15- Reflexes & Simple Circuits

What are the characteristics of a reflex?

Signal sent via an efferet (motor) neuron in respinse to afferent (sensory) stimulation

- rapid response

- typically invoulantary (can be under voluntary control)

- stereotyped

- graded response

What are the components of a reflex arc?

- sensory receptor

- afferent (sensory) neuron

- CNS connection (monosynaptic: 2 neurons connecting or polysynaptic: 3 neurons connecting)

- efferent (motor) neuron

- effector muscle

Steps of reflex arch

1) Receptor: skin

2) Sensory neuron travels into CNS

3) In CNS: something happens either Mono or polysynaptic

Motor neuron sends response to effector muscle

Response seen

(All happening at the same level)

extrafusal fibers are:

- causes the contraction

- Primary force-generating muscle fibers

- makes up the bulk of muscle tissue

- attach to tendons

Extrafusal fibers are innervated by:

Alpha motor neurons

Intrafusal fibers are:

- sensory receptors that provide information about muscle length and stretch

- connect to tendon, tendon to bone

- found within muscle spindles

Intrafusal fibers are innervated by

gamma motor neurons

Where are Intrafusal fibers housed?

Intrafusal fibers are housed within muscle spindles sends info about stretch and length

Muscle spindle

- in all skeletal muscles

- low threshold (doesn't need a lot to activate)

- parallel to voluntary skeletal muscles fibers

- stretch receptors: monitor length and speed of length

- 3-4 mm long spindle -shaped receptors

- both motor and sensory innervation

Sensory innervation of muscle spindles are:

Afferent (1a) nerve fibers

Golgi Tendon Organ (GTO)

- response to muscle tension!

- mechanoreceptors located at the musclulotendinous junction

- high threshold receptors

-response to shortening of muscle during consternation and tighten collagen fibers and squeezes nerve ending

- 1b nerve fibers

Nociception is

Pain. Free nerve endings

What are 3 types of noxious stimulus:

• Mechanoreceptors

• Thermal

• Polymodal receptors

Mechanical receptors

Mechanical stimuli (i.e. sharp, pricking)

Thermal

Slow burning, cold sharp, pricking

Polymodal

Mechanical stimuli and temperature (i.e. hot and cold, mechanical) BOTH fired together

Lying under a warm weighted blanket is an example of what?

Polymodal

Walking on hot sand in bare feet is an example of?

Thermal

Doing an ice bath plunge is an example of?

Thermal

Where in the spinal cord do the afferent neurons synapse?

Dorsal horn

Which sensory input would stimulate the mechanoreceptors only?

a. Walking on hot sand in bare feet

b. Doing an ice bath plunge

c. Stepping on a Lego in bare feet

d. Lying under a warm weighted

blanket

C. Stepping on a Lego in bare feet

Myotatic reflex (aka stretch or deep tendon reflex)

- sensory receptor = muscle spindle

- Quick tap delivered to tendon by reflex hammer causes stretch of tendon which deforms muscle spindles to initiate action potential

• Fast-conducting afferent fibers (1a) synapse directly on 𝛼

motor neurons in the spinal cord, which innervates the

muscle fibers of the same muscle (homonymous muscle)

Muscle contraction

• ↑ excitability of agonist

• ↓ excitability of antagonist

• Monosynaptic

• Two neuronal arc

• Ipsilateral

Homonymous muscle means

Same muscle (ex. Biceps)

Agonist means

Primary mover (synonymous with homonymous) (ex. Biceps)

Antagonist

Relaxed muscles (Ex. Triceps)

Reciprocal inhibition of myotatic reflex

• Fibers from muscle spindle also synapse with an inhibitory

interneuron in the spinal cord

• Makes an inhibitory synapse with an motor neuron 𝛼 innervating the antagonist muscle (heteronymous muscle)

• Activation of muscle spindle afferents cause contraction of

homonymous muscle and reciprocal inhibition of the

antagonist (heteronymous) muscle

• Di-synaptic

Reciprocal inhibition example

Quad reflex but tight hammis! Reflex inhibition comes in and turns off the antagonist so even though hammis are tight, reflex is still made

Inverse myotatic reflex

• Sensory receptor = Golgi Tendon Organ (GTO)

• Makes excitatory synapse with an interneuron that inhibits

motor neuron that innervates the homonymous muscle 𝛼

group —>Reduces contraction of homonymous muscle

• Protects muscles from excessive force or tension

• a.k.a. Autogenic inhibition or reciprocal activation

Inverse myotatic reflex example

I pick up something really heavy and fire biceps significantly. This stimulates the GTO, GTO through 1b fiber goes to dorsal horn. Inhibits biceps

(Helps prevent us from getting hurt! Turns off muscle when you’re using too much force!)

Gamma motor neurons are located where?

Ventral horn of the spinal cord and mixed eight aplpha motor neurons

Gamma motor neurons are LMN or UMN?

LMN!

Gamma motor neurons innervate

Muscle spindle. skeletal muscle and produce contraction —>

Stretches muscle spindle in the middle —> Stimulates 1a

afferent fibers —> Indirectly causes muscle contraction

Other characteristics of gamma motor neurons

Under supraspinal control

• Reticular formation

• Cerebral cortex (Corticospinal tract)

• Vestibular nucleus

Descending motor fibers work together to regulate muscle

tone via gamma motor neurons

*Explain the neurological process that occurs when a physical therapist assesses the patellar tendon reflex.

1) Tap patellar tendon and tendon is quickly stretched

2) Muscle spindles are activated

3) sends signal 1a fibers into dorsal horn

4) 1a fiber synapse on alpha motor neuron of effector muscle (quads) = cause contraction

5) 1a fibers also synapse with inhibitor interneurons, inhibits reciprocal inhibition of antagonist (hammis)

***GTO (1b fibers sensitive to tension) is not stimulated because there is no inverse myotatic reflex

**RE WATCHWhat is the difference between reciprocal inhibition and the inverse myotatic relex?

Reciprocal inhibition: inhibits antagonist

Inverse myotatic reflex: happened bc of stimulus of GTO and 1b

Flexor (withdrawal) reflex

• Sensory receptor = Nociceptors (Free nerve endings)

• Noxious stimulus applied to the skin stimulates free nerve endings —>Impulses conducted through myelinated small-diameter afferent fibers and unmyelinated afferent

fibers

• Synapse with several motor neurons 𝛼 in spinal cord (3-4 interneurons) —>Contraction of ipsilateral flexor muscles and relaxation of ipsilateral antagonist extensor muscles

• Net effect = Withdrawal of limb away from noxious stimuli

• Polysynaptic

Flexor (withdrawal) reflex example

Crossed extension reflex

• Sensory receptors = Nociceptors (Free nerve endings)

• Noxious stimulus activates pain fibers —>Impulses conducted to the spinal cord via afferents

• Collateral signals sent through anterior commissure (ventral white commissure) of spinal cord —>Multi-synaptic connections with 𝛼 motor neurons that innervate contralateral flexor and extensor muscles —>Produces contraction of extensors contralateral to the site

of noxious stimulation

• Polysynaptic

We need crossed extension reflex to balance out flexor withdrawal reflex! Example

Keeps you standing on one leg as you lift your leg off of a sharp shell

Reflex modulation

• Reflexes can be suppressed or modified during functional

activities or postures (sitting vs standing, ambulation)

• Affected by consciousness (sleeping, awake, comatose)

• Supraspinal control and ongoing sensory input can affect impulse transmission through spinal pathways

Clinical abnormal stretch reflexes

Hyperexcitability/Hyper-reflexia: UMN lesion

• Alpha and gamma motor neuron hyperexcitability

• Loss of descending cortical control

Hypoexcitability/Hyporeflexia: LMN lesion

Name three [3] conditions that would results in hyper-reflexia?

Stoke, TBI, spinal cord injury, MS

Name three [3] conditions that would result in hyporeflexia?

ALS, peripheral nerve damage, Bell's palsy, herniated discs (puts pressure on nerve root)

A patient presents with an intervertebral disc herniation

between C6-7 on the left. What examination findings are MOST LIKELY on a neurological screen (myotome, dermatome, and reflex testing)?

C7 nerve root. Weak elbow extension, wrist flexion. What areas are hypo=middle finger, posterior forearm and posterior arm. Change in triceps reflex, hyporeflexive because it's a LMN!

A patient presents with the following

examination findings. Sensation testing: Hypoesthesia across the patella, anterior-medial lower leg, foot, and great toe on the left. All other regions intact. Myotome testing: Intact BUE/BLE except for left ankle dorsiflexion

weakness Deep tendon reflex testing: Intact throughout BUE/BLE except for hyporeflexia left patellar tendon reflex.

Which nerve root is MOST LIKELY involved?

L4 nerve root is affected. Myotome: L4 does dorsiflexion. Quad reflex (L2, 3, 4)