Lecture 6

Sensorimotor Hierarchy in the Brain

• Stimulation of the Posterior Parietal Association Cortex (PPC) during neurosurgery can evoke strong intentions and desires to move contralateral limbs or facial muscles.
• Low intensity stimulation (5 mA) of the PPC can lead to a strong intention and desire to move, while high intensity stimulation (8 mA) can cause an illusory experience of actual movement.
• Damage to the PPC can result in apraxia, a difficulty planning and performing requested voluntary movements, often caused by stroke, brain injury, or neurodegenerative disease.
• Contralateral neglect, a disrupted ability to attend to stimuli on one side of the body, can result from posterior parietal damage, leading to a bias in conscious attention.
• The Dorsolateral Prefrontal Association Cortex (DLPFC) receives projections from the PPC and is involved in facilitating decisions to make overt actions.
• The Secondary Motor Cortex primarily receives input from the association cortex and sends information to the primary motor cortex, playing a role in creating more complex movements.
• Mirror Neurons in the Secondary Motor Cortex are involved in understanding and imitating the actions of others.
• The Primary Motor Cortex is a major point of convergence for cortical sensorimotor signals and is responsible for sending specific motor commands to the body.
• Wilder Penfield's work with epileptic patients and brain stimulation contributed to neural cartography, mapping brain function through electrical stimulation.
• The Motor Homunculus in the primary motor cortex reflects a somatotopic layout and contralateral activation of muscles, scaled to the intricacy of movement.
• The Cerebellum, referred to as the "Little Brain," receives diverse inputs and is thought to compare various inputs and correct movements online as they happen.
• Cerebellum damage can lead to loss of precise control over movement, difficulty in maintaining posture and balancing, as well as broader cognitive, sensory, and emotional deficits.

The Sensorimotor System: Understanding the Brain-Body Connection

• The sensorimotor system is organized hierarchically, with the association cortex involved in abstract goal intentions and more concrete execution of movements happening as we descend down the sensorimotor pathways.
• Some sensorimotor activities can be achieved entirely within lower levels of the hierarchy, using central sensorimotor programs that can be both innate or learned.
• The spinal cord has computational complexity and can execute some movements and reflexes independently without the need for input from the brain.
• Motor neurons, motor units, and muscles are essential components of the sensorimotor system, with motor units comprising a motor neuron and the muscle fibers it innervates.
• The relationship between muscles matters, with flexors and extensors, as well as synergistic and antagonistic muscles, playing key roles in movement and coordination.
• Reciprocal innervation is a key principle of spinal cord circuitry, ensuring smooth limb movements by inhibiting one of the antagonistic muscles when the other is activated.
• Reciprocal innervation also facilitates reflexes, and many reflexes are more complex and plastic than simple withdrawal reflexes.
• Walking movements may not rely on hierarchical input from the brain, as shown in a study where cats with their spinal cords separated from the brain were able to make walking movements when stimulated.
• Central sensorimotor programs, stored in lower areas of the hierarchy, can be activated independently once learned, freeing higher-level brain structures to handle more demanding aspects of performance.
• Some sensorimotor programs develop without practice, while others require practice, with the development of expertise often taking around 10,000 hours.
• Shifting control to lower levels in the sensorimotor system allows for faster responses and frees higher-level brain structures to deal with more demanding or abstract aspects of performance.
• The sensorimotor system involves parallel processing, with different areas of the hierarchy handling various aspects of movement and control, allowing for efficient and coordinated motor activities.