Reflexes Flashcards
What is a reflex?
A specific involuntary unpremeditated, built in response to a particular stimulus
What are the classes of reflexes?
- Somatic reflexes – somatic motor neurons and skeletal muscles
- Autonomic (visceral) reflexes – baroreceptor reflex
- Monosynaptic/polysynaptic
What is homeostasis?
- A stimulus-response sequence
- Reflexes may be a component of homeostatic control systems
- Sometimes aware of stimulus and/or response conscious awareness many are autonomic
What is the pathway to mediating a reflex?
- Stimulus - A detectable change in internal/external environment
- Receptor - Detects the change
- Passes the signal down the afferent pathway
- Signal is received (along with those from other stimulus/receptor interactions) at the integrating centre and is compared to set point
- Signal is passed down the efferent pathway to the effector
- Effector produces a response
- Sometimes response gets negative feedback
What are the classes of movement?
- Voluntary
- Reflexes
- Rhythmic motor patterns
Describe voluntary movement
- Complex actions e.g. reading/writing
- Purposeful goal directed
- Learned
Describe reflex movement
Involuntary rapid
Describe rhythmic motor pattern
- Combines voluntary and reflexive acts (chewing, walking, running)
- Initiation and termination voluntary
- Once initiated repetitive and reflexive
Describe Lamina I-VI of the spinal cord
Terminations for primary afferent neurons and neurons of reflex circuits
What are the different rexed laminae of the spinal cord?
- Laminae I-VI (dorsal horn)
- Lamina VII (lateral horn)
- Lamina VIII and IX (ventral horn)
What is the function of Laminae I-VI?
Involved in sensory processing, receiving primary afferent input and participating in reflex circuits.
What is the function of Laminae VII?
Contains autonomic neurons, specifically preganglionic sympathetic (T1-L2) and parasympathetic (sacral) neurons.
What is the function of Laminae VIII and IX?
House motor neurons, including alpha motor neurons ( lie in clumps within the ventral horn that activate motor units. Some axons synapse with Renshaw cells, which help modulate motor neuron activity via inhibitory feedback.
Describe the myotatic reflex (Knee jerk)
- Tap patella tendon produces a transient streching of the quadriceps muscle
- Stimulates dynamic nuclear bag receptors of muscle spindle
- Increase in rate of firing of group Ia afferents leads to contraction of quadriceps
- Ia fibres also stimulate inhibitory interneurons which inhibit antagonistic (flexor) muscle of knee joint
- Reflex is lost if lower lumber dorsal roots of spinal cord are damaged
What is a static respone?
Receptor portion of the muscle spindle stretched slowly, the number of impulses transmitted from both the 1o and 2o endings increases almost directly in proportion to the degree of stretching and the endings continue to transmit these impulses for several minutes.
What is a dynamic response?
When the length of the spindle receptor increases suddenly, the 1o ending (but not the 2o ending) is stimulated powerfully.
What important role do muscle spindles play?
as comparators for maintenance of muscle length during goal directed voluntary movements
What are voluntary changes in muscle length initiated by?
motor areas of brain, orders include changes to set point of muscle spindle
What is a-g co-activation?
Simultaneous activation of extrafusal fibres and (a-motor neurons) and intrafusal fibres (g-motor neurons)
This process readjusts sensitivity of muscle spindle continuously as muscle shortens
Describe Inverse Myotatic (Golgi Tendon) reflex
- Protective to prevent tearing of muscle / detachment of tendon
- Contributes to maintenance of posture
- GTO stimulated by increasing tension in muscle
- Signals transmitted to spinal cord - Reflex entirely inhibitory
- Negative feedback mechanism which prevents development of excessive tension in muscle
- Inhibitory effect from GTO can be so great leads to sudden unloading of muscle
Describe the Golgi tendon organ
- Main proprioceptors that provide information about the state of musculature
- Golgi tendon organs lie within tendons in series with contractile fibres
- Respond to degree of tension within the muscle
- Group Ib afferent fibres relay information to CNS (particularly spinal cord and cerebellum)
Describe Withdrawal Reflex and Crossed Extensor Reflex
- Protective reflex of rapidly removing limb from damaging stimuli
- Stimulation of withdrawal reflex frequently elicits extension of the contralateral 250ms later
- Helps maintain posture and balance
