MSS32 Somatic And Autonomic Reflexes Flashcards
Reflex
Reaction to a specific stimulus
- Rapid
- Short neural pathway
- Automatic / Involuntary
- Predictable
Brain stem
- Midbrain
- Pons
- Medulla oblongata
–> continuous with spinal cord
Proprioceptors
sense of self-movement and body position
Reflex arc
- Receptor
- detects stimulus - Sensory (afferent) neuron
- transmits sensory (afferent) impulse ***from receptor to CNS - Integration centre
- CNS interprets sensory information and coordinates the events in response to stimuli - Motor (efferent) neuron
- transmit motor (efferent) impulses ***from CNS to effector - Effector
- muscle / gland responds to instruction of motor neuron
Monosynpatic reflex vs Polysynaptic reflex
Monosynaptic: NO interneuron
Polysynaptic (>= 2 synapses): have interneuron
***Functions of reflexes
- Maintenance of balance and posture
- Stretch reflex
- Flexor and Crossed-extensor reflex
- Vestibulo-spinal reflex - Protection
- Golgi tendon reflex
- Withdrawal reflex - Digestive and renal functions
- Defecation reflex
- Micturition reflex - Automatic actions
- Pupillary light reflex
- Reflexes in deglutition (swallowing)
- Cough reflex
- Sneeze reflex - Homeostasis
- Chemoreceptor reflex
- Baroreceptor reflex - Coordination of complex movements
- Vestibulo-ocular reflex
Classification of reflexes
- Innate (inborn) vs Acquired (learning-dependent)
- Spinal (process in spinal cord) vs Cranial (process in brain)
- Somatic (control skeletal muscles) vs Autonomic (control smooth / muscles / glands)
- Monosynaptic vs Polysynaptic
***Skeletal muscle
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)
***Muscle spindle
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
Type 1a fibre vs Type 2 fibre
Type 1a:
- Annulospiral ending
- Detect muscle length during ***movement
Type 2:
- Flower spray ending
- Detect muscle length during ***static state
Spinal reflexes (somatic)
- Stretch reflex (e.g. knee-jerk, ankle-jerk, biceps, triceps reflex)
- monosynaptic (except reciprocal inhibition) - Golgi tendon reflex
- polysynaptic - Flexor reflex
- polysynaptic - Crossed extensor reflex
- polysynaptic - Superficial cord reflex
- Stretch reflex
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
Patellar reflex (knee-jerk)
- Tap patellar tendon
- Quadriceps stretched
- Muscle spindle activated
- Impulse in 1a sensory neuron
- Monosynaptic (spinal cord) + Polysynaptic (Stimulation of inhibitory interneuron of antagonist muscle)
- Impulse in α motor neuron + Inhibition of antagonist muscle
- Muscle contract + Antagonist muscle relax
Reciprocal inhibition:
antagonistic pair of muscle (Hamstrings) must be inhibited / relax to prevent working against the agonist (Quadriceps)
Test for localisation of site of lesion / neuropathy
∵ 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
***Alpha-Gamma Co-activation
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
Hyperreflexia and Hyporeflexia
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)
Type 1b fibre
- 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
- Golgi tendon reflex
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
- Flexor reflex (withdrawl reflex)
Noxious stimulus
- -> ***Pain receptor activation
- -> Pain afferent impulse
- -> Spinal cord (polysynaptic)
- -> ↑ Firing rate of α motor neuron
- -> Flexor muscle contraction (+ Extensor muscle relax)
- Crossed extensor reflex
***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
- Superficial cord reflex
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
Cranial reflexes (autonomic)
- Pupillary light reflex
- Chemoreceptor reflex
- Baroreceptor reflex
- Pupillary light reflex
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
Clinical significance: Defect in Pupillary light reflex
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
- Chemoreceptor reflex
Blood CO2↑ / O2↓ / pH↓
- -> Chemoreceptor activation (carotid, aortic bodies, medulla oblongata)
- -> **Vagus nerve, **Glossopharyngeal nerve CN9
- -> Medulla
- -> Cardiovascular centre (medulla)
- -> Vasomotor centre (medulla)
- ↑ Sympathetic nerve
- -> ↑ Heart rate, ↑ Force of contraction, Vasoconstriction - ↓ Parasympathetic nerve
- -> ↑ Heart rate
Chemoreceptors
Carotid, Aortic bodies: monitor O2, CO2, pH
Medulla oblongata: monitor CO2, pH
- Baroreceptor reflex
↑ Blood pressure
- -> Baroreceptor activation (Aortic bodies, Carotid sinuses)
- -> **Vagus nerve, **Glossopharyngeal nerve CN9
- -> Medulla
- -> ↓ Sympathetic nerve + ↑ Parasympathetic nerve
- -> ↓ Heart rate + ↓ Force of contraction + Vasodilation
Spinal reflexes (with autonomic component: autonomic ganglion) --> involves 2 neuron: pre-, post-ganglionic neuron
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
Short reflex (in enteric nervous system ENS)
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
***Summary
Location of reflex arc:
- Spinal vs Cranial
- Somatic vs Autonomic
Spinal reflexes
- Stretch (somatic) (muscle spindle: type 1a sensory fibres as receptor)
- Golgi tendon (somatic) (golgi tendon organ: type 1b sensory fibres as receptor)
- Flexor, Cross extensor (somatic) (pain receptor)
- Superficial cord (somatic)
- Defecation (autonomic)
- Micturition (autonomic)
Cranial reflex:
- Pupillary (autonomic)
- Chemoreceptors (autonomic)
- Baroreceptor (autonomic)
