UNIT 9: CONTROL OF MUSCLE MOVEMENT

Question 1

Reflexes

Answer 1

are rapid automatic responses to stimuli
that can have skeletal muscle, smooth muscle,
cardiac muscle or glands as effectors

help to protect the body and
help maintain homeostasis.

Question 2

what happens during a reflex?

Answer 2

a stimulus is detected by a
receptor and triggers signaling in 2 or more
neurons of a reflex arc to cause a consistent
motor response in an effector (target).

Question 3

Basic Structure of a Reflex Arc:

Answer 3

1. Sensory receptor – detects a stimulus
2. Sensory neuron – a first order unipolar neuron that carries afferent signals (action potential) created by the stimulus into the central nervous system (brain or spinal cord).
3. Integration center – primarily located in the central nervous system (brain or spinal cord). Ranges from a single synapse between a sensory and motor neurons to complex arcs involving multiple synapses between interneurons
4. Motor neuron – carries efferent signals (action potentials) away from the integration center towards the effector.
5. Effector (Target Cell) – Muscle fibres (skeletal, cardiac or smooth muscle) or glands create a response (by contraction or secretion respectively)

Question 4

How can neural reflexes be classified?

Answer 4

1. By the efferent division of the nervous system that
   controls the response
2. By the CNS location where the reflex is integrated
3. By whether or not the reflex is innate or learned
   (conditioned)
4. By the number of neurons in the reflex pathway
5. By which sides of the body the sensory and motor neurons are located:

Question 5

Describe the efferent division that controls the response

Answer 5

a) Somatic (skeletal muscle) reflexes – effectors are skeletal muscles that contract in response to signals from the somatic lower motor neurons.
Ø E.g.1: Knee-jerk reflex
Ø E.g. 2: withdrawal reflex
Ø E.g. 3:crossed-extensor reflex

b) Autonomic (Visceral) Reflexes – effectors are smooth muscle, cardiac muscle or glands that contract or secrete in response to signals from sympathetic and/or parasympathetic postganglionic neurons
Ø E.g.1: Micturition (urination) reflex
Ø E.g. 2: sneezing
Ø E.g. 3: vomiting
Ø E.g. 4: dilation of the pupil

Question 6

Describe the CNS location where the reflex is integrated:

Answer 6

a) Spinal reflexes – sensory neurons synapse with interneurons
or motor neurons in the dorsal, lateral or ventral horn of the
spinal cord. Occur without input from the brain.
Ø E.g.1: Knee-jerk reflex
Ø E.g. 2: Withdrawal reflex
Ø E.g. 3:Crossed-extensor reflex
Ø E.g. 4: Micturition (Urination) reflex

b) Cranial reflexes – sensory neurons synapse with interneurons or motor neurons in the brain where integration occurs.

Ø E.g. 1: Sneezing
Ø E.g. 2: Vomiting (effector is smooth muscle of the stomach wall)
Ø E.g. 3: Pupillary light reflex (constriction of pupil when exposed
to light – involves smooth muscle of iris).
Ø E.g. 4: Corneal reflex (bilateral blinking of eyes when an object
comes into contact with the cornea – involves skeletal muscle
of the eye – the orbicularis oculi.

Question 7

whether the reflex is innate or learned

Answer 7

a) Innate reflexes – genetically predetermined reflexes that are present at birth.
Ø Includes all of the previously listed examples: Knee-jerk reflex, Withdrawal reflex, Crossed-extensor reflex, Micturition (Urination) reflex, Sneezing, Vomiting, Pupillary light reflex Corneal reflex, etc.

b) Learned (Conditioned) Reflexes – automatic reflexes that are gained over time through repeated experience (learning)
Ø E.g.1: Putting on a jacket/mitts when you see it will be -40 C with the windchill in Winnipeg.
Ø E.g.2: Potty training – learning how to use a toilet whenever you feel the urge to urinate (which is caused by the innate micturition reflex) or defecate.
Ø E.g.3: Classic example = Pavlov’s dogs who were trained to start salivating in response to the ringing of a bell.

Question 8

By the number of neurons in the reflex pathway

Answer 8

a) Monosynaptic reflexes – the reflex arc consists of 2 neurons (a sensory neuron and a motor neuron) with a single synapse that connects them (NO interneuron is present).
Ø E.g.1: All stretch reflexes like the knee jerk (patellar) reflex and Achilles tendon (plantar) reflex.

b) Polysynaptic Reflexes – the reflex arc consists of 3 or more neurons with 2 or more synapses in the pathways (i.e. one or more interneurons is present in the reflex pathway).
Ø E.g.1: All withdrawal reflexes
Ø E.g.2: All crossed extensor reflexes.
Ø E.g.3: Corneal Reflex
Ø E.g.4: All Autonomic reflexes like the Micturition (Urination) reflex and pupillary light reflex

Question 9

By which sides of the body the sensory and motor neurons are located

Answer 9

a) Ipsilateral reflexes – the sensory and motor neurons are on the same side of the body.
Ø E.g.1: All stretch reflexes like the knee jerk (patellar) reflex
Ø E.g.2: All withdrawal (flexor) reflexes - for example putting your right hand on a hot pan handle and pulling it back right away even before you feel the pain of the burn.

b) Contralateral Reflexes – the sensory and motor neurons are on opposite sides of the body
Ø E.g.1: All crossed extensor reflexes – interneurons carry the signal to the opposite side of the spinal cord.

Ø Note: Some reflexes are both ipsilateral and contralateral (the
stimulus causes a simultaneous response on both sides of body)
Ø E.g.1: Pupillary light reflex – shining a light in one pupil causes both
pupils to constrict; E.g2: The corneal reflex – touching the cornea of
one eye causes blinking in both eyes.

Question 10

Where do autonomic reflexes occur/

Answer 10

in the smooth muscles, cardiac muscle and
glands of the body that form many of the internal
organs (heart, stomach, intestines, pancreas, etc).

Question 10

Where do autonomic reflexes occur/

Answer 10

in the smooth muscles, cardiac muscle and
glands of the body that form many of the internal
organs (heart, stomach, intestines, pancreas, etc).

Question 11

Describe the components of an autonomic reflex using the micturition reflex as an example

Answer 11

a spinal autonomic reflex

a) Stimulus = stretch in the urinary bladder as it fills with urine

b) Receptors = stretch (pressure) receptors in the
bladder wall send signals to the spinal cord
(and to the brain to give you that “need to pee” feeling).

c) CNS integration = occurs in the sacral segment
of the spinal cord (which contains the cell
bodies of preganglionic neurons of the
parasympathetic nervous system and these
relay the signals to the postganglionic neurons
that innervate the muscle of the bladder wall

d) Effector = detrusor muscle (bladder wall) contracts and internal urethral sphincter opens

e) Response = urine is pushed out of the bladder and enters into the urethra
Ø Without potty training, this urine would then exit the body through the relaxed External urethral sphincter (a skeletal muscle). However through a learned reflex, we have been taught to keep the external urethral sphincter
contracted, until we can safely expel the urine)

Question 12

proprioceptors

Answer 12

Sensory receptors,

located in muscles, joint capsules and ligaments.

Question 13

What are Proprioceptors responsible for providing information about?

Answer 13

1. position of the different parts of the body in space
   (especially the limbs).
2. movements and actions the body is doing.
3. amount of force (tension) being produced by a muscle

Question 14

Match the specific components of a skeletal muscle reflex to the general steps of a reflex

Question 15

Describe a Golgi tendon organ, and describe their role in body movements

Answer 15

Found at the junction between tendons and muscle fibers

Ø Composed of the free nerve endings of sensory
neurons that wind between the collagen fibers inside the connective tissue capsule of the tendon that attaches muscle to bones of the skeleton.

Ø Monitor the tension (force) in a muscle, particularly during isometric contraction, and trigger a reflex (the autogenic reflex) that will relax the muscle in response to higher levels of tension

Ø Applying more tension to a muscle activates the Golgi tendon organ reflex (autogenic reflex).
Fig. 13.2

Question 16

Golgi tendon (autogenic ) reflex:

Answer 16

a) Stimulus: Increased tension of the muscle fibers pulls on the collagen fibers in the GTO organ which deforms them and causes them to squeeze the dendrites of the sensory neuron in the GTO.

b) Receptor: The sensory neuron in the GTO acts as a
mechanoreceptor (mechanically gated channels in the dendrites of the sensory neuron open response to being squeezed).

c) Integration: Sensory neuron synapses with an inhibitory interneuron in the spinal cord. The interneuron inhibits signals in the lower (alpha) motor neuron that to the same muscle fibers that
are developing relatively high tension.

d) Efferent: Alpha Lower Motor Neuron is inhibited, so no efferent signal present.

e) Response: Relaxation (decreased tension) of muscle fibers

Question 17

Describe or draw a muscle spindle

Answer 17

the most important proprioceptor.

Ø Are the most abundant proprioceptor in skeletal muscles and are found in every skeletal muscle in the body.

Ø Are spindle shaped stretch receptors that detect muscle length and changes in muscle length.

Ø Are made of several modified (small) skeletal muscle cells called intrafusal fibers

Question 18

Describe the Properties of intrafusal fiber

Answer 18

a) They are enclosed in a connective tissue capsule.

b) The center regions of the fibres in the spindle lack actin and myosin (non-contractile region).
This region is wrapped in the dendrites of first order afferent (sensory) neurons.

c) The ends of the fibers have actin and myosin and are innervated by gamma motor neurons and
can contract (contractile region).

d) They are embedded throughout the extrafusal muscle fibers of skeletal muscles (these are the
larger, main fibers of a muscle that are responsible for generating most of the force during
muscle contraction; they are stimulated by alpha lower motor neurons).

Question 19

Explain alpha-gamma coactivation

Answer 19

In order to ensure that muscle spindles remain sensitive to stretch at all muscle lengths, the muscle spindle must be kept stretched tight
Slack/limpness in the spindle will make it less sensitive to stretch.

Ø When a muscle contracts due to signaling in the alpha lower motor neuron, appropriate compensatory signals are sent through gamma motor neurons to the intrafusal fibers in the spindle
When the ends of the intrafusal fibres contract, the spindle tightens and is prevented from going slack (loose) when the muscle contracts.

Ø The excitation of both the alpha and gamma lower
motor neurons to maintain the sensitivity of the muscle to stretch is a result of this alpha-gamma coactivation

Question 20

diagram the stretch reflex and explain the purpose of this reflex

Answer 20

a) Stimulus: stretch in the muscle increases tension in
spindle

b) Receptor: muscle spindle stretch squeezes intrafusal fibers and triggers an increased rate of signaling in the afferent (sensory neuron)

c) CNS Integration: in spinal cord or cranial nerve nuclei – no interneuron is needed, the sensory neuron connects directly to the alpha lower motor neuron (monosynaptic reflex).

d) Efferent: signals travel through the alpha lower motor neuron to the extrafusal fibres of the skeletal muscle

e) Response: Contraction of the same muscle in which stretch was detected.

f) Negative feedback loop: Contraction of the muscle reduces the tension (stretch) in the muscle spindle and afferent signals will decrease (negative feedback

Question 21

Define a myotatic unit.

Question 22

explain the purpose of this reflex

Question 23

Describe and name all components of a monosynaptic stretch reflex including its reciprocal inhibition using the knee jerk reflex as an example.

Answer 23

Ø An extensor muscle, the quadriceps femoris,
contracts to extend the leg in response to stretch in
the quadriceps muscle:
a) Stimulus: tapping the patellar tendon which stretches
the quadriceps femoris muscle.
b) Receptor: muscle spindle (in the quadriceps)
c) CNS Integration: in spinal cord the sensory (afferent)
neuron synapses directly to the alpha lower motor
neuron (neurotransmitter = glutamate).
d) Efferent: signals travel through the alpha lower motor
neuron to the extrafusal fibres of the quadriceps.
e) Response: Contraction of the quadriceps
Ø Like all stretch reflexes, this is an ipsilateral and
monosynaptic reflex.

Ø Reciprocal Inhibition during the Knee-Jerk Reflex
a) Stimulus: tapping the patellar tendon which stretches
the quadriceps femoris muscle.
b) Receptor: muscle spindle (in the quadriceps)
c) CNS Integration: in spinal cord the sensory (afferent)
neuron synapses with an inhibitory interneuron and
the interneuron releases the inhibitory
neurotransmitter glycine onto the alpha lower motor
neuron.
d) Efferent: none.
e) Response: Relaxation of the antagonist muscles (the
hamstrings

Question 24

Describe and name all components of a polysynaptic reflex pathway including its reciprocal inhibition using the withdrawal (flexion) reflex as an example