صدق الله العظيم الاسراء اية 58

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

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

Pathway of Reflex Arc

a)Physiological importance: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:a)Clinical importance:  They test the integrity of the pathway (afferent, center in CNS, and efferent)

Local Reflexes Their centers outside CNS Systemic or General Reflexes Their centers inside CNS

Their centers are present outside CNS Examples: Their centers may be present in; a)Dorsal root ganglia (DRG) as those responsible for the flare and allodynia (1ry 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

Conditioned Reflexes Their centers present in cerebral cortex Unconditioned Reflexes Their centers present in CNS outside cerebral cortex

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

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

Hypothalamic Midbrain Pontine Medullary Spinal

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

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

2) Classification: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.

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

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

Its pathway consists of 3 main components; 1.Afferents 2.Interneurons 3.Efferents

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.

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

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

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

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

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

Some of them are excitatory and the others are inhibitory.

Interneurons form different types of circuits that perform the following functions; 1. Divergence2. Convergence3. After-discharge4. Inhibitory circuits

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.

May be on the same side and may be on the opposite side.

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.

May come from single or multiple sources

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.

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.

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

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

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

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.

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

More active neurons less active neurons Collateral Inhibitory interneuron 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.

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. Ach Glycine

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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