Reflexes Flashcards
What is a reflex ?
A rapid, automatic, involuntary, inbuilt, hardwired and unconscious response
What is the reflex pathway ?
Sensory receptor > afferent pathway > integrating centre > efferent pathway > effector organ
Summarise the reflex arc
A stimulus is detected by a receptor (specialised cell or dendrites of sensory neurone)
This activates an afferent/sensory neurone leading to the propagation of action potentials along the axon of sensory neurones
A motor neurone is then activated and the axon of the stimulated neurone carries action potentials to an effector organ
The effector organ responds as a result of neurotransmitter being released by the motoneuron
Do reflex responses require conscious perception ?
They are not dependent on conscious perception but can be inhibited by conscious control
Give some examples of receptors in the skin :
- Meissner’s corpuscle (rapidly adapting - touch and pressure)
- Merkle’s corpuscle (slowly adapting - touch and pressure)
- free nerve ending (slowly adapting)
- Pacinian corpuscle (rapidly adapting - vibration and deep pressure)
- Ruffini corpuscle (slowly adapting - skin stretch)
What do mechanoreceptors detect ?
Touch and pressure
What is a receptive field ?
A receptive field is when each sensory neurone responds to stimulus within a specific region of the skin
What does a small receptive field indicate ?
That there is a high receptor density and so greater acuity
How do you find the receptive field size ?
Adjust the distance between the tips and determine the minimum distance at which 2 points can be felt
What do sensory receptors do ?
They transduce stimuli into generator potentials and therefore action potentials
What are the properties of sensory receptors ?
They have :
• high specificity - respond more readily to one particular stimulus
• high sensitivity - extremely sensitive to a specific stimulus
What do olfactory receptors respond to ?
They respond to a few odour molecules in the air
What do (dark adapted) rod photoreceptors respond to ?
They can respond to a single photon
Where are effectors found ?
They are found in muscles (skeletal, smooth and cardiac) and glands (endocrine and exocrine)
What is a stimulus ?
A stimulus causes local depolarisation in nerve endings and then action potentials are propagated along sensory axons
Give some examples of types of reflexes :
1) spinal reflex e.g. withdrawal reflex
2) cranial reflex e.g. pupil constriction
3) innate and conditioned reflexes
4) somatic reflexes e.g. withdrawal reflexes
5) autonomic reflexes
6) monosynaptic and polysynaptic reflexes
What is the difference between innate and conditioned synapses ?
Innate synapses are natural whereas conditioned synapses need to be learned
Where are autonomic reflexes carried out ?
Carried out by the autonomic nervous system to smooth muscle, cardiac muscle or glands
Why are monosynaptic reflexes quicker than polysynaptic reflexes ?
Because they involve less neurones
What is the complexity of reflexes dependent on ?
Dependent on the number of synapses involved
What is a monosynaptic reflex ?
It is a rapid response to a specific stimulus. The sensory neurone synapses directly on to a motoneurone.
What is a polysynaptic reflex ?
There is a longer delay between the stimulus and response. There is atleast 1 interneurone between sensory and motoneuron.
What type of reflex is the knee jerk stretch reflex ?
It is a monosynaptic spinal reflex
Summarise the method of the knee jerk stretch reflex
1) The patellar tendon is tapped with a reflex hammer
2) The spindle receptors in the quadriceps muscle are stretched
3) Distortion of receptors stimulates sensory neurones that extend to the spinal cord
4) The sensory neurone synapses with the motoneuron in the spinal cord that controls motor units in stretched muscle
5) The knee extends in a brief kick
6) The collaterals of the afferent fibres make synaptic contact with inhibitory interneurones which in turn inhibit the antagonistic flexor muscles of the knee joint
Summarise the withdrawal reflex
- the stimulated afferent neurone stimulates excitatory interneurones which stimulate the biceps
- contraction of the biceps causes flexion of the elbow joint which leads to withdrawal of the hand from the stimulus
- the afferent neurone also stimulates inhibitory interneurones which inhibits efferent interneurones supplying the triceps to prevent them from contracting
- this is known as reciprocal innervation
What type of reflex is the flexor/crossed extensor reflex ?
A polysynaptic spinal reflex
Give some examples of neonate reflexes
Rooting reflex - happens when the corners of the baby’s mouth is stroked or touched
Grasp reflex - happens when you put something in a baby’s hand
Moro reflex - also called the startle reflex
Summarise muscle spindle reflexes
Muscle spindles are bundles of small specialised intrafusal muscle fibres which are innervated by both sensory and motor neurones.
The afferent neurones are activated by stretch and they detect changes in the length of muscle fibres which cause contraction of that fibre.
It is a negative feedback loop.
