Spinal Cord Circuitry and Reflexes Flashcards
Definition of reflexes
Stereotyped, involuntary response to a stimulus
Definition of an autonomic reflex
Reflexes mediated by the ANS which activates smooth, cardiac muscle and glands
Definition of a somatic reflex
Reflexes mediated by the somatic nervous system which activates skeletal muscle
What is a reflex
What characterises an autonomic and somatic reflex
Stereotyped involuntary responses to a stimulus
Autonomic
- Mediated by the ANS
- Activates smooth, cardiac muscles and glands
- often bilateral
Somatic reflexes
- Mediated by the somatic NS
- Activates skeletal muscle
What are the 4 main properties of any reflex
Simplest neural circuit
Fast automatic behaviour
Very old evoluntionary to help us survive
Unconscious
What are the basic steps in any reflex
Receptor Sensory neurone Interpretation center Motor neurone Effectot
Describe the monosynaptic reflex pathway in the knee jerk reaction
What could go wrong in this pathway
Muscle spindle surrounded by extrafusal fibres
Myelinated Aa, large diameter fibre in peripheral nerve
Cell body in DRG
Synapse in ventral horn with myelinated Aa motor
Transduces electrical => chemical energy onto NMJ => muscle
Myelinated fibres can get demyelinated => decreased conduction velocity Cell body in DRG can get damaged NT problem NT receptor problem Muscle problems
Describe the structure of a peripheral nerve
Whole nerve surrounded by epineurium
Nerve fascicles in perineurium
Individual fibres surrounded by endoneurium
Peripheral nerve contains many afferent and efferent fibres
What is the average speed of a Aa fibre
120m/s
What is the 2 functions of a monosynaptic reflex
What is an example of a monosynaptic reflex
Controls stretch leg muscles
Maintains upright posture
Muscle spindles in the stretch reflex
What are the 3 properties of a polysynaptic reflex
Have interneurons More synapses => 0.5ms synaptic delay => slower reflex Synapse signals can be changed -Spatial summation => AP -Temporal summation => AP -EPSPs, IPSPs => SP
What are the 2 properties of properioceptors in reflexes
- where are they found
- what do they do
Found in skeletal muscles, tendons, joint capsules, ligaments
Carry sensory input to CNS
Describe the 3 main properties of muscle spindles
- location
- stimulus
- type of synaptic reflex
-pathway of function
-Parallel and inbetween extrafusal muscle fibres
-Respond to stretch, prevent over stretching
Tonically active sensory neurones around intrafusal fibres => CNS
-MONOSYNAPTIC
- Extrafusal loses tone/stretches too much
- Intrafusal stretched
- Sensory afferent from sensory neurone => CNS
- a motor contracts extrafusal muscle
How is the muscle spindle reflex stopped when you want to move voluntarily
Descending pathway coactivates
a motor => extrafusal
y motor => intrafusal
Both contract so length change is not detected by the sensory afferent
Describe the 3 main properties of golgi tendon organs
- location
- stimulus
- type of synaptic reflex
-pathway of function
- Series at musculotendinous junction with collagen fibres
- Responds to tension
- POLYSYNAPTIC
-Extrafusal muscle contraction stretches golgi tendon organ
-GTO fires => CNS
Interneurone between afferent and efferent inhibits muscle contraction
-Muscle relaxes, load dropped
What is the withdrawal reflex
3 properties
Total flexor pattern => retraction of the whole limb from danger
Hardwired in spinal cord
Many interneurons
Why are interneurons important
If both flexors and extensors are excited => both contract => rigidity
Inhibitory interneuron onto antagonist => relaxes => movement
Describe the pathway in the reciprocal innervation reflex to move 1 limb
Sensory receptor in muscle spindle
Afferent to SC
Excitatory synapse => contract flexor
Inhibitory interneurone => relax extensor
Describe the pathway in cross cord reflexes in order to coordinate 2 limbs when painful stimulus detected
Sensory receptor in 1 limb (nociceptor)
Afferent of R leg to CS
4 different effects happen at the same time
- Excitatory interneuron => contract R flexor
- Inhibitory interneuron => relax R extensor
Decussation from R dorsal => L ventral
- Excitatory interneuron => contract L extensor
- Inhibitory interneuron => relax L flexor
How would you coordinate the whole limb in a reflex
Ascending and descending fibres in the ventral horn of the SC to innervate different segments of the SC
What are spinal pattern generators
How is their input changed/stopped
Allows for walking via the crossed extensor reflex
No sensory/descending inputs
Precise temporal sequences of muscle contraction => coordinated mv
Can change timing via rhythm generators
Stopped by descending input from cortex
What are the 2 types of complex movement and what areas of the nervous system do they involve
Walking, intersegmental at SC
Turning, intersegmental with the brainstem
Describe how cortical control can interfere with reflexes
How can you override this cortical control
From cortex =long tracts=> conscious descending inhibition to override reflex
To override this, clench teeth
- Increases ion movement in CSF
- Increases membrane excitability => easier to reach AP
What is recurrent inhibition
What cell is involved
How does it work
Motor neurone is excited, acts on muscle and Renshaw cell
Renshaw cell (inhibitory interneuron) inhibits further excitation of same motor neurone
Allows for self regulation of effectors within SC
What are the 2 pyramidal reflexes
Where do they act
Corticospinal
-cortex => SC
Corticobulbar
-cortec => brainstem
What are the 4 extrapyramidal reflexes
Where do they act
Vestibulospinal (maintains balance)
-vestibular nuclei => SC
Tectospinal
-sup colliculus (visual mv) => cervical spine
Reticulospinal
-pons => midbrain
Rubrospinal
-red nucleus in midbrain => SC
What are the 2 tracts in the vestibulospinal system
What do they do when you fall
What do they positively and negatively innervate?
Lateral VS
- ipsilateral action
- controls balance and posture
- innervates antigravity muscles
When falling
+ve ipsilateral leg extensors (monosynaptic)
-ve ipsilateral leg flexors (monosynaptic)
+ve ipsilateral upper arm flexors (polysynaptic)
Medial VS
- bilateral action
- only in the neck and shoulder
When falling
+ve bilateral neck muscles
What is the function of the tectospinal tract
Coordinates head movements in relation to visual stimuli
What is the function of the reticulospinal tract
How does the medial RS and the lateral RS differ
Medial RST => increases muscle tone in limbs, excites voluntary movement
Lateral RST => decreases muscle tone in limbs, inhibits voluntary movement
What is the function of the rubrospinal tract
Output for cerebellum
Contralateral
Role in fine movement
Describe the pathway involved in vestibular reflexes when you fall
Inner ear receives orientation info via vertical vestibular apparatus
Vestibular nuclei in pons
Spinal motor neurones
Postural reflex body movement
Describe the pathway involved in visual reflexes when you fall
Eye sees movement Lateral geniculate nucleus Pretectal area in pons Descending to SC Spinal motor neurones act to maintain balance
Describe pressure receptors in relation to posture
What happens if you damage the peripheral nerves connected to the Pacinian corpuscles in your feet and close your eyes
Monitor weight distribution in posture
If you damage peripheral nerves connected to the Pacinian corpuscles in the feet and close your eyes => fall
What are the 4 functions of cerebellar reflexes
What tracts do they operate with
Integrates sensory info
Body position
Complex movement coordination
Maintains posture
Acts via VST and RST
What happens when the reflex response goes wrong
Why are they lost
What can happen as a consequence
- Absent
- Distorted
- Exaggerated
- Change of symmetry
Lost due to
- degeneration
- demyelination
- synaptic problems
Reflexes are lost before weakness
What are the 4 potential causes of a weak/absent reflex
Damage to nerves outside SC
-Peripheral neuropathy
Damage to motor neurones
-Motor neurone disease
Problems with the NMJ
-Myasthenia gravis
Muscle disease
-Myopathy
What are the 2 potential causes of an excessive reflex
Spinal cord damage above the level controlling the hyperactive response
Higher CNS damage
What are the 2 potential causes of an asymmetric response
Early sign of a progressive disease
Localised nerve trauma
How would you test for spinal cord injurioes
Reflex tests determine area injures
- Motor neurones above injury => unaffected
- Motor neurones below injury => absent
How would you test for closed head injuries
Why do this test
Increased pressure in skull and on the oculomotor nerve
-Changed eye reflex response with the light (oculomotor reflex)
Describe the Babinsky reflex in the foot
What is normal
What is patholoigcal/normal for neonates
Normal reaction when lateral plantar stroked
-toes curl down
In hemi/paraplegic patients (lost upper motor neurone control) and neonates
-toes curl up
What controls the 3 flexor reflexes found in neonates and young infants when they
- stand
- walk
- develop
Stand
-flexor reflex controlled by RST
Walk
-flexor synergy with cortex and pattern generator
As they develop, Babinsky reflex is lost
Describe how reflexes develop as you develop
- at birth
- as CNS matures
At birth
- no voluntary control
- primitive and developmental reflexes only
As CNS matures
- higher brain areas override primitive reflexes
- allows development of mature responses (postural reflex)
Any disinhibitiom is pathological
Describe reflexes in patients with cerebral palsy
Movements are random and uncontrolled
Higher control development failure
-retain primitive reflexes
-affects sensory mv perception
