week 5 Flashcards
(30 cards)
What is the hierarchical organization of the motor/sensorimotor systems?
The motor/sensorimotor systems are organized hierarchically. Higher-level planning and decision-making occur in secondary motor areas (such as the supplementary motor area [SMA] and premotor cortex [PMC]) and posterior parietal cortex, while the primary motor cortex (M1) sends execution signals. Subcortical structures—including the basal ganglia and cerebellum—and lower centers (brainstem, spinal cord) further integrate planning, coordination, and execution of movements.
How is the cerebral cortex organized in terms of layers?
The cerebral cortex is made up of six distinct layers (laminae):
Lamina 1: Contains dendrites.
Lamina 2: External granular layer.
Lamina 3: Pyramidal cells.
Lamina 4: Internal granular layer (often the main recipient of thalamic sensory input).
Lamina 5: Pyramidal cell layer containing output neurons (e.g., Betz cells).
Lamina 6: Internal granular layer. Inputs and outputs are highly layer-specific, and the cortex forms the outer surface (grey matter) of the forebrain, covering subcortical structures.
Who was responsible for defining distinct cortical areas and how many did he describe?
The neuranatomist Brodmann defined distinct cortical areas based on cytoarchitectural differences. He described approximately 52 areas, each of which is functionally relevant.
What characterizes the primary motor cortex (M1) anatomically and functionally?
Location and Function: The primary motor cortex, known as M1 or Brodmann Area 4, is like the brain’s control room for voluntary movements. It sends the “go” signals that make your muscles move.
Special Cells: Inside M1, there are special large nerve cells called Betz cells. Think of them as the powerful messengers that quickly transmit movement commands from your brain to your muscles.
The Body Map: M1 is organized like a map of your body. But rather than matching the exact size of your body parts, this map gives more space to areas that need detailed control. For example, even though your hand is smaller than your leg, it gets a larger area on this map because it performs more precise, fine movements.
Contralateral Control: Also, the motor cortex on one side of your brain controls the opposite side of your body. So, the left side of M1 directs movements for your right side, and vice versa.
What is the role of Betz cells in motor function?
Betz cells are large pyramidal neurons located in layer 5 of M1. They project their long axons via the corticospinal tract to the spinal cord, targeting both alpha and gamma motor neurons (and spinal interneurons), thus initiating, regulating, and controlling voluntary, skilled movements. Their projections cross at the medulla, ensuring contralateral control of the limbs, and they also send signals to the brainstem.
How did early researchers map the motor cortex?
Fritsch and Hitzig (1870): Used electrical stimulation in dogs to show that localized stimulation produces simple movements.
Penfield (1940): Stimulated the motor cortex during surgery in epileptic patients, observing that such stimulation evokes simple movements. These studies led to the establishment of the somatotopic motor map (the “homunculus”).
What does the somatotopic organization of the motor cortex imply?
Somatotopic organization means that specific body parts are represented in distinct, contralateral locations in the motor cortex. However, rather than reflecting the physical size of the body part, the size of the cortical representation is proportional to the precision or fine motor control required by that part.
How realistic are the traditional “homunculus” cartoons of the motor cortex?
The traditional homunculus cartoons are an oversimplification. In reality, the cortical motor maps are more interdigitated and less discrete than the cartoons suggest. Effector-specific regions may be interwoven with regions that show different connectivity, structure, and function—especially those involved in planning rather than direct execution.
What differences are observed when applying short versus prolonged electrical stimulation to M1?
Brief microstimulation (around 50 ms) produces simple movements or muscle contractions, while prolonged stimulation can evoke complex, goal-directed, coordinated actions that resemble natural movements.
What motor deficits occur following a lesion in the motor cortex?
Lesions in the motor cortex, such as those occurring in strokes, can lead to a permanent loss of fine motor control—for example, impairing the precision grip and the independent use of fingers.
In discussions about what is represented in the motor cortex, what is the debate between muscles versus movements?
There is ongoing debate on whether the motor cortex exclusively codes for individual muscles or for more complex movements. Although M1 can be electrically stimulated to produce specific muscle contractions, it also contributes to complex actions. The exact nature of its coding remains unclear; it sends the signals necessary for execution without exclusively detailing either muscles or movement patterns.
Where is the primary motor cortex (M1) located in the brain?
M1 is located in the frontal lobe of the brain, directly anterior to the primary somatosensory cortex.
What roles do secondary motor areas (such as SMA and PMC) play in movement?
the Supplementary Motor Area (SMA) and the Premotor Cortex (PMC)—are primarily involved in planning, preparation, and coordination of movement. They integrate sensory, cognitive, and decision-making information from other brain regions (including the frontal cortex and posterior parietal cortex) to prepare complex, goal-directed actions before they are executed by M1.
How does the posterior parietal cortex (PPC) contribute to sensorimotor integration?
The PPC integrates sensory input from various modalities (visual, tactile, and proprioceptive) and is instrumental in planning and guiding movements in space. It helps determine potential actions based on environmental cues and spatial orientation.
What distinctions exist within the Supplementary Motor Area (SMA)?
The SMA is now functionally divided into two areas:
SMA Proper: More involved in learning sequential movements.
Pre-SMA: Plays a larger role in the initiation and planning of internally generated movements as opposed to stimulus-driven actions.
What functions are attributed to the dorsal premotor cortex (PMd)?
The dorsal premotor cortex (PMd) is crucial for the preparation of movements through setting up motor plans. It is particularly important in learning conditional actions (e.g., associating a red light with braking and a green light with accelerating) and in preparing the motor system during “set” periods before movement execution.
What are mirror neurons, and where were they first identified?
Mirror neurons are cells that fire both when an individual performs a goal-directed action and when the individual observes or hears the same action performed by someone else. They were first reported in the ventral premotor cortex (PMv) and are believed to play a role in learning by imitation and understanding others’ intentions.
How are mirror neurons functionally linked to Broca’s area in humans?
In humans, mirror neurons in PMv are functionally connected with Broca’s area (Brodmann Areas 44/45). They exhibit stronger responses during actions related to food or objects and are thought to contribute to the learning and understanding of complex actions and intentions.
What is the role of the Basal Ganglia in motor control?
The Basal Ganglia are involved in planning, programming, integrating, and executing movement. They interact with the motor cortex, cerebellum, and brainstem to fine-tune and regulate the initiation and coordination of voluntary movements.
What is neuroplasticity, and why is it important in the sensorimotor system?
Neuroplasticity is the brain’s ability to form and reorganize synaptic connections in response to learning, experience, or injury. In the sensorimotor system, this is crucial for remapping somatotopic representations, skill acquisition (e.g., musicianship), and recovery after lesions such as strokes or amputations.
How do somatotopic maps change to reflect neuroplasticity?
Somatotopic maps can expand or shrink based on sensory input and training. For instance, training (as seen in pianists) can expand the representation of fingers, while denervation or amputation can lead to a reduction in the corresponding map area. Even co-activation (such as surgically fusing two digits) can lead to fused representations.
What is long-term potentiation (LTP) and what role does it play in learning?
LTP is an activity-dependent, long-lasting strengthening of synapses. It results from high-frequency stimulation that causes increased efficacy in signal transmission between neurons and is considered fundamental for learning and memory, including aspects of motor learning.
What is long-term depression (LTD) and how is it different from LTP?
LTD is an activity-dependent reduction in synaptic strength. While LTP increases signal transmission by inserting more AMPA receptors into the post-synaptic membrane, LTD typically involves a decrease in synaptic efficacy (for example, through a reduction in AMPA receptors). In the cerebellum, LTD is believed to be a major mechanism of motor learning and is not necessarily NMDA-dependent.
What are the three key principles of LTP?
-Cooperation: Only a simultaneous activation of multiple axons produces the large depolarization required for LTP.
-Associativity: A weak stimulus can undergo potentiation if paired with a strong one.
-Synapse Specificity: LTP occurs only at synapses that are actively engaged, leaving inactive synapses unchanged.
