By Dr. Abdel Aziz M. Hussein Assist. Prof of Physiology Polysynaptic Reflexes
Synaptic transmission is inhibited by all the following, except :- a- oxygen lack b- alkalosis c- acidosis d- prolonged activity of synapse
Post-tetanic Potentiation in synapses :- a- is due to increased Ca ++ concentration in postsynaptic neurons b- is due to increased Ca ++ influx into presynaptic neurons c- results from slow prolonged stimulation of synapse d- causes fatigue of the synapse
Long-term potentiation of synaptic transmission :- a- is involved in pain control system b- is caused by increased Ca ++ concentration in presynaptic neurons c- results from fast repetitive stimulation of synapse d- is associated with decreased Ca ++ concentration in postsynaptic neurons
Def It is a polysynaptic reflex that moves the affected limb away from the painfull stimulus;Pathway 1.Stimulus → painful stimulus 2.Receptors → free nerve endings 3.Afferents→ A delta 4.Center → AHCs 5.Response Flexion of the stimulated limb Increased extension of the contralateral limb
Significance 1) Withdrawal reflex: 1) Withdrawal reflex: Is of survival or protective value →help to immediately remove the stimulated part of the body away from sources of painful stimuli. 2) Crossed extensor reflex: 2) Crossed extensor reflex: supportive reflex Is supportive reflex i.e. supports the body weight which is shifted to the opposite limb.
Properties of polysynaptic spinal reflexes are due to the presence of interneurons and synapses in the reflex arc. They include;
The impulse is conducted from receptors to afferents to interneurons then to efferent i.e. one way direction At synapse, conduction only in one direction from the presynaptic neuron to the post-synaptic neuron, because the transmitter releasing vesicles present only in the pre-synaptic membrane
Total reflex time: Def., Is the period or time that passes between application of the stimulus and the onset of the reflex response. Causes: ? Central Delay : It is the time needed for transmission of impulses through central synapses in the reflex pathway. It depends mainly on the number of synapses in reflex pathway.
It is a gradual decline in the reflex response when its sensory nerve is stimulated for a relatively prolonged period. It is due to fatigue of synapses in the reflex pathway.
The site (locality) of the stimulus determines the nature of response e.g.: In withdrawal reflex, the pattern of flexion that occurs varies with the site of the stimulus. a) Stimulus at the lateral aspect of the thigh leads to adduction and medial rotation. b) Stimulus at medial aspect of the thigh leads to abduction and lateral rotation
Mechanism: 1.Pain fibers from the lateral aspect of the thigh centrally connect with and excite the motor neuron pool of the flexors and adductors 2.Pain fibers from medial side are connected with flexors and abductors
Def. The extent of the response is determined by the strength of stimulation, so; 1)Weak stimuli produce limited responses. 2) Stronger stimuli produce more widespread responses.Example When applying painfull stimulus to sole
1.Mild painful stimulus 1.Mild painful stimulus at the sole of the foot leads to dorsiflexion of the ankle only. 2.Moderate painful stimulus at the sole of the foot leads to dorsiflexion of the ankle and flexion of the knee. 3.Severe painful stimulus at the sole of the foot leads to dorsiflexion of the ankle and flexion of both knee and hip.
Mechanism: 1) Strong stimuli cause stimulation of more receptors → activate more afferents whose central terminals diverge to activate many interneurons at various levels of CNS→ spreading of inputs in the CNS → excite greater numbers of efferents. 2) Divergence of central terminals of afferents and axons of interneurons
Def., Reflex contraction of a group of ms is usually associated with inhibition of the antagonistic ms. Mechanism: Afferents give excitatory inputs to the MNs of a ms, activate at the same time inhibitory interneurons,which inhibit the MNs of antagonistic ms. Significance: It provides a coordinated ms contraction around the joints and reduces the resistance to movements.
It is also found in other areas of the nervous system other than spinal cord e.g. A) In Medulla Oblongata: pressor area and depressor area and inspiratory area and expiratory area B) In the Hypothalamus: heat gain center and heat loss center and feeding center and satiety center
Exception: +ve supporting reaction in which all groups of ms of lower limbs are contracted at the same time. Deep pressure applied to the sole of foot by the body weight during standing results in reflex contraction of both flexors and extensors of the lower limb to support the body against gravity
2 phenomena which are studied by comparing motor and reflex tetanus
Cause: Motor neurons are said to be gradually recruited (activated), one after the other i.e. not excited at the same time. Due to; A.Different velocities of nerve impulses due to different thickness of afferent neurons. B.Presence of interneurons.Recruitment Def., It is a gradual ↑ in the magnitude of the reflex response at the onset of afferent stimulation until the response reaches its full magnitude.
Cause: is explained by the repetitive discharge interneuron circuitsAfter-discharge Def., It is the persistence of the reflex response for a period of time after stoppage of sensory stimulation.
Test yourself A flexor withdrawal reflex shows all the following properties, except :- a- recruitment b- prepotent c- non-fatigable d- irradiation
Test yourself After-discharge of reflex responses :- a- increase the magnitude of the reflex responses b- delays the onset of fatigue of reflex responses c- involves interneuron circuits d- depends upon spatial summation