Spinal Reflexes

Spinal Reflexes

1. Afferent Fibre Classification

   • Spinal Cord and Rexed's Laminae
     • Dorsal is at the back (think of animal on all fours)
     • Grey matter
       • Where cell bodies lie
     • White matter
       • Where afferent fibres run through
     • Spinal cord splits into 10 different laminae
       • Motoneurones lie largely in laminae IX
     • Dorsal root ganglion (DRG)
       • Cell body of the afferent fibre
       • Lies outside the spinal cord
         • Terminates in lamina I
         • Pathways go through the laminae to the efferent pathway
   • Erlanger and Gasser system
     • Fibres are A, B or C
   • Lloyd-Hunt system
     • Fibres grouped by I, II, III and IV

2. Muscle Spindle Stretch Reflex

   • Afferent Innervation
     • Single Ia afferent fibres
       • Large myelinated A-α fibre
       • Signals dynamic and static response
       • Primary (annulospiral) endings
         • Wrap around the swollen portion of the NBF
       • Also activated by tap or vibration of the tendon that the muscle is attached to
       • Firing Characteristics
         • Constant level of stretch shows a constant level of firing
         • Increase in stretch shows an increase in frequency of firing
           • Increases further with faster stretching
           • After stretch, firing is increased but is consistent
     • Group II afferents
       • Large, myelinated A-β fibres
       • Secondary (flower-spray) endings
       • Signals static response only
       • Firing Characteristics
         • Consistent frequency of firing before and during stretch
         • Increased frequency with sustained stretch
   • Efferent Innervation
     • Extrafusal fibres are innervated by α-motoneurones
     • Intrafusal fibres are innervated by γ-motoneurones
       • Ensure the muscle spindle is always in direct contact with the changes occurring in the muscle
         • Tension in the spindle is altered to adjust for changes in muscle length due to extrafusal fibre contraction
     • α- and γ- motoneurones are usually co-activated during voluntary movements
       • Iα fibres signal the length of the muscle
       • γ-motoneurones cause contraction of the spindle
         • Ensures no slackness
   • Myotatic Stretch Reflex
     • Monosynaptic
       • Simplest form of reflex
       • Afferent fibres terminate on the motoneurones which innervate the same muscle
     • Iα muscle spindle afferents directly synapse onto α-motoneurones in the ventral horn
       • No interneurons are used
       • Motoneurones synapsed onto can be homonymous muscle (same) or heteronymous (synergistic)
       • Glutamate is the neurotransmitter
     • e.g. knee Jerk Reflex
       • Tapping provides the stimulus that causes the contraction
     • e.g. standing upright
       • Spindles in the leg detect stretch when leaning forwards or back
         • They fire and stimulate α-motoneurones to cause the muscle to contract and bring the body back upright
     • Antagonistic inhibition
       • Opposing antagonistic muscles about a joint are inhibited/relaxed
       • Disynaptic Pathway
         • Occurs via Iα interneurons
         • Homonymous muscle is stimulated to contract via glutamate, whilst the antagonist muscle is inhibited by glycine
   • Key Terms
     • Flexors
       • Bend limbs into the body about a joint
     • Extensors
       • Straighten limbs about a joint
     • Synergists
       • Contract, working together
     • Antagonists
       • Relax, working against each other
   • Muscle Spindles
     • Regulate the level of stretch in the skeletal muscles
       • Feedback mechanism to the nervous system about the length of the muscle, showing it's state of contraction
     • Made up of intrafusal fibres
       • Encapsulated bundles of specialised muscle fibres that make up the intrafusal fibres
       • Nuclear bag fibres (NBF)
         • Swollen central portion
           • Nuclei of the fibres positioned centrally
         • Very little sarcoplasm between myofibrils
       • Nuclear chain fibres (NCF)
         • Thin and evenly shaped
           • Nuclei are positioned along the length of the fibre
         • Lots of space between myofibrils
     • Centre is non-contractile
       • Afferent fibres found here
         • Sends action potentials to the CNS
     • Dynamic sensitivity
       • Muscle spindles detect the rate at which something is changing
         • e.g. length of the muscles

3. Golgi Tendon Organs (GTOs)

4. Withdrawal Reflexes

5. One reflex or many?

6. Also known as sensorimotor integrations. Organised within the spinal cord into the spinal reflex arc

   • Automatic rapid response to a stimulus
   • Initiated by a stimulus, which is detected by a sensor, causing the generation of action potentials
     • Action potentials are passed along afferent (sensory) neurones to the CNS
       • Central processing takes place, causing the release of neurotransmitter
         • Neurotransmitter acts upon interneurons, which link afferent and efferent pathways
           • Can be short, acting within the spinal cord
           • Can send projections to the brain
         • Interneurons are not used in monosynaptic reflexes, where the sensory pathway links directly to the motor pathway
         • Efferent (motor) pathway is connected to the effector and causes it to create an output
   • The reflex arc can be modulated by other neurones and propriospinal pathways (interneurones within the spinal cord) as well as the brain

Images and files in the map