Reflexes Flashcards
What is homeostasis at its most brief?
A stimulus-response sequence
What are reflexes relationship to homeostasis?
Reflexes may be a component of homeostatic control systems
Are reflexes a conscious choice or no?
Sometimes aware of stimulus and/or response
For many reflexes, no conscious awareness
describe the pathway mediating a reflex arc?
- A detectable change in internal/external environment
- Detects the change
- Signal received (along with those from other stimulus/receptor interactions)
- Sometimes response gives negative feedback
What reflexes involve special senses?
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 within images on the retina – used to make postural adjustments to preserve balance
What are reflexes other than those involving special senses involved in?
Other types of reflex are concerned with homeostatic functions
- blood pressure regulation (baroreceptor reflex)
What are the three classes of movement?
Voluntary
Reflexes
Rhythmic motor patterns
What is voluntary motion?
Complex actions (reading, writing)
Purposeful goal directed
Learned
What is reflex motion?
Involuntary, rapid, stereotyped (knee jerk, eye blink)
What are rythmic motor patterns?
Combines voluntary & reflexive acts (chewing, walking, running)
Initiation & termination voluntary
Once initiated repetitive & reflexive
What are the two types of nerve fibres in somatosensory nerve fibres?
Myelinated and unmyelinated
What is the axon diameter and conduction velocity of somatosensory nerve fibre Ia?
12-20µm
80-120m/s
What is the axon diameter and conduction velocity of somatosensory nerve fibre Ib?
12-18µm
80-120m/s
What is the axon diameter and conduction velocity of somatosensory nerve fibre II?
Aβ
5-12µm
35-75m/s
Aγ - motor to spindle
3-9µm
15-30m/s
What is the axon diameter and conduction velocity of somatosensory nerve fibre III?
1-5µm
5-30m/s
What is the axon diameter and conduction velocity of somatosensory nerve fibre B?
1-3µm
3-15m/s
What is the axon diameter and conduction velocity of somatosensory nerve fibre IV (the unmyelinated one)?
- 2-1.5µm
0. 5-2m/s
Describe the positional order of the spinal cord
Lamina I-VI
- terminations for primary afferent sensory neurons & neurons of reflex circuits
Lamina VI
- Sensory input from joints and muscle
Lamina VIII & IX
- Cell bodies of motor neurons
Motor neurons supplying muscles of trunk situated medially while those supplying distal muscles situated laterally
Muscles that flex limbs lie dorsal to those that extend limbs
Where do alpha motor neuron cell bodies lie?
(alpha)-motor neuron cell bodies lie in clumps within ventral horn of spinal cord (lower motor neurons)
How many skeletal muscle fibres does a motor neuron activate?
Each motor neuron activates a motor unit (6-1500 skeletal muscle fibres)
What type of neuron helps to provide economical movement?
Some axons branch back into cord and synapse with interneurons called Renshaw cells (recurrent or feedback inhibition)
- Suppresses weakly firing motor neurons and dampening strongly firing ones - Produces economical movement
Provide an example of the importance of Renshaw cells
Importance reflected in strychnine poisoning
- disables Renshaw cell inhibition - leads to convulsions
What is the myotatic reflex?
Knee jerk
Describe the myotatic (knee jerk) reflex
Example of a monosynaptic stretch reflex
Tap of patellar tendon stretches quadriceps muscle
Stimulates dynamic nuclear bag receptors of muscle spindle
Increase* in 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 knee joint
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 do muscle spindles lie?
Muscle spindles lie within muscles in parallel with skeletal muscle fibres
Particularly numerous in fine motor control muscles (e.g. eyes, hands)
How are muscle spindles innervated?
Innervated by (gamma)-motoneurons (efferents) and group Ia and II afferent fibres
Afferents respond to muscle stretch while (gamma)-efferent activity regulates the sensitivity of the spindle
What two responses do muscle spindles undergo?
Static Response
Dynamic Response
What is the static response of spindle muscle fibres?
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.
Gamma-static excites nuclear chain fibres
What is the dynamic response of spindle muscle fibres?
When the length of the spindle receptor increases suddenly, the 1o ending (but not the 2o ending) is stimulated powerfully.
Gamma-dynamic excites nuclear bag fibres
What do muscle spindles do exactly?
Muscle spindles play important role as comparators for maintenance of muscle length during goal directed voluntary movements
Voluntary changes in muscle length initiated by motor areas of brain, orders include changes to set-point of muscle spindle
Simultaneous activation of extrafusal fibres (alpha-motor neurons) and intrafusal fibres (gamma-motor neurons) is called alpha-gamma co-activation
This process readjusts sensitivity of muscle spindle continuously as muscle shortens
Discuss the inverse myotatic (Golgi Tendon) reflex
Movement is simply an inverted knee jerk as name implies
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
Discuss muscle fatigue in the thigh during maintained posture
During maintained posture (e.g. standing), quadriceps muscle will start to fatigue
Force in patellar tendon will decline, thus activity in afferent Ib fibres will decline
Normal inhibition of motor neurons supplying quadriceps will be removed
Muscle will contract more strongly, so upping force in patellar tendon
What are Golgi tendon organs?
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)
What is the withdrawal reflex and crossed exterior reflex?
In leg that feels pain - reflex inhibits in spinal cord the motor neurons to the extensor muscle and stimulates motor neurons in the flexor muscle
While in the opposite leg the reflex does the opposite: stimulates motor neurons to extensor and inhibits those to flexor
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
What are the stages of the step cycle?
Right heel contact -> left toe-off surface
Left heel contact -> Right toe-off surface
Right heel contact
What are the phases of the step cycle?
Stance phase
- Foot touching ground, flexion of knee & ankle - Finishes with extension about all joints for forward movement
Swing phase
- Bending (flexion) of hip, knee & ankle followed by knee and ankle straightening (extension)
Alternate swing and stance that are out of phase
What is the Central Pattern Generator (CPG)?
a neuronal network in which interconnected excitatory and inhibitory neurons produce an oscillating, rhythmic output in the absence of sensory feedback
2 half centres which activate flexors and extensors respectively and which mutually inhibit each other
Can be modelled using inhibitory 1a interneurons and Renshaw cells (RC)
Where is the Central Pattern Generator (CPG) located?
Located in spinal cord - capable of autonomous signals
Modulated by proprioception input
Thought to be initiated by mesencephalic locomotor region
- Output through reticular nuclei & reticulospinal tracts
