CPG

Question 1

Central Pattern Generators

Answer 1

Networks of interneurons in the spinal cord and brainstem that create rhythmic, patterned motor activity

Example: walking, breathing

When triggered they run automatically without conscious thought

Question 2

Why are CPGs needed?

Answer 2

Reflexes alone can’t restore posture or coordinate complex sequences — CPGs provide the rhythm and sequencing.

Question 3

Complex/Volitional Movement

Answer 3

Movements planned and refined by higher brain centers

Motor cortex, basal ganglia, cerebellum.

Examples: Playing piano, sports, speech, etc.

Adds precision, learning, and voluntary control on top of reflexes and CPGs.

Question 4

Where are CPGs located?

Answer 4

• Mainly in spinal cord (lumbar for legs, cervical for arms).
• Also in brainstem (respiration, chewing).
• Linked with hypothalamus and reticular formation.

Question 5

Pacemaker neurons

Answer 5

Generate rhythmic bursts

Question 6

Diffuse Excitation

Answer 6

Neurons switch on/off to sustain rhythm

Question 7

Reciprocal inhibition

Answer 7

Flexor and extensor networks shut each other off to alternate phases.

Question 8

Phase‑dependent reflexes

Answer 8

Reflexes can be modulated depending on the step phase.

Question 9

Flexor Burst Generator (Leg Step Cycle)

Answer 9

• Activates flexor motor neurons → leg bends → swing phase.
• Fixed duration (swing is always about the same length).
• Eventually inhibited → stops flexion

Question 10

Extensor Burst Generator (Leg Step Cycle)

Answer 10

• Activates extensor motor neurons → leg straightens → stance phase.
• Variable duration (stance depends on speed and load).
• Reciprocal inhibition ensures only one is active at a time

Question 11

Rhythm of the Leg Step Cycle

Answer 11

• Flexion → Extension → Flexion → Extension.
• Creates the basic walking cycle.

Question 12

Sensory control of walking

Answer 12

• Reflexes adjust contraction to match load (how much weight the leg bears).
• Sensory cues (proprioceptors, mechanoreceptors) tell CPG when it’s safe to start the next swing.
• Transition rules:
• Leg must not be bearing weight.
• Hip extended.
• Opposite leg in stance.
• Crossed projections link left and right leg CPGs for alternating gait.

Question 13

Arm Swings

Answer 13

• CPGs in cervical cord coordinate arm motion.
• Arm swing is diagonal with opposite leg (right arm forward when left leg forward) → cancels trunk torque.

Question 14

Upper Body Balance

Answer 14

• Reticular formation CPGs coordinate trunk and head posture.
• Essential for bipedal stability.
• Spinal injury patients need trunk support because these CPGs are disrupted.

Question 15

Head Angle

Answer 15

• Despite body motion, head angle stays stable.
• Achieved by combined visual, vestibular, and proprioceptive reflexes

Question 16

Postural Maintenance

Answer 16

• Organized in reticular formation of pons/medulla.
• Relies on three sensory inputs:
• Somatosensory (proprioception): limb/joint position.
• Vestibular: gravity reference.
• Visual: slower, but provides vertical and motion cues

Question 17

Postural Maintenance

Answer 17

• Organized in reticular formation of pons/medulla.
• Relies on three sensory inputs:
• Somatosensory (proprioception): limb/joint position.
• Vestibular: gravity reference.
• Visual: slower, but provides vertical and motion cues

Question 18

Toe-off (start of SWING)

Answer 18

The foot leaves the ground. The leg transitions into flexion to clear the ground and advance forward

Question 19

SWING (flexion)

Answer 19

The hip and knee flex, ankle dorsiflexes; the leg moves forward. This phase has a relatively fixed duration across speeds

Question 20

E1 (transition into STANCE)

Answer 20

Begins at heel strike. Initial contact and loading response; extensor activity ramps to accept body weight

Question 21

HEEL STRIKE (start of STANCE)

Answer 21

The heel contacts the ground; the limb begins supporting load.

Question 22

STANCE (extension)

Answer 22

The limb bears weight and pushes the body forward.

Question 23

E1 (early stance/loading):

Answer 23

Rapid increase in extensor drive to stabilize the limb as weight is transferred.

Question 24

E2 (mid-stance)

Answer 24

Sustained extensor activity to support posture and allow forward progression; center of mass passes over the foot.

Question 25

E3 (late stance/propulsion)

Answer 25

Extensor activity peaks for push-off; then declines as the ankle plantarflexes and the forefoot unloads

Question 26

Swing and Stance main idea

Answer 26

SWING is flexion-dominant to lift and advance the limb; STANCE is extension-dominant to support and propel

Question 27

Half-centers

Answer 27

Flexor and extensor burst generators reciprocally inhibit; FBG produces fixed-duration swing, EBG produces variable-duration stance.

Question 28

Transition rules

Answer 28

Unloading + hip extension + contralateral stance are permissive signals to start swing

Question 29

Coordination

Answer 29

Leg CPGs couple across sides; arm CPGs couple diagonally with legs; reticular formation coordinates trunk and head for stability.

Question 30

E1: Initial contact and loading response sensory inputs

Answer 30

- Golgi tendon organs (GTOs): Sense tension in extensors; can facilitate extensor activity under load (phase-dependent modulation). - Muscle spindles: Stretch of extensors reinforces extensor activation. - Cutaneous receptors: Plantar contact signals support.

Question 31

E2: Mid-stance sensory input

Answer 31

Load-related afferents keep extensors engaged; reflex gains are tuned to maintain stability

Question 32

E3: Terminal stance and pre-swing (propulsion)

Answer 32

Forefoot loading and hip extension signal readiness to transition; unloading of the limb is a key permissive cue

Question 33

Stretch reflex

Answer 33

Can switch from flexor-facilitating during swing to extensor-facilitating during stance, depending on phase

Question 34

GTO reflex

Answer 34

- Classically inhibitory in static conditions, but during stance can be excitatory to support loaded extensors (phase-dependent gain)