Physiology of CNS Reflex Action By Dr Abdel

Physiology of CNS Reflex Action By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

Reflex Action Def: • It is an automatic (involuntary) specific response of an organ caused by an adequate sensory stimulus

Pathway of Reflex Action (1) Stimulus (2) Receptor sensory neuron (3) CNS (spinal cord) motor neuron (4) Effector (5) Response 4

Pathway of Reflex Arc

Significance of Reflexes a) Physiological importance: • Differs according the type of reflex. • Some reflexes are protective such as flexion withdrawal reflex , and others are supportive such as stretch reflex, so its importance differs according to the type of reflex. a) Clinical importance: They test the integrity of the pathway (afferent , center in CNS, and efferent)

Classification of Reflexes Local Reflexes Systemic or General Reflexes • Their centers outside CNS • Their centers inside CNS

Local Reflexes • • • a) Their centers are present outside CNS Examples: Their centers may be present in; Dorsal root ganglia (DRG) as those responsible for the flare and allodynia (1 ry hyperalgesia) b) Enteric nervous system as those regulating the GIT activities c) Prevertebral sympathetic ganglia as those responsible for inhibitory enterogastric reflexes

Local axon reflex

Systemic Reflexes Condition ed Reflexes Unconditi oned Reflexes • Their centers present in cerebral cortex • Their centers present in CNS outside cerebral cortex

Conditioned Reflexes • Are acquired reflexes i. e. need previous experience or training • Need intact cerebral cortex → their centers are present in cerebral cortex. • Initiated by visual, olfactory, auditory stimuli or even thinking of the stimulus • Play a role in regulation of G. I. T. , respiratory and cardiovascular functions

Conditioned Reflex

Pathway of Conditioned Reflex

Unconditioned Reflexes • They are inherent (inborn) reflexes • Don’t need intact cerebral cortex → their centers are not present in cerebral cortex • Don’t need training. • According to the site of their centers, they include; 1. Hypothalamic reflexes 2. Midbrain reflexes 3. Pontine reflexes 4. Medullary reflexes 5. Spinal reflexes

Unconditioned Reflexes Hypothalamic Midbrain Pontine Medullary Spinal

Unconditioned Reflexes 1. Hypothalamic reflexes include reflexes of regulation of food intake, body temperature, etc… 2. Midbrain reflexes include pupillary light reflex and righting reflexes 3. Medullary reflexes include reflexes of regulation of CVS function e. g. baroreceptor reflex and GIT functions such as mastication, vomiting and GIT secretions and motility

Spinal Reflexes

Spinal Reflex 1) Def. , • Are reflexes their centers are present in the spinal cord. 2) Classification: • Are classified into 3 types; A) Superficial spinal reflexes: Their receptors are present on body surface (skin). • Examples; a. Scratch reflex b. Flexion withdrawal reflex c. Abdominal reflex d. Cremasteric reflex. e. Plantar reflex.

Spinal Reflex 2) Classification: B) Deep spinal reflexes: • Their receptors are present in deep structures e. g. ms, bone and joints. • e. g. Ms Stretch reflex C) Visceral spinal reflexes: • Their receptors are present in visceral organs. Examples; • i) Micturition reflex: → is initiated by distension of urinary bladder. • ii) Defecation reflex: → is initiated by distension of rectum.

Spinal Reflex A) Somatic reflexes: • Mediated by somatic efferent supplying skeletal ms

Spinal Reflex B) Autonomic reflexes: • Mediated by autonomic efferent supplying blood vessels, glands and visceral organs

Spinal Reflex Pathway or Arc • 1. 2. 3. Its pathway consists of 3 main components; Afferents Interneurons Efferents

Afferent Neurons • Are monopolar neurons which present in the DRG. • The axon of each neuron divides into 2 branches; 1. Peripheral branch → terminates in sensory receptors 2. Central branch → enters into the spinal cord.

Afferent Neurons • The central branch divides into several terminals which ends on; a. Gray matter of spinal cord e. g. posterior horn cell as MSN, SGR , etc…, interneurons and anterior horn cells b. Ascend or descend to higher or lower segments respectively

Afferent Neurons Functions: a)Conduction of the sensory signals to the spinal centers to excite the subsequent neurons in the reflex pathways.

Afferent Neurons Functions: b)Divergence of the incoming sensory signals into wider areas in the NS.

Afferent Neurons

Interneurons • Are small highly excitable neurons • Are located in the gray matter between afferent neurons and the efferent neurons.

Interneurons • All reflex arcs include interneurons except the stretch reflex i. e. monosynaptic

Interneurons • Some of them are excitatory and the others are inhibitory.

Functions of interneurons • Interneurons form different types of circuits that perform the following functions; 1. Divergence 2. Convergence 3. After-discharge 4. Inhibitory circuits

Interneurons Circuits

1. Divergence • A single input (afferent) neuron or (more commonly) interneuron divide to give several collaterals and reach a larger number of efferent neurons Significance : • Help in spread of a single afferent signal to a large No. of postsynaptic neurons in the spinal cord.

1. Divergence • May be on the same side and may be on the opposite side.

1. Divergence

2. Convergence • Multiple collaterals from input (afferent) neurons or more commonly interneurons converge (collect) on a relatively fewer number of output (efferent) neurons Significance: • This help spatial summation of EPSPs → discharge of impulses on the postsynaptic neurons.

2. Convergence • May come from single or multiple sources

3. After-discharge Circuits • Def. • It is prolonged (continuous) discharge from an efferent neuron even after stoppage of stimulation of the afferent nerves • Mechanism: • Because the afferent impulses do not reach efferent directly but through interneurons circuits • There are 2 types of these circuits; 1. Open interneuron circuits 2. Closed interneuron circuits.

3. After-discharge Circuits

3. After-discharge Circuits A) Parallel ( Open ) Chain Circuits: • In this circuit an afferent neuron stimulates an efferent neuron both directly and indirectly through an interneuron which is anatomically arranged in parallel with the afferent neuron.

3. After-discharge Circuits A) Parallel ( Open ) Chain Circuits: • Impulses from inputs not reach to output at the same time due to delay 0. 5 ms at each synapse • Duration of discharge depends upon the No. of interneurons

3. After-discharge Circuits B) Closed-chain (Reverberating) Circuits: Principal Interneurons Efferent Afferent 1 2 3 Collaterals

3. After-discharge Circuits B) Closed-chain (Reverberating) Circuits: • The activity of these circuits stop by either; - -- 1. Fatigue of synapse 2. Inhibitory interneurons

3. After-discharge Circuits B) Closed-chain (Reverberating) Circuits: Examples : Reticular activating system (RAS) • Wakefulness depends upon the activity of RAS (contains many reverberating circuits) • Single sensory stimulation causes activation of RAS for long time (16 -18 hours). • RAS, in turn stimulate the cerebral cortex which by its turn re -stimulate it & so on. • RAS activity continues till fatigue of the synaptic transmission occurs and then sleep occurs.

4. Inhibitory Circuits • In this circuit, an excitatory input is converted into an inhibitory output. • Types: • 2 types of these circuits; 1. Lateral inhibitory circuits 2. Recurrent inhibitory circuits

4. Inhibitory Circuits A) Lateral inhibitory circuits: Inhibitory interneuron Collateral less active More active neurons

4. Inhibitory Circuits A) Lateral inhibitory circuits: • In this circuit the afferent neuron activates an inhibitory interneuron which in turn inhibit the adjacent efferent neurons. Site: • Ascending sensory pathways Importance: • These circuits are important in focusing on or sharpening of the most important sensation.

4. Inhibitory Circuits B) Recurrent inhibitory circuit: • Nerve fiber gives a collateral branch which excites (via Ach) an inhibitory neuron which in turn, inhibits (via glycine) the original neuron as well as the surrounding neurons. Glycine Ach

4. Inhibitory Circuits B) Recurrent inhibitory circuit: Renshaw cells : • Small inhibitory interneurons present in AHCs of the spinal cord • Stimulated A. H. C. gives a recurrent collateral branch which stimulates the Renshaw cells. • Renshaw cells in turn inhibits either: stimulated or surrounding A. H. C. Importance: a)Focus the motor activity to the most facilitated motor neurons b)Stops the activity of the stimulated motor neurons.

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