WEEK 4: Reflexes & Posture Flashcards
State properties of reflexes
Properties
-Stereotype: ALWAYS REMAINS THE SAME.
-Specific (stimulus & response)
-Adaptable (involvement of other centers)
Revise the structure of a reflex arc.
A reflex arc is a neural pathway that mediates a reflex action. It typically consists of five essential components:
Receptor:
The process begins with a stimulus acting on a sensory receptor. Sensory receptors are specialized cells or nerve endings that respond to specific stimuli, such as pressure, temperature, or pain.
Sensory Neuron:
The sensory neuron, also known as the afferent neuron, carries the signal from the receptor to the central nervous system (CNS). The axon of the sensory neuron transmits the nerve impulse toward the spinal cord or brain, depending on the reflex.
Integration Center (Spinal Cord or Brain):
Upon reaching the CNS, the sensory neuron communicates with interneurons (association neurons) within the spinal cord or brain. The integration center processes the incoming signal and determines the appropriate response.
Motor Neuron:
The processed signal is then transmitted to a motor neuron, also known as the efferent neuron, which carries the response from the CNS back to the effector organ or muscle.
Effector Organ:
The motor neuron stimulates the effector organ, which carries out the response to the original stimulus. This could be a muscle contracting or a gland releasing a substance.
In summary, the reflex arc follows a sequence of events:
Stimulus → Receptor → Sensory Neuron → Integration Center (CNS) → Motor Neuron → Effector Organ → Response
Describe reflex action.
Outline Characteristics of Reflex Actions.
A reflex action is an involuntary and rapid response to a stimulus that occurs without conscious thought or decision-making.
Characteristics of Reflex Actions:
Involuntary: Reflex actions are automatic and occur without conscious control.
Rapid: Reflexes are typically swift, enabling a quick response to the stimulus.
Predictable: The response to a specific stimulus is consistent and predictable.
Protective: Reflexes often serve a protective function, helping to avoid harm or injury.
Basic Survival Mechanism: Reflex actions are fundamental to survival, as they allow the body to react swiftly to potential dangers.
Describe the function of the various parts of the brain to control motor behavior (revision).
Primary Motor Cortex (M1):
Location: Located in the precentral gyrus of the frontal lobe.
Function: M1 is responsible for the voluntary control of fine and discrete movements. Neurons in this region send signals to the spinal cord, influencing muscle contractions.
Premotor Cortex:
Location: Situated anterior to the primary motor cortex in the frontal lobe.
Function: Involved in the planning and coordination of movements. It plays a role in organizing sequences of movements and integrating sensory information for motor planning.
Supplementary Motor Area (SMA):
Location: Located on the medial surface of the hemisphere, anterior to the primary motor cortex.
Function: SMA is involved in the planning and initiation of complex movements, especially those requiring coordination between both sides of the body. It contributes to the control of posture and bilateral movements.
Basal Ganglia:
Components: Includes structures like the striatum, globus pallidus, and substantia nigra.
Function: The basal ganglia contribute to the planning and initiation of movements, as well as the modulation of movement patterns. They play a role in suppressing unwanted movements and refining motor output.
Cerebellum:
Location: Situated at the back of the brain, below the occipital lobe.
Function: The cerebellum is crucial for the coordination, precision, and accurate timing of movements. It receives information about ongoing movements and helps fine-tune motor output by comparing intended and actual movements.
Primary Sensory Cortex (S1):
Location: Found in the postcentral gyrus of the parietal lobe.
Function: S1 is involved in processing somatosensory information, providing feedback about the position, movement, and touch sensations of body parts. This information is crucial for motor control and coordination.
Somatosensory Association Cortex:
Location: Adjacent to the primary sensory cortex in the parietal lobe.
Function: Integrates sensory information from S1 to create a more comprehensive perception of the body’s position and movements. This information contributes to motor planning and coordination.
Frontal Eye Fields (FEF):
Location: Situated in the frontal lobe.
Function: FEF is involved in the control of eye movements, coordinating visual attention and gaze shifts.
Primary Auditory Cortex and Association Areas:
Location: In the temporal lobe.
Function: While primarily associated with auditory processing, these areas can contribute to motor control, especially in tasks involving auditory-motor integration.
Association Areas and Parietal Cortex:
Function: These regions integrate information from various sensory modalities and contribute to spatial awareness, object recognition, and the coordination of movements based on environmental cues.
Describe and understand the hierarchical nature of the motor control system (revision).
Example: Reaching for an Object:
Higher Brain Centers: The cortex plans the reach, considering the object’s location and the body’s position.
Intermediate Centers: The basal ganglia and cerebellum refine the movement, adjusting for accuracy and coordination.
Brainstem: The brainstem ensures postural stability and coordinates eye and head movements during the reach.
Spinal Cord: The spinal cord executes the motor commands, activating the appropriate muscles for reaching.
The effector organs are both the extensor and flexor muscles (in somatic reflexes)
Therefore, neurological examination tests these muscles to assess what?
The effector organs are both the extensor and flexor muscles (in somatic reflexes)
Therefore, neurological examination tests these muscles to assess the following:
-Spinal cord injury
-Sensory neuron damage
-Motor neuron damage
(revise the concept of UMN & LMN lesions)
State Some essential components of proper motor control.
Ability to generate movement for the desired action and goal (volition)
Coordination of signals to many muscle groups & joints
Proprioception
Requires receptors (length of muscles & force applied to the muscles)
Postural adjustments (must respond to shifts in center of mass)- head, torso and limb.
. Sensory feedback (& sensory input at all 4 levels-spinal cord, brain stem, motor cortex & supplementary motor area)
- Compensation for the physical characteristics of the body
- Adaptability (over time & to tasks)
- Unconscious processing (‘automaticity’)
Discuss the following concepts of
(a)functional segregation
(b)hierarchical organization of motor control. 4 levels & 2 side loops involved.
Outline them.
(a) Areas that play a role in the control of movement are scattered throughout the nervous system.
(b) The hierarchical organization of motor control refers to the structured arrangement of control centers in the nervous system, where each level has distinct functions and responsibilities.
spinal cord, brain stem, motor cortex and supplementary motor area
Basal ganglia and cerebellum
All of the messages from the 4 levels and 2 side loops involved in motor processing eventually are channeled to the muscles via only one route.
What is it called?
= the alpha (α) motor neurons = ‘final common pathway’ of motor control
State 2 areas that are directly connected with the spinal motor neurons.
State 2 tracts that cross at the brainstem to descend contralaterally, synapse with motoneurons supplying the distal muscles (fine motor control) on the opposite side.
State 2 tracts that DO NOT cross at the brainstem (ipsilateral) & synapse with motor neurons supplying the proximal muscles involved in balance & locomotion.
Only the motor cortex and brainstem motor areas are directly connected with the spinal motor neurons
Rubrospinal & corticospinal tracts cross at the brainstem to descend contralaterally, synapse with motorneurons supplying the distal muscles (fine motor control) on the opposite side.
Vestibulospinal & reticulospinal tracts DO NOT cross at the brainstem (ipsilateral) & synapse with motor neurons supplying the proximal muscles involved in balance & locomotion.
Define the following terms.
1. Motor neuron pool.
2. Motor unit
3. Innervation ratio
4. Motor unit recruitment
All explain the relationship between motor neurons and muscles
Motor Neuron Pool= several motor neurons supply a muscle.
Motor Unit=1 motor neuron + all muscle fibers innervated
Innervation ratio is the number of muscle fibers innervated by a motor unit (no. of m. fibers/no. of motor units)
low= fine motor control
high= posture & coarse control
Motor unit recruitment =incremental activation or stimulation.
Motor unit recruitment is the process by which the nervous system activates and controls motor units to produce varying levels of muscle force.
Describe the All or none response of motor units.
All or none response of motor units = “when the neuron impulse fires, all (of) the muscle fibers innervated by the nerve will contract”
Describe the innervation ratio for fine motor control and posture & course control.
Low= fine motor control
High= posture & coarse control
State the 2 general ways of Motor neuron control of force
Two general ways:
Size principle (Henneman size principle)
small motor neurons first recruited.
(b) Rate code
low frequency= small force (NB! Though not expressed this way this is explained very well in Sherwood).
Tetanus= max. state of contraction
Discuss the Hennemann size principle.
The size of the motor unit recruited depends on the size of force demand.
Smaller, low-threshold motor units are activated first, followed by larger, high-threshold motor units as the demand for force increases.
