7.2 Descending Pathways Flashcards
Define ‘reflexes’
Automatic, subconscious responses to changes in the external environment (Involuntary response to a stimuli)
Does not require the brain!
List 3 instances where reflexes are important!
1) Reflexes maintain homeostasis E.g. balance, posture
2) Reflexes are protective E.g. touching hot kettle, tripping on the kerb
3) Reflexes are part of vital body processes E.g. HR, BP, digestion
What is the ‘the reflex arc’ termed when it travels through the spinal cord?
spinal reflex
What are the 5 components of ‘the reflex arc’?
1) receptor: detects the stimulus
2) sensory neuron: carries the impulse (afferent)
3) integration centre: processes impulse from sensory to motor neuron
4) motor neuron: conducts the output to the periphery (efferent)
5) the effector: e.g muscle contraction
What is meant by a monosynaptic vs polysynaptic reflex?
Monosynaptic: some simple reflexes may simply be 2 neurones and step 3 is a synapse
- Eg. muscle spindles ➞ α-MN
Polysynaptic: some reflexesmay be more complex where step 3 may be an interneuron
- Eg. Golgi tendon reflex, withdrawal reflex (flexor reflex)
List 4 ways in which reflexes can be classified
1) by developement
2) by response
3) by complexity
4) by processing site
List 2 ways reflexes can be classified by developement
1) Innate: genetically or developmentally determined
2) Acquired: learned
List 2 ways reflexes can be classified by response
1) Somatic: control of skeletal muscle contraction and superficial and stretch reflexes
2) Visceral (autonomic): control of SM, cardiac muscle and glands
List 2 ways reflexes can be classified by complexity of circuit
1) Monosynaptic: one synapse
2) Polysynaptic: multiple synapses
List 2 ways reflexes can be classified by processing site
1) Cranial: processes in the brain
2) Spinal: processes in the spinal cord
List 5 important reflexes and the nerve roots being tested
1) Supinator C5-6
2) Biceps C5-6
3) Triceps C7
4) Quadriceps (patella) L3-4
5) Ankle (archilles) S1
How is a recording for reflexes recorded?
Either: 0-4, or absent, +,++,+++
Not lecture… (for 0-4 grading)
- 0 = no response; always abnormal
- 1+ = slight but present response; may be normal
- 2+ = a brisk response; normal
- 3+ = a very brisk response; may or may not be normal
- 4+ = a tap elicits a repeating reflex; always abnormal
What are Stretch (myotatic) reflexes and why are they important
Give an example of strech reflex and state its importance
Reflex that resists stretching of muscle and maintains its length (usually named after the muscle being tested)
Eg. Postural reflex ➞ Import for posture, balance, holding heavy objects
Explain the Strech reflex
1) 1a afferent fibres within the muscle spindle respond to stretch within the muscle
2) travels to the spinal cord and synapses onto LMN supplying synergist and also interneurons which Inhibit LMN supplying antagonist
3) Causes contraction of the synergistic muscles and and reciprocal relaxation of the antagonist muscle
What is the opposite of the Stretch (myotatic) reflex?
Golgi Tendon reflex
What type of reflex is the patella reflex in terms of complexity and processing site?
Monosynaptic spinal reflex (means it occurs independant of the brain, brain only recieves info after reflex has occured)
Explain the Patella reflex (incl action of agonist and antagonist)
1) stretch to the patella tendon of the quadricep muscle is sensed by muscle spindles
2) this stimulates 1a afferent neurone (sensory) which travels to spinal cord and synapses in the ventral horn with alpha motor neurones (efferent)
3) alpha motor neurones stimulate quadriceps to contract causing knee to extend
4) At the same time, when sensory neurons travel into spinal cord, they ALSO stimulate inhibitory interneurons which inhibit the actvity of motor neurons supplying antagonists (hamstring)
5) this causes hamstring to relax (allowing the action of the quadriceps to be un-opposed)
What type of reflex is the Golgi tendon reflex in terms of complexity?
Polysynaptic
When does the Golgi tendon reflex occur?
In response to overstretch of a tendon due to contraction of the muscle. Negative feedback mechanism preventing too much tension on the muscle and tendon in order to protect it.
** Also known as the ‘inverse stretch reflex’
Explain the Golgi tendon reflex
1) 1b afferent sensory fibres in the golgi tendon organ detect over stretch of the tendon
2) travel to spinal cord and synapses with inhibitory and excitatory interneurons
3) motor neurons stimulated cause relaxation of the muscle and reciprocal contraction of the antagonist muscle
Compare ‘stimulation’ and ‘inhibition’ of the stretch vs golgi reflex
Strech
- 1) stimulates synergistic muscles
- 2) Inhibits LMN supplying antagonist
Golgi
- 1) stimulates antagonist
- 2) Inhibits LMN supplying synergistic
What is the withdrawal reflex also known as?
What type of reflex is this in terms of complexity?
Flexor reflex. It is a polysynaptic reflex
Describe the Withdrawal reflex (flexor reflex)
1) Stimulation of nociceptors by painful/noxious stimuli
2) afferent sensory neurone travels to spinal cord and synapses with multiple interneurons (excitatory and inhibitory) enabling the sensory neurone to connect to many neurones. (further reciprocal inhibition)
3) This affects 3 different pathways in the spinal cord:
a) activates ascending pathways for sensation (pain) and postural adjustment
b) activates the withdrawl reflex- contracts flexors and inhibits extensors (ipsilateral side) which pulls foot away from painful stimuli
c) activates the crossed extensory reflex- contracts extensors and inhibits flexors (contralateral side) which supports body as weight shifts away from painful stimuli
** Each pathway above will synaps onto interneurons in various segments which then synaps onto efferent neurons to elicit the response
Give 4 important neonatal reflexes and briefly describe each incl CNS origin and timeframe
*** Study others attached on image
1) Tonic neck reflex: ‘fencing posture when supine’. Origin = brainstem vestibular nuclei and lasts from birth to 4-6 months
2) Grasp reflex: finger in palm of baby’s hand ➞ hand, elbow and shoulder flexion. Origin = brainstem vestibular nuclei and lasts from birth to 4-6 months
3) Step reflex: ‘walking or dance reflex’ steps up with oppite foot when dorsum of ipsilateral foot is stimulated. Origin = cerebral cortex and lasts from birth to 4-6 months
4) Crawl reflex: when baby is placed on stomach, they will pull legs under their body and kick them out in a crawling motion. Lasts from birth to few weeks
List 6 things we must examine when assessing the motor system
1) Appearance
2) Gait
3) Tone
4) Power
5) Coordination
6) Reflexes
When inspecting the motor system what 8 things are we looking for? (DWAARFSS)
Deformities, Wasting, Asymmetry, Atrophy, Rahes, Fasiculations, Scars, Swelling
What is meant by assessing muscle tone?
Give 4 examples
Resistance of a muscle to passive movement
Eg.
- Shake hand, roll legs
- Spasticity: ‘clasp-knife’
- Ridgidity: ‘cog-wheel’
- Hypotonia: flacidity
How do we assess power during a motor exam
Assess myotomes and compare sides
Use MRC grading
State what is meant by a score of 0-5 in the MRC grading scale
List 4 things we are specifically observing in a patients GAIT
1) Walking: arm swinging, smoot turning, symmetry, size of steps
2) Pigion steps?
3) Walking on tiptoes: assess S1/S2 myotome
4) Heels: assess L4/L5 myotome
List 4 abonormal types of GAIT
Hemiplegic, Paraplegic, Cerebellar, Ataxic, High stepping, Shuffling, Antalgic
Normal motor function relies on what?
The transmission of a signal from the cortex to the muscle
(goes via an UMN in the brain or spinal cord to a LMN in the PNS and cranial nerves)
Via what 2 areas may neurons travel through?
1) via the pyramids of medulla in the brain stem (pyramidal)
2) originate in the basal ganglia and cerebellum and dont travel via the pyramids of medulla (extrapyramidal)
List 4 locations where lesions may occur?
If a lesions occurs in the pathway via the pyramids of medulla what are they known as?
Lesions can occur at any stage of pathway
- Brain, spinal cord, anterior horn, peripheral nerve, NMJ, muscle
Lesions in neurons of the pyramids of medulla are referred to as pyramidal lesions
List a way in which the following may be damaged:
- Cerebral cortex
- Spinal cord
- LMN
- NMJ
- Muscle
Cerebral cortex: CVA (cerebral vascular accident- ie. stroke)
Spinal cord: trauma, cauda equina syndrome, tumour, MS
LMN: Trauma, nerve injury ie. carpal tunnel, Motor neurone disease
NMJ: Myasthenia gravis
Muscle: Duchenne’s muscular dystrophy, Myotonic dystrophy
How may reflexes help determine the level of a lesion?
Reflexes are altered with UMN and LMN lesions and thus the presence or absence can confirm level of lesion
Give 4 specific examples of LMN lesions?
Carpal tunnel, cauda equina, infection (polio), MND
Give 4 specific examples of UMN lesions?
Stroke, MS, spinal trauma, traumatic brain injury, CP
What specific loss is present in an UMN vs LMN lesion?
UMN – no inhibitory descending pathways
LMN – lack of functioning efferent neurone
List 4 signs seen in an UMN lesion and explain the reason for each
1) Weakness/paralysis: AP cant propagate from UMN to LMN
2) Increased tone: spasticity caused by overexaggerated stretch reflex
3) Increased tendon reflexes (Hyperreflexia): caused by lack of descending inhibitory pathways
4) Extensor plantars (Babinski sign): because normally extension is inhibited but when UMN is damaged, inhibition is blocked resulting in extension of great toe
List 5 signs seen in an LMN lesion and explain the reason for each
1) Weakness and flacid paralysis: due to loss/decreased innervation to effector muscles
2) Decreased tone: since tone is partially dependent on the monosynaptic reflex arc that links muscle spindles to LMN
3) Loss of tendon reflexes: due to loss/decreased innervation to effector muscles
4) Wasting and atrophy of muscles: due to damage to alpha motor neurones = lack of trophic factors needed by the muscles fibres
5) Fasciculations: Spontaneous AP ➞ firing of the motor unit ➞ twitch
Compare UMN vs LMN innervation to the face
The forehead recieves innervation from BOTH the right and left hemispheres of the brain
The lower face recieves innervation ONLY from the opposite side of the brain
Patient presents with inability to move one side of face
How can we distinguish Bell’s palsy from a stroke and explain why
Ask patient to wrinkle forehead:
If both sides can be wrinkled = stroke (forehead sparing and lip droop) because the forehead has dual innervation.
If only one side can be = Bell’s palsy (forehead and lip droop) because facial nerve is damaged, no innervation reach the forehead or lower face
Are Erb’s Palsy and thoracic nerve injury UMN or LMN lesions?
What is seen in each?
Both LMN
long thoracic nerve injury: Winged scapula
Erb palsy: arm extended, internally rotated, adducted arm and wrist flexed (brachial plexus injury following shoulder dystocia at delivery)
Give 2 types of abnormal posturing that may be seen in severe brain injury
Decorticate and Decerebrate posturing
These are primitive motor reflexes (automatic movement patterns) that occur as a result of severe brain injury and are associated with very poor prognosis
Describe the appearance of Decorticate posturing (2)
1) Fexion in the upper extremities
2) Extension of the lower extremities
Explain the cause of decorticate posturing
Mummy position
Caused by:
Dis-inhibition of the red nucleus ➞ results in overactivity of the rubrospinal tract ➞ flexion in the upper extremities
Disruption of the lateral corticospinal tract ➞ pontine reticulospinal and medial and lateral vestibulospinal tracts overwhelm ➞ extension of the lower extremities
Describe the appearance of Decerebrate posturing
1) Head arched back
2) Arms extended by sides
3) Elbows extended
4) Legs extended and internally rotated
Explain the cause of Decerebrate posturing
Due to brainstem damage below the red nucleus
If a Patients progressed from decorticate to decerebrate posturing what may this indicate?
Brain herniation
Motor neurone disease causes degeneration of what structures?
Degeneration of the motor cortex, spinal tracts, anterior horn grey matter and LMNs
Give the 3 main patterns/ clinical features that occur with Motor neurone disease
What is common about them all?
1) Stiffness/weakness of hands spreading distal to proximal. fasciculations
2) Speech and swallowing difficulties
3) Slow, progressive muscle wasting ➞ distal to proximal
ALL are progressive BUT at different rates
What is Myasthenia gravis and explain the pathophysiology
Disorder of the NMJ: Autoimmune disease causing antibodies against ACh receptor
(65% associated with thymic hyperplasia but cause of this is unclear)
What is the main clinical feature of Myasthenia gravis?
Variable muscle weakness and fatiguability
Incl muscles of: eyes, speech, swallowing and arms
How is Myasthenia gravis diagnosed and treated?
What is the prognosis with treatment?
Diagnosis:
- antibodies, EMG, edrophonium test
Treatment:
- cholinesterase inhibitors for symptomatic treatment
- Immunosuppression / thymus removal
Prognosis: normal life but long term treatment
