MOTOR SYSTEM I & II LECTURES 12, 13

Chapter 5 Overview

• Topics Covered:

  • Muscle structure

  • Types of movements

  • Receptors of muscles

  • Involvement of motor cortex

Overview of Sensory Information

• Sensory Pathways:

  • Sensory information (touch & pain) travels via sensory neurons in ascending tracts to the brain.

  • Key pathways: Dorsal Column and Spinothalamic Pathway.

  • Touch processed in the Primary Somatosensory Cortex while pain processed in the Cingulate Gyrus.

• Communication with Motor Cortex:

  • Sensory cortex communicates with Primary Motor Cortex (MI) to coordinate motor activities.

• Motor Command Transmission:

  • Commands from the brain sent via descending tracts to spinal cord, where motor neurons carry them out.

Neuromuscular Junction

• Definition: Area where motor neuron connects with muscle cells (fibers).

• Functioning:

  • Muscles contract when neurotransmitters are released at the neuromuscular junction.

• Acetylcholine (ACh):

  • ACh binds to nicotinic receptors on muscle fibers, triggering contraction.

  • Absence of ACh leads to no muscle movement.

Muscle Contraction and Behavioral Changes

Types of Behavioral Changes

1. Reflexes:

   • Fast, automatic responses to stimuli.

   • Involves only spinal cord; e.g. constriction of pupils in response to light.

2. Movements:

   • Simple movement due to brief muscle contraction, usually deliberate.

3. Acts:

   • Complex behaviors (e.g., walking, talking) consisting of series of muscle contractions.

   • Requires a motor plan established prior to action.

Motor Plan

• Definition: A structured sequence of muscle contractions for a specific movement.

• Process:

  • Planning: Brain strategizes how to perform the movement.

  • Coordination: Motor cortex organizes when and how muscles contract.

  • Execution: Neural commands sent through descending pathways to trigger contractions.

Muscle Interplay

• Skeletal Muscles:

  • Connect to joints via tendons.

  • Can act as antagonists (opposing muscles) or synergists (muscles working together).

• Muscle Characteristics:

  • Muscles have a striated appearance due to actin & myosin proteins.

Proprioception

• Monitors body movement and positions through proprioceptors (receptor cells in muscles/tendons).

• Types of Proprioceptors:

  1. Muscle Spindles:

     • Send signals to CNS when muscle is stretched, prompting contraction.

  2. Golgi Tendon Organs:

     • sends signal to CNS when muscle contracts too much, preventing damage.

Movement Control and Motor Cortex

Movement is the act of changing physical position or location, which involves coordinated contractions of muscles controlled by the nervous system.

Motor Unit

• A motor unit consists of a motor neuron and the muscle fibers it controls.

• When activated, it triggers contraction in all associated fibers simultaneously.

• Motor units allow for muscle control and strength by varying the number of active units.

• Primary Motor Cortex (MI):

  • Located in the precentral gyrus; initiates motor commands.

  • Body map reflects regions needing more precise control (like hands).

• Non-primary Motor Cortex:

  • Consists of Supplementary Motor Cortex and Premotor Cortex; modulates MI activity.

  • Mirror Neurons:

    • Located in the premotor cortex and provide insight into motor behavior analysis.

    • A system for analyzing other neurons behaviors like imitation.

Pathways for Motor Commands

• Descending Pathways:

  • Pyramidal System: Lateral corticospinal tract (direct control of peripheral muscles, crossed in medulla).

  • Extrapyramidal System: Medial corticospinal tract (controls axial & proximal muscles, both sides of the spinal cord).

Pyramidal Tract (Corticospinal Tract)

• Responsible for direct control of voluntary movements, especially fine motor skills.

• Originates in the Primary Motor Cortex.

• Descends through the brainstem and crosses over at the medulla oblongata.

• Continues down the spinal cord to synapse with motor neurons.

• Essential for executing precise voluntary movements, influencing muscles in limbs and trunk.

Extrapyramidal System:

• Controls axial & proximal muscles, both sides of the spinal cord.

• Allows for balance and posture adjustments in movements.

• Involves various brain structures, including the basal ganglia, substantia nigra, and cerebellum.

• Coordinates involuntary movements and reflexes to ensure smooth, controlled actions.

• Modulates motor commands initiated by the pyramidal system, providing a balance between voluntary and involuntary control of movement.

• Important for the execution of complex, coordinated motor patterns such as walking and reaching, and for maintaining posture during movements.

Motor Control Areas

• Basal Ganglia and Cerebellum:

  • Both adjust commands from the motor cortex to ensure smooth movements and coordinate complex muscle actions.

Basal Ganglia

• Basal Ganglia: Group of nuclei in the brain involved in coordinating voluntary movements and motor learning.

• Parts include caudate nucleus, putamen, globus pallidum. (Caudate nucleus + putamen + striatum)

  • Modulates movements in conjunction with the motor cortex.

  • Involved in cognitive functions alongside motor control.

  • Assists with muscle tone and postural regulation.

  • Dysfunction linked to movement disorders like Parkinson's Disease and Huntington's Disease.

Cerebellum

• Location: The cerebellum is located at the back of the skull, beneath the occipital lobes.

• Functions:

  • Coordinates voluntary movements.

  • Maintains posture and balance.

  • Ensures smooth motor control.

  • Assists in motor learning by adjusting movements based on past experiences.

• Sensory Input: Receives sensory information from the spinal cord and other brain areas to fine-tune motor commands.

• Dysfunction Effects: Issues can lead to balance problems and coordination difficulties.

Movement Disorders

Parkinson’s Disease (PD):

• Major symptoms include rigidity, tremors, and slow movements.

• Causes: Head trauma and exposure to toxins

• Treatment Options: L-DOPA and alternative therapies (e.g., exercise, deep brain stimulation).

• Usually onsets about 65 years or older.

Huntington’s Disease (HD):

• An inherited hyperkinetic disorder with involuntary movements and cognitive impairments.

• Symptoms: Excessive jerks, facial twitches, eventually affecting mobility and cognitive functions.

• Common in between 30-50 years

• Drugs or alcohol abuse can cause early onset of disease