MSS32 Somatic And Autonomic Reflexes

Question 1

Reflex

Answer 1

Reaction to a specific stimulus

• Rapid
• Short neural pathway
• Automatic / Involuntary
• Predictable

Question 2

Brain stem

Answer 2

1. Midbrain
2. Pons
3. Medulla oblongata

–> continuous with spinal cord

Question 3

Proprioceptors

Answer 3

sense of self-movement and body position

Question 4

Reflex arc

Answer 4

1. Receptor
   - detects stimulus
2. Sensory (afferent) neuron
   - transmits sensory (afferent) impulse ***from receptor to CNS
3. Integration centre
   - CNS interprets sensory information and coordinates the events in response to stimuli
4. Motor (efferent) neuron
   - transmit motor (efferent) impulses ***from CNS to effector
5. Effector
   - muscle / gland responds to instruction of motor neuron

Question 5

Monosynpatic reflex vs Polysynaptic reflex

Answer 5

Monosynaptic: NO interneuron

Polysynaptic (>= 2 synapses): have interneuron

Question 6

***Functions of reflexes

Answer 6

1. Maintenance of balance and posture
   - Stretch reflex
   - Flexor and Crossed-extensor reflex
   - Vestibulo-spinal reflex
2. Protection
   - Golgi tendon reflex
   - Withdrawal reflex
3. Digestive and renal functions
   - Defecation reflex
   - Micturition reflex
4. Automatic actions
   - Pupillary light reflex
   - Reflexes in deglutition (swallowing)
   - Cough reflex
   - Sneeze reflex
5. Homeostasis
   - Chemoreceptor reflex
   - Baroreceptor reflex
6. Coordination of complex movements
   - Vestibulo-ocular reflex

Question 7

Classification of reflexes

Answer 7

1. Innate (inborn) vs Acquired (learning-dependent)
2. Spinal (process in spinal cord) vs Cranial (process in brain)
3. Somatic (control skeletal muscles) vs Autonomic (control smooth / muscles / glands)
4. Monosynaptic vs Polysynaptic

Question 8

***Skeletal muscle

Answer 8

Extrafusal muscle:

• Contractile
• innervated by α motor neuron (efferent)

Intrafusal muscle (Muscle spindle):

• Sensory + Contractile
• Sensory (non-contractile, central): receptive surface, wrapped with type 1a (primary), 2 (secondary) sensory nerve fibres
• Contractile region (peripheral): innervated by γ motor neuron (efferent)

Question 9

***Muscle spindle

Answer 9

Stretching extrafusal muscle

• -> Activates intrafusal muscle spindle (拉長左)
• -> ↑ action potential firing rate in type 1a sensory fibres

Contracting muscle

• -> Reduce tension muscle spindle (縮短左)
• -> ↓ action potential firing rate in type 1a sensory fibres

Question 10

Type 1a fibre vs Type 2 fibre

Answer 10

Type 1a:

• Annulospiral ending
• Detect muscle length during ***movement

Type 2:

• Flower spray ending
• Detect muscle length during ***static state

Question 11

Spinal reflexes (somatic)

Answer 11

1. Stretch reflex (e.g. knee-jerk, ankle-jerk, biceps, triceps reflex)
   - monosynaptic (except reciprocal inhibition)
2. Golgi tendon reflex
   - polysynaptic
3. Flexor reflex
   - polysynaptic
4. Crossed extensor reflex
   - polysynaptic
5. Superficial cord reflex

Question 12

1. Stretch reflex

Answer 12

Stretching muscle

• -> **Muscle spindle activation (detecting change in muscle **length)
• -> ↑ Firing rate in type 1a sensory fibres
• -> Spinal cord (Monosynaptic)
• -> ↑ Firing rate in α motor neuron
• -> Muscle contraction

Reciprocal inhibition:

• antagonist inhibited –> accommodate contraction of agonist
• Excitatory signal from muscle spindle
• -> Spinal cord
• -> Stimulation of inhibitory ***interneuron
• -> Inhibition of motor neuron that innervates antagonist

Alpha-Gamma co-activation:
maintains sensitivity of muscle spindles

Question 13

Patellar reflex (knee-jerk)

Answer 13

1. Tap patellar tendon
2. Quadriceps stretched
3. Muscle spindle activated
4. Impulse in 1a sensory neuron
5. Monosynaptic (spinal cord) + Polysynaptic (Stimulation of inhibitory interneuron of antagonist muscle)
6. Impulse in α motor neuron + Inhibition of antagonist muscle
7. Muscle contract + Antagonist muscle relax

Reciprocal inhibition:
antagonistic pair of muscle (Hamstrings) must be inhibited / relax to prevent working against the agonist (Quadriceps)

Question 14

Test for localisation of site of lesion / neuropathy

Answer 14

∵ different stretch reflexes are mediated by neural pathways at different segments of spinal cord
e.g. No knee-jerk reflex –> problem with L4 level
Biceps reflex: C5
Triceps reflex: C7
Brachioradialis reflex: C6
Ankle jerk: S1

Question 15

***Alpha-Gamma Co-activation

Answer 15

When extrafusal muscle fibres actively shorten

• -> intrafusal muscle fibres are NOT shortened accordingly
• -> muscle spindle becomes slack
• -> ↓ sensitivity of muscle spindles

Co-activate γ motor neuron by descending pathways

• -> intrafusal muscle fibres now ALSO shortened
• -> keeping muscle spindles taut
• -> ↑ sensitivity of muscle spindles
• -> Muscle spindles are still responsive during muscle contraction to detect further changes in muscle length
• -> in turn ↑ α motor neuron discharge (enhance activity), allowing the continued firing of α motor neurons

–> Alpha-Gamma loop

Question 16

Hyperreflexia and Hyporeflexia

Answer 16

Upper motor neuron lesion (cerebrum, brainstem, spinal cord)

• -> loss of inhibition of γ motor neuron
• -> ↑ activity of γ motor neuron
• -> ↑ sensitivity of muscle spindles
• -> ↑ muscle tone (Hypertonia)
• -> Hyperactive stretch reflex (Hyperreflexia)
• -> Spasticity

(damage to descending pathways –> increasing the total responsiveness of α motor neurons from muscle spindle 1a sensory input)

Lower motor neuron lesion (anterior horn, nerve roots, peripheral nerves, NMJ) / Impairment of reflex arc integrity

• -> direct damage to α motor neurons / 1a afferents
• -> ↓ muscle tone (Hypotonia)
• -> Hypoactive stretch reflex (Hyporeflexia / Areflexia)

Question 17

Type 1b fibre

Answer 17

• Innervate muscles of Golgi tendon organ (senses changes in muscle tension)
• ***Contracting muscle (vs stretched muscle in 1a fibre)
• -> activates Golgi tendon organ
• -> ↑ firing rate of action potential in 1b fibres

Question 18

2. Golgi tendon reflex

Answer 18

Contracting muscle
—> Golgi tendon organ activation (detecting ***tension)
—> ↑ Firing rate in 1b fibres
—> Spinal cord (polysynaptic):
—> Inhibitory interneuron to Agonist + Excitatory interneuron to Antagonist
—> ↓ Firing rate of α motor neuron (agonist)
—> Agonist muscle relaxation

e.g. Protect biceps from excessively heavy loads by causing muscle to relax and drop the load

Question 19

3. Flexor reflex (withdrawl reflex)

Answer 19

Noxious stimulus

• -> ***Pain receptor activation
• -> Pain afferent impulse
• -> Spinal cord (polysynaptic)
• -> ↑ Firing rate of α motor neuron
• -> Flexor muscle contraction (+ Extensor muscle relax)

Question 20

4. Crossed extensor reflex

Answer 20

***Excitatory neurons (that cross spinal cord) stimulate motor neurons that innervate extensor of opposite leg

Ipsilateral side = Flexor reflex (***Flexor contract, Extensor relax)
–> move foot away from stimulus

Contralateral side = Opposite of Flexor reflex (Flexor relax, ***Extensor contract)
–> allow body weight shifts to non-stimulated leg

Question 21

5. Superficial cord reflex

Answer 21

Mediated by neural pathway **locally organized at the spinal cord
- but requires descending activity from **Supraspinal level for normal manifestation (descending signals modulate reflex arc action)

Clinical significance: Test for ***Corticospinal tract lesion

Gentle cutaneous stimulation:
stroking sole (lateral aspect) of the foot

Positive Babinski reflex:

• Extension of great toe
• Fanning out of other toes
• Infant: normal (corticospinal tract not fully myelinated / mature)
• Adult: pathological

Normal plantar reflex (Negative Babinski reflex):

• Flexion of all toes
• Adult: normal

Question 22

Cranial reflexes (autonomic)

Answer 22

1. Pupillary light reflex
2. Chemoreceptor reflex
3. Baroreceptor reflex

Question 23

1. Pupillary light reflex

Answer 23

Light

• -> Photoreceptor activation (from intrinsically photosensitive retinal ganglion cells)
• -> ***Optic nerve
• -> Midbrain (**Pretectal nucleus (unilateral) –> Accessory oculomotor nucleus / **Edinger-Westphal nucleus (bilateral))
• -> ***2 Oculomotor nerves CN3 (Parasympathetic)
• -> ***Direct response + Consensual response
• -> Pre-ganglionic parasympatheic fibre –> ***Ciliary ganglion –> Post-ganglionic parasympatheic fibre
• -> Pupillary constrictor muscle (Circular muscle) contraction
• -> ↓ pupil size
• radial muscle (control by sympathetic division) –> Superior cervical ganglion

Question 24

Clinical significance: Defect in Pupillary light reflex

Answer 24

Motor defect (一隻有, 一隻無反應):
- impaired pupillary constriction of one eye REGARDLESS which eye stimulated

Sensory defect (兩隻都無反應):
- Impaired pupillary constriction of both eyes when ONE eye is stimulated

Question 25

2. Chemoreceptor reflex

Answer 25

Blood CO2↑ / O2↓ / pH↓ - -> Chemoreceptor activation (carotid, aortic bodies, medulla oblongata) - -> ***Vagus nerve, ***Glossopharyngeal nerve CN9 - -> Medulla - -> Cardiovascular centre (medulla) - -> Vasomotor centre (medulla) 1. ↑ Sympathetic nerve - -> ↑ Heart rate, ↑ Force of contraction, Vasoconstriction 2. ↓ Parasympathetic nerve - -> ↑ Heart rate

Question 26

Chemoreceptors

Answer 26

Carotid, Aortic bodies: monitor O2, CO2, pH Medulla oblongata: monitor CO2, pH

Question 27

3. Baroreceptor reflex

Answer 27

↑ Blood pressure - -> Baroreceptor activation (Aortic bodies, Carotid sinuses) - -> ***Vagus nerve, ***Glossopharyngeal nerve CN9 - -> Medulla - -> ↓ Sympathetic nerve + ↑ Parasympathetic nerve - -> ↓ Heart rate + ↓ Force of contraction + Vasodilation

Question 28

``` Spinal reflexes (with autonomic component: autonomic ganglion) --> involves 2 neuron: pre-, post-ganglionic neuron ```

Answer 28

``` 1. Defecation reflex Feces in rectum --> ***Stretch receptor activation --> Sensory nerve --> Spinal cord --> ***Parasympathetic nerve --> Contraction: Rectum, Sigmoid colon + Relaxation: Internal anal sphincter ``` ``` 2. Micturition reflex Urine in bladder --> ***Stretch receptor activation --> Sensory nerve --> Spinal cord --> ***Parasympathetic nerve --> Contraction: Bladder + Relaxation: Internal urethral sphincter ```

Question 29

Short reflex (in enteric nervous system ENS)

Answer 29

Processing at peripheral ganglia of ***local nerve plexus*** without involving CNS Sensory fibres directly synapse onto ganglion (post-ganglionic cell) --> Post-ganglionic fibre (skipped Pre-ganglionic fibre) --> effector Characteristics: - Skip pre-ganglionic cell - NO spinal cord / CNS processing - NOT considered autonomic (since no CNS involved) Example: Internal GI stimuli --> Chemoreceptor, Osmoreceptor, Mechanoreceptor (visceral afferent) --> Short reflex via local nerve plexus (ENS) --> Smooth muscle / Gland --> Contractile / Secretory activity to help digestion

Question 30

***Summary

Answer 30

Location of reflex arc: - Spinal vs Cranial - Somatic vs Autonomic Spinal reflexes 1. Stretch (somatic) (muscle spindle: type 1a sensory fibres as receptor) 2. Golgi tendon (somatic) (golgi tendon organ: type 1b sensory fibres as receptor) 3. Flexor, Cross extensor (somatic) (pain receptor) 4. Superficial cord (somatic) 5. Defecation (autonomic) 6. Micturition (autonomic) Cranial reflex: 1. Pupillary (autonomic) 2. Chemoreceptors (autonomic) 3. Baroreceptor (autonomic)