somatosensory mechanisms of motor control

sensory receptors

• gather internal (body) and external (environment) info

  • somatosensory, vestibular, visual, auditory

somatosensory

• info from mechanoreceptors in skin and body

• internal info about body and limb movement from proprioceptors

• external info about environment from cutaneous receptors

vestibular

• info from mechanoreceptors in inner ear

• internal info about head and body movement

visual

• info from photoreceptors in our eyes

• ext info about environment

auditory

• info from mechanoreceptors in inner ear

• ext info about environment

3 types of proprioceptors

muscle spindles, golgi tendon organs, joint receptors

muscle spindles

• proprioceptors located in muscles

  • 2 types of muscle spindles transmit distinct types of info: type Ia and II

type Ia receptors

transmit info abt rate of muscle lengthening

type II receptors

transmit info abt muscle length

golgi tendon organs

• proprioceptors located within tendons

• free nerve endings intertwined within tendon fascicles

• detect and transmit info on forces exerted on tendons

joint receptors

• proprioceptors located in joint capsule, ligaments, and meniscus

• include free nerve endings and receptor organs similar to cutaneous receptors

• activity of joint receptors increases as joints approach limits of their ROM

CNS response to joint receptor activity

• to protect joints, ligaments, tendons, and muscles from injuries occuring at extreme joint angles

  • ruptured ligaments and dislocations

muscle spindle anatomy

• muscles contain extrafusal and intrafusal muscle fibers

extrafusal muscle fibers

• innervated by alpha motor neurons

  • produce contractile forces for movement

intrafusal muscle fibers

• innervated by gamma motor neurons

  • house muscle spindles used to sense movement

active control of muscle spindle sensitivity

• CNS actively controls sensitivity of muscle spindles

• alpha gamma coactivation: couples activation of alpha (extrafusal fibers) and gamma (intrafusal fibers) motor neurons

  • tightens intrafusal fibers to retain spindle sensitivity at diff muscle lengths

sensory function

• CNS uses sensory info for perception and motor control

perception

used for awareness and interpretation of our body and world

motor control

used to plan, initiate, monitor and regulate motor skills

proprioceptors and perception

• proprioceptors contribute to perception of body and limbs

  • position sense, kinesthesia, sense of forcep

position sense

refers to perception of body and limb position/oreintation

kinesthesia

refers to perception of body and limbs movement

sense of force

refers to perception of forces produced by body and limbs

proprioceptors and motor control

• proprioceptors contribute to motor control of body and limb

  • motor planning, monitoring, regulation

motor planning

uses info abt body and limb position and movement

monitoring

gathers body and limb info to predict movement errorsre

regulation

uses body and limb info to correct movement errors

sensorimotor integration

proprioceptors are critical for normal motor control of everyday motor skills

reflex

• repeatable, stereotyped responses to sensory stimulus

• several reflexes initiated by proprioceptors

5 typical steps of reflexes

1. sensory receptors detect stimulus

2. sensory neurons transmit signals about stimulus

3. interneurons integrate sensory and motor info

4. motor neurons transmit motor commands to muscles

5. muscles product stereotyped response

• excitatory reflex = muscle contraction

• inhibitory reflex = muscle relaxation

stretch reflex

• induces muscle contraction in response to lengthening (stretch)

• helps maintain posture and balance in response to unexpected perturbations

  • influenced by: age, health, sensory input, motor output, activation history, task goals, prior experience

reciprocal inhibition

• often coupled w stretch reflex

• inhibition of antagonist muscles

  • facilitates stretch reflex response

golgi tendon organ reflex

• induces muscle inhibition in response to large tendon forces

• prevents musculoskeletal injuries

  • inhibits agonist muscles

  • activates antagonist muscles

  • overrides stretch reflexes

CNS & GTO reflex

• CNS actively inhibits GTO reflex during motor skills

• enables motor skills that require high levels of force

  • can contribute to serious injury

arthrogenic muscle inhibition

• involves inhibition of agonist and antagonist muscles in response of joint pain

• used to prevent joint injuries

• primarily initiated by joint receptors

  • often involves integration of info from multiple sensory sources (joint receptors, GTOs, cutaneous receptors, muscle spindles

withdrawal and crossed extensor reflexes

painful stimuli under foot produce coordinated reflexes involving both legs

withdrawal reflex

• involves muscle actions that withdraw leg on same side as stimulus

  • lifts foot away from painful stimulus

crossed extensor reflex

• involves muscle actions that extend the leg opposite stimulus

• maintains balance during withdrawal reflex