Part 2 lecture M101A WHITE

reflexes and rythmic movements (walking or breathing)

(1)sensory feedback helps guide involuntary (reflexes) and voluntary (rhythmic) behaviors

(2)sensory and motor systems are closely connected, hard to fully separate them since behavior is continuously motivated and guided by sensory input

(3) muscles have sensory endings called muscle spindles that detect changes in muscle length and send feedback to the nervous system to help regulate movement.

Afferents for reflex circuits is IIa fibers from muscle spindles which send info about muscle stretch to spinal cord as seen in myotatic reflex ( involuntary stretch ex; patellar reflex)

Gamma motor neurons adjust the sensitivity of muscle spindles, allowing dynamic range and control so spindles can accurately signal changes in muscle length

Golgi tendon organs provide feedback on muscle contraction force, helping protect muscles from excessive force

muscle splindles that detect change in muscle lenght and send feedback to nervous system, causing movement

dye injected into the soma and travels down axon, allowing full reconstruction of axonal pathways

dye injected into target area (ex nerve terminal) and travels back to soma which helps trace connection from nerve terminal back to cells control center

Pro: traces axonal pathway from cell body to muscle, providing complete visualization of axon

Con: harder to visualize the entire motor unit and may not show all terminal connections

Pro: shows connection from muscle back to cell body, making it easier to identify the neuron controlling the muscle

Con: Won’t show the full axon pathway.

anterograde: inject into cell body (soma) of neuron

Retrograde: inject into muscle (soles) target zone

start with retrograde dye to identify the neurons connection to muscle, then follow up with anterograde dye to trace full axon pathway

representation of how different areas of the brain (like motor cortex) control specific muscles or body regions

Tontopy: mapping of sound frequencies

Retinotopy: Mapping of visual information from retina

Somatotopy: Mapping of body parts onto the brain or spinal cord

lower motor structures which are organized somatotopically onto the spinal cord

Motor map runs from medial to lateral; motor neurons for distal muscles (like thumb) located laterally, while proximal muscles (triceps) are more medial. Trunk muscles (axial muscles) positioned the most medially

experience (motor learning) injury, or rehabilitation, allowing for adaptation to new movement or recovery from damage.

Antagonist's muscles are inhibited (relax) to allow movement of agonist's muscles

Synergist muscles contract & work together w/ agonist to create movement

happens on both sides of the body, but the opposite limb, antagonist muscles may be activated for balance

the muscle that assists the prime mover muscle or agonist during movement

EX: soleus and gastrocnemius muscles both contributed to plantar flexion on the ankle

muscle that produces the opposite motor action to another muscle

EX: biceps (flexor) and triceps (extensor) muscles work against each other; when one contracts other relaxes

muscles or actions of the opposite limb

EX: if you flex your right arm, the contralateral muscle would be involved in the movement of the left arm

order in which motor units are activated based on the force

states that smaller motor units are recruited first during low-force activities, large motor units are recruited as more force is needed

recruitment order follows size principle, meaning smaller motor units are recruited before larger ones during muscle activation

neural circuit that produces rhythic activity (Ex: walking or breathing)

they have simple nervous systems, making it easier to study CPGs, which control rhythic movements like swimming or walking

Lower motor neurons: in the spinal cord and brainstem, connected directly to muscles

Local Circuit Neurons: in the spinal cord, provide coordination between muscle and most input of LMN

they send signal directly to muscles, acting as the “final common path“ for movement

muscle tone, posture, adjusting eyes, head, and body in response to sensory input (ex; visual or auditory)

Primary Motor Cortex (Brodmann's Area 4): initiated voluntary response

Premotor cortex (Broadmann's Area 6): plans and directs movement sequences

detects and reduces “motor errors” (differences between intended and actual movements) in real-time and long term

continuously adjecting to reduce motor errors, helping movements become more precise over time

prevents unwanted movements and gets the mottor system ready to intiate voluntary movements

Parkinsons Disease: difficulty initiating movements

Huntingtons Disease: Problems with movement transitions and control

group of lower mottor neurons that all innervate a single muscle

organized along the spinal cord to match the muscles they control, forming spatial map

located medically in the ventral horn of the spinal cord

Medial: Axial (trunk) muscles

Next Lateral: shoulder and pelvis muscles

Further lateral: Arm and leg muscles

Most lateral: distal muscles (ex; hands, fingers)

helps identify where specific-muscle controlling neurons are located in the spinal cord

control posture and balance by targeting medial lower motor neuron pools.

Lateral descending pathways from the motor cortex control skilled movements, especially for distal muscles like those in the hands.

They are located medially in the ventral horn and control axial (trunk) muscles for posture.

Medial circuit neurons coordinate bilateral, rhythmic movements and have long axons crossing multiple spinal segments and often the midline.

Medial: Long axons, coordinate bilateral movements.

Lateral: Short axons, control fine movements, stay on the same side.

It's the arrangement of motor neurons in the spinal cord, with medial neurons controlling trunk muscles and lateral neurons controlling distal muscles.

alpha mottor neurons innervate extrafusal muscle fibers, generating force for posture and movement

gamma motor neurons innervate intrafusal muscle fibers in muscle spindles to adjust their sesnsitity, allowing muscle to sense stretch

Extrafusal: Main muscle fibers for force and movement, innervated by α motor neurons.

Intrafusal: Specialized fibers in muscle spindles that detect stretch, innervated by γ motor neurons.

Muscle spindles are embedded within the muscle and act as sensory receptors to detect changes in muscle length.

They coordinate to maintain smooth movement by adjusting muscle length and force as the muscle changes.

In the brainstem, specifically in eight motor nuclei of cranial nerves.

A motor unit is a single α motor neuron and all the muscle fibers it innervates, forming the smallest unit of force activation in a muscle.

Slow motor units have small, red fibers, contract slowly, generate small forces, and are fatigue-resistant. They’re important for postural activities.

FF motor units generate large, quick forces but fatigue easily, making them ideal for brief, powerful actions like sprinting or jumping.

FR motor units provide moderate force and speed and are fatigue-resistant, suitable for activities requiring moderate, sustained force.

The innervation ratio is the number of muscle fibers per α motor neuron, influencing muscle function:

Soleus: 180 fibers for sustained posture.

Gastrocnemius: 1000-2000 fibers for quick movement.

Extraocular: 3 fibers for precise eye movement.

Motor units adapt based on use; for example, sprinters develop fast, fatigable fibers for power, while marathon runners develop slow, fatigue-resistant fibers for endurance.

Motor units are recruited in order of size, with small S units activating first, followed by FR units for moderate force, and FF units for maximum force.

Increased firing rates in motor neurons lead to temporal summation of muscle contractions, increasing overall muscle force.

state where muscle contractions are smooth and forceful, and no peaks and droughts

max firing rate 20-25 Hz

it averages out tension changes, allowing smooth movement

Phasic response to small stretches, sensitive to velocity

Static nuclear bag and nuclear chain fibers, signaling sustained stretch.

Excitatory input to a muscle and inhibitory input to its antagonist.

Deep tendon or myotatic reflex (e.g., knee-jerk response).

Group II afferents.

It adjusts α motor neuron activity to maintain desired muscle length.

Control spindle sensitivity by modulating excitability of intrafusal fibers.

Dynamic enhances Ia dynamic response; Static enhances Ia and II static response.

Force generated in response to stretch; high gain = more force for small stretch.

Standing on a moving bus - modulated by upper motor neuron pathways.

To allow muscle fiber lengthening, essential for warming up in athletic performance.

They are co-activated to prevent muscle spindles from becoming unloaded or overactivated.

Independently of α motor neurons, for precise movement control and fine adjustments.

During difficult, precise movements or in unpredictable situations.

The excitability level of α motor neurons.

They suppress excitatory signals to specific α motor neurons, influencing reflex gain.

It acts as a feedback system to reduce muscle force and prevent excessive tension.

It increases Ib afferent firing, sensitive to muscle tension.

Spindles monitor and maintain muscle length; Golgi tendon organs monitor and maintain muscle force.

They signal hyperextension or hyperflexion, enhancing the protective response of Ib interneurons.

Withdrawal of a limb from a painful stimulus by activating ipsilateral flexor muscles and inhibiting ipsilateral extensor muscles.

the activation of contralateral extensor muscles and inhibition of contralateral flexor muscles, providing postural support during withdrawal.

They modulate responsiveness to sensory inputs, sometimes enhancing or suppressing reflex withdrawal based on the stimulus type.

CPGs are specialized local circuits that coordinate rhythmic muscle activation for movements like walking, independent of sensory input or higher brain signals.

The stance phase, where the limb is in contact with the ground, and the swing phase, where the limb is lifted and moved forward.

At higher speeds, the limb movement pattern changes from a progressive sequence to synchronized movements (e.g., trotting, galloping).

CPGs in the spinal cord allow coordinated movement, and inputs like treadmill speed can regulate the rate of locomotion.

Dopamine (from l-DOPA injection) activates local circuits, supporting the CPG function even without upper motor neuron input.

Human locomotion relies more on upper motor neuron pathways and postural control, which spinal CPGs alone can't fully support.

They consist of excitatory and inhibitory neurons and are connected with modular circuits for coordinating left-right and forelimb-hindlimb actions.

It is a set of signs and symptoms resulting from damage to the lower motor neurons in the brainstem or spinal cord, leading to issues such as paralysis, muscle weakness, and loss of reflexes.

Symptoms include paralysis or weakness (paresis), loss of reflexes (areflexia), loss of muscle tone, spontaneous muscle twitches (fibrillations and fasciculations), and eventual muscle atrophy.

Fibrillations result from increased excitability of individual denervated muscle fibers, while fasciculations stem from abnormal activity in injured α motor neuron units.

Long-term denervation and disuse of the affected muscles result in their gradual wasting or atrophy.

The stretch reflex is a monosynaptic circuit where sensory fibers from muscle spindles connect with α motor neurons, helping control muscle tone and maintaining muscle length.

Damage interrupts reflex pathways, leading to areflexia (loss of reflexes) and decreased muscle tone, as reflex arcs are crucial for maintaining baseline muscle activity.