The Motor System PDF

**The Motor System** - The motor system enables voluntary, complex movements, which are carefully timed and coordinated. **Extrapyramidal disorders are common in psychiatric patients**. **Side effects of prescribed medications mimic extrapyramidal disorders** It operates in a **hiera...

**The Motor System** - The motor system enables voluntary, complex movements, which are carefully timed and coordinated. **Extrapyramidal disorders are common in psychiatric patients**. **Side effects of prescribed medications mimic extrapyramidal disorders** It operates in a **hierarchical and parallel organization** where information flows across multiple control centers. It uses two main mechanisms: - **[Negative Feedback]:** **Adjusts movements by comparing intended and actual movement**. - **[Feed Forward]:** Utilizes memory to anticipate and adjust for rapid movements. **How the Motor System Works** Motivation for movement appears to be **subcortical** → **limbic areas, hypothalamus & cerebellum (dentate nucleus) and areas of basal ganglia** Motor program (sequence of intended movement) drawn up in ***premotor + supplementary motor areas and relayed to 1° motor cortex*** Motor commands from the brain are sent along two primary pathways, which ***decussate (cross sides) before activating muscles:*** - **Upper Motor Neuron (UMN):** Initiates movement from the brain cortex. - **Lower Motor Neuron (LMN):** Connects to muscles, enabling movement. Clinical significance: - **UMN lesions:** Cause ***spasticity***, **exaggerated** reflexes, and **positive Babinski sign**. - **LMN lesions:** Lead to **flaccid paralysis**, **muscle atrophy**, and ***loss of reflexes***. **Major Pathways of Axons from the Brain** - The loop → cerebral cortex → basal ganglia, thalamus & back to cortex has two components, one direct (facilitating movements) and one indirect ***(inhibiting movements***). **Output via GPi & SNr is inhibitory to thalamus** 1. **Lateral Pathways (Direct):** **Control fine, voluntary movements.** The direct pathway -- inherent /spontaneous **activity of GPi is ↓ by striatal activity results in disinhibition!** -- thus facilitating movement 2. **Ventromedial Pathways (Indirect):** **Influence posture and reflexive movements via brainstem control**. **The Cerebellum** Located in the back of the brain, the cerebellum is crucial for ***movement coordination***. It receives sensory information and adjusts muscle actions accordingly. - The 4 deep nuclei are dentate, globose, emboliform & fastigial **(latter two = interpositus nucleus)** largely responsible for output **GlEFaDe** - **Organization:** Divided into **midline (trunk control),** **paravermal (limbs),** and **lateral (digits) zones**. 1. **Mossy Fibers:** - **Origin:** Diverse sources, including the cerebral cortex, spinal cord, and brainstem. - **Target:** **Granule cells in the cerebellar cortex**. - **Signal:** **Weak** excitatory input to Purkinje cells. 2. **Climbing Fibers:** - **Origin:** **Inferior olivary nucleus**. - **Target:** Directly to Purkinje cells. - **Signal:** **Strong excitatory** input to Purkinje cells, leading to complex spikes. - **Purkinje Cells:** - **Target:** **Deep cerebellar nuclei (DCN).** - **Signal:** Inhibitory GABAergic output. - **Deep Cerebellar Nuclei (DCN):** - **Target:** Various brain regions, including the thalamus, red nucleus, and vestibular nuclei. - **Signal:** Excitatory output, modulating motor control and coordination. - **Timing and Coordination:** The cerebellum plays a crucial role in timing and coordinating movements. It receives input from various sensory modalities and uses this information to fine-tune motor commands. - **Motor Learning:** The cerebellum is involved in motor learning, allowing us to acquire and refine new motor skills. - **Balance and Posture:** The cerebellum helps maintain balance and posture by integrating sensory information from the vestibular system and proprioceptors. - **Functions:** Coordinates movements, prevents over-corrections, and adjusts learned motor skills. - **Dysfunction Effects:** Causes ***ataxia (uncoordinated movement),*** tremors, and slurred speech. - \- **Lesions result in ataxic movement**: Action tremor, abnormal path of movement, - In mild dysfunction, visual cues essential for smooth & purposeful movement. - **Slurred speech & dysarthria.** - Hypotonia - Posture -- **face turns contra- laterally & ipsilateral shoulder lowered**. - Often affected by **alcohol & thiamine deficiency** **The Basal Ganglia** Comprises **5 prominent subcortical nuclei**: The basal ganglia consist of interconnected nuclei **(caudate, putamen, globus pallidus, subthalamic nucleus, and substantia nigra**) that are essential ***for initiating and regulating movement.*** - Extensive interconnection + links to motor areas of brain - **Primary input from cerebral cortex, output via thalamus back to cortex (NB ∆ cerebellum!)** - **Output (GPi/SNr) tonic inhibition on thalamus** - **Direct Pathway:** Facilitates movement **by reducing inhibition on motor activity**. - **Indirect Pathway:** **Suppresses unnecessary movement by increasing inhibition.** - **Motor Dysfunction:** Lesions can lead to hyperkinetic (excessive movement, e.g., Huntington\'s disease) or hypokinetic disorders (reduced movement, e.g., Parkinson\'s disease). - The loop → cerebral cortex → basal ganglia, thalamus & back to cortex has two components, one direct (facilitating movements) and one indirect (inhibiting movements). Output via GPi & SNr is inhibitory to thalamus - **Parkinson\'s Disease** - Caused by damage to **dopamine-producing neurons** in the substantia nigra, leading to **overactivity in inhibitory pathways**. - **Symptoms:** Tremors, stiffness, slow movement, and **expressionless face**. - The loop → cerebral cortex → basal ganglia, thalamus & back to cortex has two components, one direct (facilitating movements) and one indirect (inhibiting movements). Output via GPi & SNr is inhibitory to thalamus **Differences Between Basal Ganglia and Cerebellum** - **Basal Ganglia:** Controls movement initiation and pattern selection. - **Cerebellum:** Refines and coordinates ongoing movement. - **Key Point:** **Basal ganglia are involved in the cognitive planning of movement, while the cerebellum adjusts movements in real time**.