reflexes Flashcards
Functions of reflexes
- automatically adjust posture and adapt motor patterns to achieve a behavioral goal
- adjusts proper amount of force needed for task
- provide fast acting safety reactions to avoid hazardous situations
some reflexes involve just sp cd and peripheral nerves, others are heavily modulated by supraspinal sites (brainstem and motor cortex)
b/c descending paths modulate spina reflexes, damage of CNS results in altered strength of spinal reflexes
how is the status of a muscle sensed
muscle receptors: components of muscle that monitor length and tension (Muscle spindles and golgi tendon organ)
afferent neuron that innervates receptor: sensory neurons that innervate muscle fibers and relay information about length and tension to sp cd (cell bodies are in the DRG and TG)
what causes an effect on the muscle
efferent neuron: motor neurons in ventral horn of sp cd
how is informations modified between sensing and causing effect
- interneurons modify reflex locally in spinal cord
- descending neurons from the cortex, brainstem, and send axons down thru spinal tracts and modify spinal reflex
- interneurons and descending neurons can be either excitatory or inhibitory
muscle spindle
- lies parallel to extrafusal fibers (force generating fibers), sense muscle length. key for proprioception: length of muscle is associated with angle of joints
- muscle spindles + joint and cutaneous afferents help sense positions of limbs with respect to body
- discharge best when muscle is stretched, fall silent when muscle shortens
- composed of intrafusal fibers which sense length of muscle
types of intrafusal fibers
1) dynamic nuclear bag fibers- response to lengthening of muscle adapts over time
2) static nuclear bag fibers/ 3) nuclear chain fibers-response is steady over time for static and nuclear chain fibers
muscle spindle sensory afferents
spiral around the intrafusal fibers in the spindle
1) Ia afferents- innervate all 3 types of intrafusal fibers in the spindle. Sense muscle length and rate of change of length- convey fast phasic dynamic responses of muscle fibers. velocity of stretch, very sensitive to small changes
2) II afferents- innervate static bag fibers and nuclear chain fibers. sense muscle length (not rate)-convey slow tonic static responses, steady state, static length. when intrafusal fibers are stretched, sensory endings are stretched and increase firing rate; unloaded-sensory endings stop firing
why do we need dynamic and static intrafusal fibers in the muscle spindle
muscles cross joints. muscle length changes when the angles of the joints change.
- when muscle is stretched, there is a dynamic phase
- when length changes and
- static phase when muscle stabilizes to new length.
need dynamic fibers to sense when the muscle is changing and static fiber to sense when the muscle has stabilized
both are very sensative to small changes and provide info about unexpected changes in length, useful to generate quick corrective measures
CNS uses muscle spindles to sense and change postion of body segments
Golgi tendon organ
- located at junction between muscle fibers and tendon in series with extrafusal fibers
- sense muscle tension
- sensory afferent: innervated by a single Ib afferent neuron branches intertwine among collagen fasicles
- stretching tendon organ straightens collagens fibers, compresses nerve endings , AP firing
- discharge best when muscle connected to golgi tendon contracts
- very sensitive to muscle contraction
- precisely measures force in the contracting muscle
purpose of golgi tendon organs
continuously measures tension from force in a contracting muscle and sends info to NS about state of contraction of the muscle
Protective against too much muscle tension, protects against muscle damage
muscle efferents
lower motor neurons that control muscle.
alpha motor neuron- neural control of workhorse, force generating extrafusal muscle fibers
Gamma motor neurons- neural control that regulates sensitivity of muscle spindle intrafusal fibers
Gamma motor neurons
dynamic- innervate dynamic nuclear bag fibers
static- innervate static nuclear bag fiber and nuclear chain fibers
Activation of gamma motor neurons causes shortening of the intrafusal fiber, stretches central region of intrafusal fiber, increases firing of afferant fiber
adjusts dynamic and static sensitivity of the muscle spindle and thei afferents. response of muscle spindles can be tuned
alpha- gamma co activation
during voluntary movements, gamma motor neurons and alpha motor neurons are co-activated so that both extrafusal and intrafusal fibers contract
This maintains spindle sensitivity as the muscle shortens
Prevents spindle sensory fiber from falling silent when muscle shortens from active contraction
Automatically maintains muscle spindle sensitivity over all muscle lengths
stretch reflex
monosynaptic excitatory reflex
