Proprioception and Movement

Joint Receptors

• found in the capsules and ligaments around the joints

• activated by distortion that accompanies the changes in the relative positioning of the flexible joints

• integration of signals takes place in cerebellum

• rapid adaptation rate

Muscle Spindle Functions

• establish muscle tone to maintain posture and balance

• response to muscle stretch to avoid damage from over-stretching

• provide damping effect for muscle contraction

Muscle Spindles at Rest

• spindles are tonically active and firing even when muscle is relaxed

1. extrafusal muscle fibers at resting length

2. sensory neuron is also tonically active

3. spinal cord integrates function

4. alpha motor neurons to extrafusal fibers receive tonic input from muscle spindles

5. extrafusal fibers maintain a certain level of tension even at rest (muscle tone)

Muscle Spindles (Alpha Activation Alone)

• in the absence of gamma motor neuron activation

1. alpha motor neuron fires

2. muscle contracts

3. less stretch on center of intrafusal fibers

4. firing rate of spindle sensory neuron decreases

Gamma Motor Neuron

• constitute about 30% of all the motor neurons

• normally, there is always some gamma excitation

• usually co-activated with alpha motor neuron

• maintain responsiveness of muscle spindle, regardless of muscle length

Muscle Spindles (alpha-gamma coactivation)

• usually occurs in a normal muscle

• allows spindle to remain active while contractile extrafusal fiber contracts

1. alpha and gamma motor neuron are both firing

2. extrafusal and intrafusal fibers both contract

3. contraction of the intrafusal fibers can maintain the stretching of the central region of the spindle while extrafusal fibers are contracting

Golgi Tendon Organ

• type of proprioceptor located at the junction of a tendon and a muscle

• responds primarily to muscle tension during an isometric contraction

• relatively insensitive to muscle stretch

• reflexes cause muscle relaxation and to prevent muscle damage from having too high a load

Vestibular Apparatus

• semi circular canals

• can move fluid around in it

• information about movement and position

• otolith organs for linear acceleration and head position

• otolith crystals on top of the organ are composed of calcium carbonate and protein particles

• crystals differentiate when you are vertical or horizontal

• when change of position occurs, crystals move which causes fluid to move and bends the hair cells, activating the receptors

Types of Reflexes

• spinal cord: referred to as the somatic reflex which involves contraction of skeletal muscles

• brain stem: eye movements for tracking objects in order to keep the image to remain stationary regardless of head movements

• ANS: heart rate control

Integration of Muscle Reflexes

1. sensory output from receptors goes to spinal cord, cerebral cortex, and cerebellum. Signals from the vestibular apparatus go directly to the cerebellum

2. postural and spinal reflexes do not require integration in the cortex. Output signals initiate movement without higher input