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
