Reflexes Flashcards
refelx
- an involuntary stereotyped motor response to a sensory stimulus
- Coordinated pattern of muscle contraction and relaxation evoked by sensory stimuli
- they are adaptable for the task they are not fixed
- Some reflexes involve just spinal cord and peripheral nerves
- simple stretch reflex
-Other reflexes are modulated by descending pathways from
cortex and brainstem
What can modulate a reflex
sensory stimuli
spinal interneurons
descending neurons
Why do we need spinal reflexes and what do they allow us to do
-To automatically keep our body upright, maintain muscle tone, allow us to hold still = postural control, keep our balance
-They allow us to automatically adapt motor patterns to achieve a behavioral goal and smooth out fine motor movements
-Adjust amount of force in muscles for a specific task
which prevents overly forceful movements
-Provide fast-acting safety reactions to avoid danger, injury
Why are spinal reflexes useful for the brain
Frees up brain to do other, more complicated
things (difficult motor tasks, cognition, memory, et
Why does measuring reflexes help to diagnose neurological disorders
damage to CNS descending pathways results in altered strength of reflexes
so altered stregnth of relexes can let a physician know that there may be CNS damage
Areflexia
(negative sign) loss of a reflex
Hyporeflexia
reduced reflex strength
Hyperreflexia
positive sign) overactive reflex
What are three things that reflexes need to do
- They need to sense the status of a muscle
- Receptors in muscle monitor length and tension of muscle Muscle spindles, Golgi Tendon Organs
- Sensory neuron (afferent) that innervates receptor. Relays information to spinal cord
* Cell bodies are in the DRG or TG - Cause an effect on the muscle: Efferent neuron
(lower motor neurons in spinal cord and brainstem) - Sometimes need to modify information between sensing ⇒ causing effect
How do reflexes modify information between sensing and causing an effect
1) Interneurons in spinal cord modify reflex locally
2) Descending neurons from cortex, brainstem project
down and modify spinal reflex
-Net effect of interneurons, descending control can be
excitatory or inhibitory
What are the two receptors that give feedback to the CNS about muscle status
- Muscle spindles
2. Golgi tendon organs
Muscle spindles
- what are they made of
- location
- function
- what makes them active/discharge
-made of intrafusal fibers
-Lie parallel to extrafusal fibers
- Sense muscle length
-Important for proprioception:
**Length of muscle correlates with angle of joints Muscle spindles give info about position of limbs
-Discharge when muscle is stretched; silent when
muscle contracts
Types of intrafusal fibers in muscle spindle and what do they sense
-Dynamic nuclear bag fibers
*Sense change in length
Response adapts
-Static nuclear bag fibers & nuclear chain fibers
*Sense static length
Response remains steady
Where are muscle spindle sensory afferents located
they spiral around the intrafusal fibers of the muscle
What are the two types of muscle spindle sensory afferents
-think of them as stretch receptors
Ia afferent: velocity of stretch
II afferents: Sense static length
Ia afferents
muscle spindle sensory afferent
- sense length and rate of change in length
- Convey fast, dynamic responses
- Code VELOCITY of stretch,
- very sensitive
II afferents
- Sense static length
- Slow, tonic response
- Code duration of stretch
Explain how the firing changes when these receptors are stretched or unloaded (contracted)
stretched: afferent are activated and increase their action potential firing rate
unloaded/contracted: afferents stop firing (bc they are stretch receptors and they aren’t stretching)
Why do we need dynamic and static intrafusal fibers in the muscle spindle?
Provide feedback about unexpected changes in muscle length. Allow quick corrective measures in muscle contraction.
-at muscle cross joints, the length of the muscle changes when the angle of the joint changes. so having both static and dynamic fibers gives the body knowledge of proprioception basically
Why do you Need dynamic fibers
to sense when muscle is changing
Why do you Need static fibers
to sense when muscle has stabilized at new length
Where are golgi tendon organs located
at the junction of muscle fibers and tendon
*In series with 15-20 extrafusal fibers
Function of golgi tenson organ
sense muscle TENSION (force)
Ib afferent
- location
- what do they innervate
- discharge
-Also called Golgi tendon organs
-in the tendon
-innervates capsule of muscle fiber
-intertwines between collagen fiber mesh
-stretch compresses nerve endings
-discharge is best when muscle
connected to Golgi contracts (bc the muscle contracting lengthens the tendon which puts more force on the golgi tendon. bc the golgi tendon senses tension this means a larger firing.
-Precisely, continuously measures force in contracting muscle
Why do we need Golgi tendon organs?
- Continuously measure force in a contracting muscle
- Give CNS precise info about state of contraction of muscles at all times
this allows you to:
- Give precise control of muscle tension so appropriate force is exerted for a task. you can pick up delicate objects without crushing them
- Provide negative feedback to CNS as a safety device. This protects against too much muscle tension and damage, by inhibiting motor neurons
ex: unintentionally dropping a weight bc it is too heavy
Muscle Spindles vs. Golgi tendon organs
Muscle spindle: sense length (spindLE)
Golgi tendon organ: sense tension
tendon=TENSION
What innervates extrafusal fibers
alpha motor neurons-key force generating
What innervates spindles
Gamma motor neurons
What is the role of gamma motor neurons
- Role is to contract and shorten intrafusal fibers when they become slack, so that sensory afferents keep firing
- **Role is to keep muscle spindle sensitive at all muscle lengths. keep muscle spindles sensitive & under tension
How doe gamma motor neurons keep muscle spindle sensitive and under tension
- fire when muscle is stretched (lengthened) Ia spindle afferents
- Muscle contraction shortens muscle and spindle becomes slack ⇒ Ia afferents stop firing
(bc the spindle senses length and when it shortens it isn’t long) - Activation of gamma motor
neurons shortens spindle by
causing contraction of
intrafusal fibers - Spindle remains taut
and Ia afferents keep firing
** Gamma motor neurons automatically maintain spindle sensitivity at all muscle lengths
What doe alpha motor neurons do
contract muscle
Gamma motor neurons
-keeps spindles sensitive as muscle shortens
-Prevent sensory afferent from falling silent during contraction
-Provides positive feedback to reinforce activation of alpha motor neurons so muscle contraction continues
-Prevent sensory afferent from falling silent during contraction
Keep muscle spindles sensitive at all muscle lengths (and angles)
What happens to alpha and gamma motor neurons during voluntary movements
they are “co-activated” by motor commands from CNS
Bc as your muscle shortens your spindles become slack and stop firing. Your gamma neurons sense this and they cause the intrafusal fibers to contract which makes the spindles taut again causing them to fire. This allows the body to sense the position and length of the muscle at all lengths and angles
Stretch Reflex: Monosynaptic excitatory reflex
- clinically called the “deep tendon reflex”
- strike the tendon with a hammer (muscle is lengthening bc there is force on the tendon) so it counters that by contracting the muscle
- this happens very fast (less than 20 ms)
Describe the different neurons that fire throughout the stretch reflex
- stimulus: hitting the tendon stretches the muscle. this lengthens the spindles and Ia afferents are excited
- Ia afferent projects to spinal cord.
- Excites α-motor neurons that project to the biceps (homonymous) and contracts.
- Also excites and contracts brachialis (synergistic). Monosynaptic.
- Ia afferent also excites inhibitory interneuron which inhibits α- motor neurons projecting to triceps (antagonist). Disynaptic. Antagonist muscle is inhibited. this is called Reciprocal innervation
Decreased or absent reflex (hyporeflexia; areflexia), decreased muscle tone
⇒ Indicates a problem with some component of reflex arc ⇒ Indicates disease of muscles, neuromuscular junction; sensory neurons, lower motor neurons
– Increased reflex (hyperreflexia or spasticity), increased muscle tone
⇒ Indicates an upper motor neuron lesion ⇒ Indicates loss of descending inhibitory control over
lower motor neurons
Why do we need the monosynaptic reflex?
-Maintain appropriate amount of tension in muscle for proper
motor control
*Maintain muscle tone for posture, balance
*Allows us to hold still
*Smoothes out movements, fluid not jerky
*Increases efficiency for walking, running
Role of Flexion and crossed-extension reflex (Flexor Reflex)
*Role: Protection: withdrawal from painful stimulus
Step on sharp glass
Automatically causes withdrawal of injured foot (contraction of flexors in leg bends, raises knee)
Simultaneously, get increased support of opposite leg
Flexion and crossed-extension reflex (Flexor Reflex)
-describe this from sensation to Spinal cord and onward
-Polysynaptic = multiple synapses, interneurons involved
Stimulus: Step on broken glass Activates A δ nociceptors
Spinal cord: Activate interneurons same side --excite ipsilateral flexor --inhibit ipsilateral extensor ⇒ Withdraws leg from glass
Activate interneurons that cross spinal cord --inhibit contralateral flexor --excite contralateral extensor Polysynaptic ⇒ Extends opposite leg for support…leg to stand on
⇒ Stronger the stimulus, the
stronger, faster the reflex Step on sharp glass vs. stepping on blunt plastic
Stimulus modifies reflex
Golgi Tendon Reflex
- Golgi Tendon Reflex Stimulus: Muscle contracts and puts tension on Golgi tendon organ
⇒ activates Ib afferent and Ib inhibitory interneuron
–Complex: other cutaneous and joint receptors
and descending pathways also converge on same Ib inhibitory interneuron
—Activation of Ib inhibitory interneuron inhibits α- motor neuron that innervates original muscle
⇒ Inhibits the muscle from contracting polysynaptic
Also prevents too much force exertion, muscle injury
How does the golgi tendon reflex help you pick up delicate things like touching a bubble without popping it
⇒ increase in muscle tension activates Golgi tendon reflex
⇒ inhibits motor neurons, grasp stops; Don’t pop bubble
Mechanisms that modulate spinal reflexes
1) Inhibitory interneurons in spinal cord help coordinate reflex actions
• coordinate reciprocal innervation (Excitation of synergistic muscles; Inhibition antagonists)
- Purpose: Simplify voluntary control so higher
commands don’t have to send separate
commands to opposing muscles
2) Descending control: Descending neurons (upper motor neurons from cortex, brainstem) synapse on neurons in spinal cord. Modulate reflexes
Spinal cord sites for descending modulation:
- α-motor neuron
- interneurons
- presynaptic terminals of sensory afferent fibers
Due to many descending influences, force of a reflex varies even though stimulus is constant
Reflexes are not constant or fixed; they can be modified…
depend on stimulus intensity, context, posture, task requirements, prior training, cognitive influences
Reflexes are not constant or fixed; they can be modified…
depends on
stimulus intensity, context, posture, task requirements, prior training, cognitive influences
