Reflexes Flashcards
What is a reflex ?
A specific, involuntary, unpremeditated, “built in” response to a particular stimulus.
Give some examples of reflexes
Removing hand from a hot object.
Lifting a foot of a sharp object.
Shutting eyes as object rapidly approaches face.
Describe a learned / acquired reflex
Many operations involved in driving a car.
Homeostasis
A stimulus-response sequence.
Reflexes may be a component of homeostatic control systems.
State the reflexes involving special senses
Rotatory Nystagmus
Post Rotatory Nystagmus
Interpretation of movement within images
Rotatory Nystagmus
Eye movements driven by moving visual images.
Post Rotatory Nystagmus
Eye movements driven by the movement of fluid in the semi-circular canals of the inner ear.
Interpretation of movement with images on the retina
Used to make postural adjustments to preserve balance.
Describe Step 1 in the pathway mediating a reflex arc
A detectable change in internal/external environment, (stimulus).
Describe Step 2 in the pathway mediating a reflex arc
The receptor detects the change
Describe Step 3 in the pathway mediating a reflex arc
Signal received (along with those from other stimulus/receptor interactions)
Describe Step 4 in the pathway mediating a reflex arc
Sometimes the response gives negative feedback
State the classes of movement
Voluntary
Reflexes
Rhythmic motor patterns
Describe voluntary movement
Complex actions (reading, writing)
Purposeful goal directed
Learned
Describe reflex movement
Involuntary
Rapid
Stereotyped (knee jerk, eye blink)
Describe rhythmic motor patterns
Combines voluntary and reflexive acts (chewing, walking, running)
Initiation and termination voluntary
Once initiated repetitive and reflexive
Describe organisation of the spinal cord
Sensory information comes in through the dorsal root of the spinal cord.
Sensory (afferent nerve) will synapse with either an interneuron or directly with the alpha motor neurones that are situated in the ventral horn.
Ventral horn - front part of the spinal cord where alpha motor neurons come out and go to a skeletal muscle
Describe Lamina I-VI (dorsal horn)
Terminations for primary afferent sensory neurones and neurones of reflex circuits.
Sensory input from joints and muscles (lamina VI)
Describe Lamina VII (lateral horn)
Pre-ganglionic sympathetic (T1-L2) and parasympathetic (sacral)
Describe Lamina VIII and IX (ventral horn)
Cell bodies of motor neurones
Where are motor neurones supplying muscles of the trunk situated ?
Medially
Where are motor neurones supplying distal muscles situated ?
Laterally
Corticospinal tract
Information from the brain CNS going down towards the spinal cord
Spino thalamic tract
Information going from the spine and ending up in the thalamus
Describe the circular route for a simple reflex arc or spinal reflex
Information comes into the spinal cord via the dorsal root ganglia, that ultimately leads into the dorsal part of the dorsal horn of the spinal cord.
These neurones ultimately either innervate directly alpha motor neurones or to an interneuron that connects sensory information to an alpha motor neurone.
Alpha motor neurones come out of the ventral horn and essentially of to the muscle to bring out contraction.
Describe the reflex connections of muscle spindles with alpha-motor neurones supplying the muscle
Alpha motor neurone cell bodies lie in clumps within the ventral horn of the spinal cord.
Each motor neurone activates a motor unit.
Describe the connection between alpha motor neurones and Renshaw cells
The alpha motor neurone in the ventral horn thats going to a particular muscle has a collateral that comes off that.
That collateral stimulates a Renshaw cell (which is inhibitory).
The Renshaw cell goes back on to the alpha motor neurone and reduces the activity of the alpha motor neurone.
What are Renshaw cells ?
Interneurons
(recurrent or feedback inhibition)
Function of Renshaw cells
Helps to produce a refinement of muscle contraction.
Describe the action of Renshaw cells
They :
- Suppress weakly firing motor neurons and dampening strongly firing ones
- Produces economical movement
Describe strychnine poisoning
Disables Renshaw cell inhibition - leads to convulsions
Give an example of myotatic stretch reflex
Knee Jerk reflex - an example of a monosynaptic stretch reflex
Describe the myotatic knee jerk reflex
Tap of patellar tendon stretches the quadricpes muscle.
This stimulates dynamic nuclear bag receptors of the muscle spindle.
Describe how myotatic reflex jerks work
An increase in the rate of firing of group Ia afferents, leads to contraction of quadriceps muscle.
Ia fibres also stimulate inhibitory interneurons which inhibit antagonistic (flexor) muscle of the knee joint.
When is myotatic reflex lost ?
Reflex is lost if lower lumbar dorsal roots of spinal cord are damaged.
What are muscle spindles ?
Main proprioceptors that provide information about the state of musculature.
Where are muscle spindles located ?
Muscle spindles lie within muscles in parallel with skeletal muscle fibres.
They are particularly numerous in fine motor control muscles (eyes, hands)
What innervates muscle spindles ?
Gamma-motorneurons (efferents) and group Ia and II afferent fibres
Function of muscle spindle afferents
Responds to muscle stretch
Function of muscle spindle gamma-efferents
Regulates the sensitivity of the muscle spindle
Function of muscle spindles
They play an important role as comparators for maintenance of muscle length, during goal directed voluntary movements.
Describe voluntary activity of muscle spindles
Voluntary changes in muscle length initiated by motor area of the brain, orders include changes to set-point of muscle spindles.
Describe alpha-gamma co-activation
Simultaneous activation of extrafusal fibres and intrafusal fibres is called alpha-gamma co-activation.
Function of alpha-gamma co-activation
The process readjusts sensitivity of muscle spindles continuously as muscle shortens.
Function of the Golgi tendon reflex
Protective to prevent tearing of muscle/ detachment of tendon.
Contributes to maintenance of posture.
What is another name for Golgi Tendon reflex ?
Inverse Myotatic Reflex
How is the Golgi Tendon Organ stimulated ?
GTO is stimulated by increasing tension in muscle
Signals transmitted to the spinal cord - reflex is entirely inhibitory
Negative feedback mechanism which prevents development of excessive tension in muscle.
Difference between muscle spindles and Golgi tendon organs
Muscle spindles monitor muscle length
Golgi Tendon organ monitors muscle tension
What does the inhibitory effect from the Golgi tendon organ lead to ?
Inhibitory effect from GTO can be so great that it leads to sudden unloading of muscle.
Describe the Golgi Tendon Organ
Main proprioceptors that provide information about the state of musculature.
Where are Golgi Tendon Organs located ?
They lie within tendons in series with contractile fibres.
Function of the Golgi tendon organ
Responds to a degree of tension within the muscle
Group Ib afferent fibres relay information to the CNS
Function of the withdrawal reflex
Protective reflex of rapidly removing limb from damaging stimuli
Stimulation of withdrawal reflex, frequently elicits extension of the contralateral limb 250ms later.
Helps maintain posture and balance
Describe locomotion step cycle
Stance phase
Swing phase
Stance phase
Foot touching the ground, flexion of knee and ankle
Finishes with extension about all joints for forward movement.
Swing phase
Bending (flexion) of hip, knee and ankle followed by knee and ankle straightening (extension)
Central Pattern Generator
Location
Modulation
Initiation
Located in the spinal cord
Modulated by proprioception unit
Thought to be initiated by mesencephalic locomotor region - output through reticular nuclei and reticulospinal tracts
Describe the structure of the CPG
2 half centres which activate flexors and extensors respectively and mutually inhibit each other
What system is involved in reflexes ?
Central Pattern Generator
