts of the upper limb, allowing for fine, precise, voluntary movement

Indirect Pathways: Red Nucleus: Role

The rubrospinal tract plays a minor role in controlling distal upper-limb muscles, but becomes more important for movement when the lateral corticospinal tract is damaged

Corpus Striatum

The corpus striatum and cerebellum influence movement through their upper motor neurons in the following ways: initiating movement, suppressing unwanted movements, regulating muscle tone, and controlling some nonmotor functions.

Corpus Striatum: Movement Initiation

Neurons in the corpus striatum receive input from sensory, association, and motor areas of the cerebral cortex, which signals to the thalamus, premotor cortex, and upper motor neurons, activating corticospinal and corticobulbar tracts to promote movement.

Corpus Striatum: Suppression of Unwanted Movements

The corpus striatum suppresses unwanted movement by continuously inhibiting neurons in the thalamus that affect upper motor neuron activity in the motor cortex.

Corpus Striatum: Suppression of Unwanted Movements: Desired Movement

When movement is desired, the corpus striatum removes the inhibition to activate appropriate upper motor neurons in the motor cortex.

Corpus Striatum: Regulation of Muscle Tone

Neurons of the corpus striatum send nerve impulses into the reticular formation that reduce muscle tone via the medial and lateral reticulospinal tracts.

Corpus Striatum: Regulation of Muscle Tone: Damage

Damage or destruction of some corpus striatum connections causes a generalized increase in muscle tone

Corpus Striatum: Regulation of Nonmotor Processes

The corpus striatum influences several nonmotor aspects of cortical function, including sensory, limbic, cognitive, and linguistic functions (e.g., the corpus striatum initiate or terminate some cognitive processes, such as attention, memory and planning)

Cerebellum

The cerebellum helps learn and perform rapid, coordinated movements by monitoring intended and actual movements, comparing motor commands with sensory feedback, and sending corrective feedback

Cerebellum: Movement Intentions

The cerebellum receives impulses from the motor cortex and corpus striatum via the pontine nuclei in the pons regarding what movements are planned.

Cerebellum: Actual Movement

The cerebellum receives information about actual movement from proprioceptors in muscles and joints, the vestibular apparatus, and the eyes, which travel mainly through the anterior and posterior spinocerebellar tracts.

Cerebellum: Comparing Command Signals

The cerebellum compares intentions for movement with the actual movement performed.

Cerebellum: Correcting Errors

If there is a difference between intended and actual movement, the cerebellum sends corrective feedback to upper motor neurons to reduce errors and smooth movements, while also learning new motor skills over time.

Cerebellum: Correcting Errors: Pathway

Corrective signals travel through the thalamus to upper motor neurons in the cerebral cortex, but travel directly to upper motor neurons in brainstem motor centers.