07-02-22 - Reflexes Flashcards
Learning outcomes
- Recall that (i) different classes of nerve fibre have different diameters (and conduction velocities); (ii) there are two different nerve fibre classification systems; (iii) how the magnitude of conduction velocity relates to the diameter of nerve fibre
- List the components of monosynaptic reflex arcs.
- Describe the structure and function of muscle spindles and their reflex connections with α-motor neurones supplying the muscle
- Describe the structure and function of Golgi tendon organs and their reflex connections with α-motor neurones supplying the muscle
- Explain how muscle spindles monitor muscle length, while Golgi tendon organs monitor tension.
- Discuss how the myotatic, inverse myotatic and cross extensor withdrawal reflex may combine to produce simple repetitive movements
- Give examples of complex reflexes mediated by the special senses (e.g. eyes and semi-circular canals) that control posture and balance
- Recall that the autonomic nervous system is involved in many reflexes
What is a reflex?
What are the 2 classifications of reflexes?
What are 3 examples of reflexes?
What are 2 examples of learned/acquired reflexes?
- A reflex is a specific, involuntary, unpremeditated, “built-in” response to a particular stimulus
- Classifications of reflexes:
1) Somatic reflexes
• Involves somatic motor neurons innervating skeletal muscles that brings about movement/action
2) Autonomic (visceral) reflexes)
• Baroreceptor reflex (monosynaptic/polysynaptic)
• Examples of reflexes (somatic reflexes):
1) Remove hand from hot object (spinal reflex)
2) Life foot off sharp object (spinal reflex)
3) Shutting eyes as object rapidly approaches face (cranial reflex)
• Learned/acquired reflex:
1) Driving a car
2) Pavlov’s dog salivating on hearing a bell
What are many reflexes?
What are examples of autonomic reflexes?
What are reflexes a part of?
What 3 autonomic reflexes might there be in response to a drop in body temperature?
- Many reflexes are autonomic, meaning they are done automatically, and we don’t have to think about them (baroreceptor reflex)
- Adjusting body temperature is an autonomic reflex
- Reflexes may be a component of homeostatic systems, where there is a stimulus-response sequence
• Autonomic reflexes might there be in response to a drop in body temperature:
1) Constriction of blood vessels
2) Curling up
3) Shivering
What are 3 reflexes involving special senses?
What are other reflexes concerned with?
• Reflexes involving special senses:
1) Rotatory nystagmus – eye movements driven by moving visual images
2) Post rotatory nystagmus – eye movements driven by the movement of fluid in the semi-circular canals of the inner ear
3) Interpretation of movement within images on the retina – used to make postural adjustments to preserve balance
• Other reflexes are concerned with homeostatic functions
What are the 5 steps in a pathway mediating a reflex arc?
- Steps in a pathway mediating a reflex arc:
- Afferent pathway:
1) Stimulus - A detectable change in the internal/external environment
2) Receptor detects the change
3) Signal receives in the integrating centre, which is normally the spinal cord or the brain, (along with those from other stimulus/receptor interactions) and compared to set point
• Efferent pathway:
4) Brain or spinal cord will bring about an activation of an efferent pathway that leads to an effector cell
5) Effector cell usually brings about a negative feedback response on the initial stimulus on the receptor, which returns the variable to its set point
What are the 3 different classes of movement?
• Classes of movement:
1) Voluntary
• Complex actions (reading, writing)
• Purposeful goal directed
• Learned
2) Reflexes • Involuntary • Rapid • Stereotyped • E.g knee jerk and eye blink
3) Rhythmic motor patterns
• Combines voluntary and reflexive acts (chewing, walking, running)
• Initiation and termination voluntary
• Once initiated, repetitive and reflexive
Classifications of somatosensory nerve fibres: • Nerve fibre • Numerical nomenclature • Alphabetical nomenclature • Fibre diameter (µm) • Conduction velocity (m/s) • Examples
What lamina of the spinal cord male up the dorsal, lateral and ventral horn?
What can be found in each horn?
- Dorsal horn
- Lamina I-VI
- Terminations for primary afferent sensory neurons and neurons of reflex circuits
- Sensory input from joints and muscle in lamina VI
- Lateral horn
- Lamina VII
- Preganglionic sympathetic (T1-L2) and parasympathetic (sacral – S2-S4) neurons
- Ventral horn
- VIII and IX
- Cell bodies of motor neurons
Where do α-motor neuron cell bodies lie in the spinal cord?
What does each motor neuron activate?
Where do some alpha motor neuron axons branch?
What are Renshaw cells?
What 2 things are Renshaw cells used for?
What does strychnine poisoning do?
- Α-motor neuron cell bodies lie in clumps within ventral horn of the spinal cord (lower motor neurons)
- Each motor neuron activates a motor unit (6-1500 skeletal muscle fibres)
- Some alpha-motor neuron axons branch back into the spinal cord and synapse with interneurons called Renshaw cells
- Renshaw cells are inhibitory interneurons that lead to feedback inhibition
• Renshaw cells can be used for:
1) Suppressing weakly firing motor neurons, and dampening strongly firing ones
2) Producing economical movement
• Strychnine poisoning disables Renshaw cell inhibition, which leads to convulsions (uncontrollable muscle contractions
What is the myotatic (knee jerk) reflex an example of?
Describe the 4 steps involved in the knee jerk reflex.
When is this reflex lost?
- The knee jerk reflex is an example of a monosynaptic stretch reflex
- Steps of knee jerk reflex:
1) Tap of patellar tendon stretches quadriceps muscle
2) Stimulates dynamic nuclear bag receptors of muscle spindle
3) Increase in rate of firing of la afferents, which leads to contraction of quadriceps muscle
4) la fibres also stimulate inhibitory interneurons which inhibit antagonistic (flexor) muscle) of knee joint
• This reflex is lost if the lower lumbar dorsal roots of the spinal cord are damaged
What is the role of muscle spindles?
Where are they found?
Where are they particularly numerous?
What are the efferent and afferent fibres that innervate muscle spindles?
What is the role of each?
- Muscle spindles are the main proprioceptors that provide information about the state of musculature
- Muscle spindles lie within muscle in parallel with skeletal muscle fibres
- Particularly numerous in fine motor control muscles like the eyes and hands
- Muscles spindles are innervated by γ-motor neurons (efferent) and group la and II afferent fibres
- γ-efferent activity regulates the sensitivity of the spindle
- Afferents respond to muscle stretch
How are muscle spindles linked to the knee jerk reflex?
What are the two types of receptors in muscle spindles?
What causes a static response when the muscle fibre is stretched?
What fibres does static response excite?
What muscle afferents are more important in static responses?
What causes a dynamic response when muscle is stretched?
What is stimulated powerfully?
What muscle afferents are more important in dynamic responses?
• For the knee jerk reflex to happen, it is the muscle spindles that are the receptors that do the detection of the change of length of that muscle
• Two types of receptors in muscle spindles:
1) Nuclear bag fibres
2) Nuclear chain fibres
- Static responses are caused when the receptor portion of the muscle spindle stretched slowly
- This causes the number of impulses transmitted from both primary la sensory fibres and secondary II muscle fibres endings to increase almost directly in proportion to the degree of stretching
- The endings continue to transmit these impulses for several minutes
- Gamma static response excites nuclear bag fibres
- Secondary Type II afferent fibres are important in static responses, as they give information about absolute changes in length of the muscles
- Gamma dynamic responses are caused when the length of the spindle receptor increases suddenly
- In this case, the primary la fibres are stimulated powerfully
- Primary Ia afferents are important for dynamic responses, as they give information about a rate of change
What is an important role of muscle spindles?
Where are voluntary changes in muscle length initiated?
What is α-y co-activation?
What does this process do?
- Muscle spindles play an important role as comparator for maintenance of muscle length during goal directed voluntary movements
- Voluntary changes in muscle length are initiated by motor areas of the brain
- α-y co-activation is the simultaneous activation of extrafusal fibres (α-motor neurons) and intrafusal fibres (γ-motor neurons)
- This process readjusts sensitivity of muscle spindle continuously as muscle shortens
What are 2 roles of the inverse myotatic (Golgi tendon) reflex.
Describe the 4 steps of this process in the quadriceps
• 2 roles of the inverse myotatic (Golgi tendon) reflex:
1) Protective to prevents tearing of muscle/detachment of tendon
2) Contributes to maintenance of posture
• During maintained posture (e.g standing), quadriceps muscle will start to fatigue
• The force in the patellar tendon will decline, so the activity in afferent Ib fibres of the GTO will decline
• Normal inhibition of motor neurons supplying quadriceps will be removed by removing inhibition of the inhibitory interneuron
• The muscle will contract more strongly, leading to an increase in force in the patellar tendon
• Golgi tendon reflex process:
1) GTO (Golgi tendon organs) stimulated by increasing tension in the muscle
2) Signals transmitted to the spinal cord via Ib sensory afferents through the dorsal route ganglion
3) There is then synapsing with an inhibitory interneuron to the quadricep (extensor) muscle, which will reduce contraction
4) There is also an increase in contraction to the flexor muscle (semitendinosus)
5) This causes a negative feedback mechanism which prevents development of excessive tension in quadriceps
What is the role of Golgi tendon organs?
Where are they found?
What do they respond to?
What nerve fibres are they innervated by?
What do these nerve fibres do?
- Golgi tendon organs are the main proprioceptors that provide information about the state of musculature
- Golgi tendon organs lie within tendons in series with contractile fibres
- They respond to degree of tension within the muscle
- Golgi tendon organs are innervated by Group IB afferent fibres
- These relay information to the CNS (particularly the spinal cord and cerebellum)
What is the withdrawal reflex and crosses extensor reflex?
What does it help to maintain?
What are the steps of this process?
How long does this process take?
- The withdrawal reflex and crosses extensor reflex is a protective reflex of rapidly removing the limb from damaging stimuli (e/g standing on something sharp)
- Helps to maintain posture and balance
• Withdrawal reflex and crosses extensor reflex:
1) In the leg that feels the pain, the reflex inhibits the motor neurons to the extensor muscle in the spinal cord
2) While doing this, it stimulates the neurons to the flexor muscle, allowing you to lift your foot up, but the weight of the body needs to be transferred to the opposite leg
3) In the opposite leg, the reflex stimulates the motor neurons to the extensor muscle and inhibits the motor neurons to the flexor muscle
• Stimulation if withdrawal reflex frequently elicits extension of the contralateral limb 250ms later
What is double support during locomotion?
What are the 3 steps of the step cycle?
- Double support is when there are 2 points of contact during walking
- Steps of the step cycle:
1) Stance phase
• Foot touching the ground
• Finishes with extension about all joints for forward movement
2) Swing phase
• Bending (flexion) of the hip, knee and ankle followed by knee and ankle straightening (extension)
3) Alternate swing and stance that are out of phase
