Chapter 11_A_3: Proprioceptive Receptors

How many different types of joint receptors are there?

Four!

Muscle Spindle Shape

Fusiform, meaning it narrows at the ends and is wide in the middle. It includes 3-12 intrafusal muscle fibers, covered by a sheath of connective tissue

Muscle spindle innervation - Secondary (II) afferent fiber location

innervates the equatorial region at both ends (more lateral than the annulospiral endings). Present in both nuclear bag and nuclear chain fibers. Both the nuclear bag and nuclear chain have tonic receptors

Alpha motor neurons go to (intrafusal/extrafusal) fibers, while gamma motor neurons go to (intrafusal/extrafusal) fibers

Alpha = extrafusal Gamma = intrafusal

What do receptors in the muscles (muscle spindle) and tendons (GTO) detect?

1. Changes in muscle length 2. The rate of the change in length 3. The force of contraction (tension)

What are the three types of proprioreceptors?

1. Joint receptors 2. The muscle spindle 3. the Golgi tendon organ

What do joint receptors detect?

1. Moment-to-moment changes in joint angle 2. Speed of joint motion 3. Amount of joint compression or distraction

What are the two types of intrafusal muscle fibers inside each muscle spindle?

1. Nuclear bag fibers 2. Nuclear chain fibers

Two types of afferent fibers that innervate intrafusal fibers of the muscle spindle?

1. Primary (Ia) afferent nerve = annulospiral nerve fiber 2. Secondary (II) afferent nerve = flower spray endings

Explain the GTO mechanism

1. When a muscle contracts, a tendon stretches 2. The GTO senses the stretch and fires (mostly with a very strong muscle contraction and with extreme passive stretch of the tendon) 3. Nerve impulses from the GTOs are transmitted by Group Ib afferent axons to the SC, then to the CB and cerebrum. 4. In the SC, GTO axons synapse on inhibitory interneurons, which inhibit the alpha motor neuron from contracting muscle, thus inhibiting the original agonist **Pink = proprioception function **Blue = Reflex function

Muscle tone (and therefore the muscle spindle) is essential for what two functions?

1. regulation of movement 2. maintenance of posture

What group are GTO fibers in?

1b afferent

Nuclear bag fibers

A larger fiber (bag shaped) with numerous central nuclei. They detect the velocity of a stretch (how fast) and are referred to as phasic receptors

Muscle spindle - reflex function

A response to stretch on a muscle, which includes: 1. response to stretch of extrafusal muscals by initiating the stretch reflex 2. Stretch reflex causes muscle contraction, thereby returning muscle to original length

Explain reciprocal innervation in the GTO reflex

At the same time of the GTO reflex, an axon collateral synapses on excitatory interneurons, which excite alpha motor neurons to the antagonist muscle, causing it to contract. Therefore, the GTO prevents over-contraction of the agonist by inhibiting further contraction of the agonist and facilitating contraction of the antagonist *important note: the interneuron requires a lot of EPSPs to fire. This allows proprioception to continuously go to the brain, but reflex does not always happen, unless there is a strong enough force

GTOs inhibit the original agonist that caused the GTO to fire. This is an example of what?

Autonomic inhibition

Describe disinhibition of the GTO

CNS centers can inhibit the GTO inhibition, thus resulting in increased muscle force. This occurs during weight training and is most noticeable during the first 2 weeks and during extremest stress situations (Did not build muscle, just inhibition of the GTO allow you to appear stronger). During extreme situations, the increased muscle force could potentially lead to injurt to the muscle/tendon unit

Somas of alpha MN and gamma MN are where?

in the anterior horn of the SC

Each intrafusal muscle fiber has (2/3/4/5) regions, which are: (list)

Each has 2 regions, which are: 1. Equatorial region 2. Ends

True or False: GTOs are stretch receptors, while muscle spindles are not

False. Both are stretch receptors, though their location and functions do differ

True or False: Both types of intrafusal fibers (nuclear bag and nuclear chain) have afferent innervation but no efferent innervation

False. Both types have both afferent and efferent innervation

True or False: The GTO, muscle spindles, and joint receptors are proprioceptors that also participate in reflexes

False. Only GTOs and muscle spindles participate in reflexes. Joint receptors do not participate in reflexes

True or False: both primary and secondary afferent fibers participate in the stretch reflex

False. Only group Ia (primary) participates in reflex. Secondary (group II) do not, they are only proprioceptors.

True or False: Joint receptors are involved in reflexes

False. The joint receptors function only as receptors and are not involved in reflexes.

GTO function - reflex

GTOs provide "self-regulation" of muscle contraction at the level of the spinal cord. They detect muscle tension and response to excessive muscle contraction (or excessive passive stretch) by inhibiting the muscle from further contraction for injury prevention

What efferent fiber innervates intrafusal fibers of the muscle spindle?

Gamma (y) motor neurons

Where are muscle spindles found?

In ALL skeletal muscles

GTOs are (parallel/in series) to a bundle of (extrafusal/intrafusal) muscle fibers

In series; extrafusal

How can GTO injury occur? How can it be prevented?

Injury can occur by an abrupt and forceful contraction or passive stretching, which can tear slips of tendon (or even the whole tendon) from attachment to bone or muscle. This can be prevented by "warming up" a muscle via stretches before forceful contractions are performed

Muscle that perform dexterous movements have (less/more) muscle spindles. Explain

More; more muscle spndles are needed to provide sensory feedback to CNS when the movement is complex. Muscle spindles are more numerous in the hand muscles

Nuclear chain - yMN innervation

Receives tonic/static y innervation; involved in tonic muscle contraction for static holding states as well as in postural reflexes

Which has slower and smaller diameter fibers: Primary (Ia) afferent fibers or Secondary (II) afferent fibers

Secondary (II) afferent fibers; they are less sensitive to stretch than larger diameter Ia afferents.

Sensory information from proprioceptors is relayed to what portions of the CNS and via what respective tracts?

Sensory information is sent to the: 1. Cerebrum via the DCML; (conscious) 2. Cerebellum via the spinocerebellar tracts (unconscious)

Intrafusal "Ends" region

Striated, contractile ends that contain acting and myosin. They receive efferent (motor) innervation via gamma MN

What proprioceptors are involved with reflexes?

The GTO and muscle spindle function in reflexes, in addition to their receptor function

GTO function - proprioception

The GTO provides information to the cerebrum and cerebellum (via the DCML and spinocerebellar tracts) about the extent of muscle contraction at all times. They fire most rapidly when the muscle is contracting

Muscle spindle - Primary (Ia) afferent fiber function

The Ia fibers are phasic in nuclear bags (e.g., detect stretch amount & velocity) and tonic in nuclear chains (e.g., detect stretch amount & duration). The Ia fibers also participate in stretch reflex.

What makes up the GTO reflex arc?

The Ib sensory fiber, the inhibitory interneuron, and the alpha motor neuron to the agonist.

True or False: GTOs are stretch receptors

True!

Why is autogenic inhibition important in the GTO reflex

The inhibition protects against injury to the muscle/tendon unit *reminder, autogenic inhibition = Stimulation of GTOs causes inhibition of further contraction of the agonist while activating the antagonist

Explain how joint receptors function in the interpretation of information

The joint receptors sends moment by moment feedback information to the CNS. The information is dependent on the location of the receptors and the direction and magnitude of deforming forces

Muscle spindle - proprioceptive function

The muscle spindle detects: 1. Length of muscle fibers and tension they produce 2. Changes in length and tension of muscle fibers 4. Rate of changes in length and tension of muscle fibers

Describe the location of the muscle spindle

The muscle spindle is located inside the skeletal muscle and has intrafusal muscle fibers.

Mechanism of joint receptors

They emit baseline action potentials (several per minute) as a resting output. They are mechano-receptors, so are deformed by either compression or stretch. When stimulated, they discharge a high frequency burst of nerve impulses. Slack may decrease baseline firing or may even be silent.

True or False: Proprioceptive receptors provide both proprioception and kinesthesia

True. They detect changes in position and movement of a segment of the body, respectively

True or False: Joint receptors are proprioceptive receptors

True

True or False: Like joint receptors, GTOs function in proprioception. Unlike joint receptors, GTOs also have a reflex function

True

True or False: The brain interprets a change from baseline whether it is the higher frequency discharge or the decreased firing/silence.

True

True or false: different nerve fibers innervate each intrafusal muscle fiber in the muscle spindle

True

True or false: the ratio of muscle spindles to extrafusal muscle fibers vary in different muscles

True

Where are joint receptors located?

in the connective tissue around a joint. In the joint capsule, connective tissue, and ligaments surrounding the joint

How can the GTO reduce muscle cramps?

When plantar flexors (gastrocnemius - agonist) are cramping, place a sustained stretch on them by pulling the ankle into dorsiflexion. Dorsiflexion causes the passive stretch of the tendon attached to the calf muscle; the GTO of the plantar flexors will become activated. It will inhibit contraction of the agonist (calf muscle), causing relaxation of the calf muscle

Muscle spindle - Primary (Ia) afferent fiber location

Wraps around the equatorial region of the intrafusal fibers (both nuclear bag and nuclear chain).

Function of the muscle spindle

a part of reflex arc; responds to and controls changes in length of extrafusal muscle fiber.

Nuclear chain fibers

a smal diameter fiber with numerous nuclei which are located along the length of the fiber; they detect the duration of a stretch adn are referred to as tonic receptors

Which of the following statements describe phasic receptors? (Hint: 6 are correct) a) respond (fire) quickly to a stimulus b) sensory receptor c) respond (fire) slowly to a stimulus d) continues to fire (produce APs) during the entire duration of the stimulus e) returns to their resting potential (stop firing) even though stimulus is still applied f) adapts quickly g) does not adapt quickly h) provides information about the amount of stretch i) provides information about the duration of stretch k) provides information about the velocity of the stretch

a, b, e, f, h, k

GTO provides autogenic inhibition, while muscle spindles provide:

autogenic excitation

Proprioceptors are receptors of: a) the special senses b) somatosensation c) both a and b

b) somatosensation

Which of the following statements describe tonic receptors? (Hint: 6 are correct) a) respond (fire) quickly to a stimulus b) sensory receptor c) respond (fire) slowly to a stimulus d) continues to fire (produce APs) during the entire duration of the stimulus e) returns to their resting potential (stop firing) even though stimulus is still applied f) adapts quickly g) does not adapt quickly h) provides information about the amount of stretch i) provides information about the duration of stretch k) provides information about the velocity of the stretch

b, c, d, g, h, i

What type of receptor are proprioceptors? a) chemoreceptors b) thermoreceptors c) mechanoreceptors d) baroreceptors

c) mechanoreceptors

Function of joint receptors

detect position (angle) and rate of movement of a joint; this allows the receptors to provide conscious and unconscious awareness of limb positions at all times, provide information of positional relationships of body parts, and assist in development of body image (psychological - can contribute to anorexia)

Which of the following is false about proprioceptors? a) All function in conscious proprioception through the DCML b) All have unconscious proprioception that is sent to the cerebellum c) All are GSAs d) All participate in spinal reflexes

d) all participate in spinal reflexes

Muscle spindle innervation - Secondary (II) afferent fiber function

detects the amount and duration of a stretch

yMN innervation

gamma MN are thin motor fibers that innervate the contractile ends of the intrafusal fibers. It varies for the nuclear bag and nuclear chain

Intrafusal

inside the muscle spindle

What happens when sensory information is relayed to the CNS?

it is integrated with information from the special senses, especially vision and vestibular. The intergrated information is then used by motor control centers to modify motor behavior (e.g., adjusting location, type, number, and frequency of motor unit activation for dvlpmnt of appropriate muscle tension to perform desired movement)

Where are proprioceptive receptors located?

joints, ligaments, tendons, and skeletal muscle.

The stretch reflex is the basis for ____________________

muscle tone

Intrafusal equatorial region

non-contractile center; receives afferent (sensory) innvervation via group Ia and II afferent neurons. the non-contractile units are made up of somas.

Extrafusal

normal muscle fibers (around spindles)

There are usually ___(number range)___ nuclear bags per muscle spindle and ___(number range)___ nuclear chain fibers per muscle spindle

nuclear bags = 2-4 nuclear chains = 4-9

Muscle spindles are (parallel/in series) to a bundle of extrafusal muscle fibers

parallel. GTOs are in series with extrafusal muscle fibers while muscle spindles are parallel to extrafusal muscle fibers

Nuclear bag - yMN innervation

receives phasic/dynamic y innervation. involved in phasic motion (stretch reflex)

What does the GTO sense?

stretch from the tendon; each GTO is selectively sensitive to the force produced by the bundle of in-series muscle fibers connected to it

Mechanoreceptors respond to what?

stretch, tension, and pressure

Where is the GTO located?

within tendons, near the musculotendinous junction (the area where the muscle fibers attach to the tendon, closer to the muscle)