Nervous System Physiology By Dr. SHAHAB SHAIKH PhD MD MBBS Lecture – 2: Spinal Cord Reflexes •••••••••••••••••••••••••••••••••• Faculty of Medicine Al Maarefa Colleges of Science & Technology

SPINAL CORD It is the elongated part of the Central nervous system Begins as Continuation of Medulla Oblongata at Foramen Magnum and extends up to upper border of L2 vertebra It contains white matter on the outside mainly and grey matter in the central region Many Interneurons in the grey matter of spinal cord are Centers for spinal reflexes Function: Conduit for Information passage To & fro Brain Center for some reflexes

TRANSVERS SECTION OF SPINAL CORD

Reflexes in the body may be … Reflex : It is an automatic response to a stimuli that occurs without conscious effort Reflexes in the body may be … Simple or Basic Conditioned or Acquired Reflexes Important in maintaining homeotsasis Examples Reflexes carry out the automatic actions of swallowing, sneezing, coughing, vomiting. Reflexes maintain balance and posture; e.g., spinal reflexes control trunk and limb muscles. Brain reflexes involve reflex center in brainstem; e.g., reflexes for eye movement.

Components of reflex arc are: SPINAL REFLEX Reflex : It is an automatic response to a stimuli that occurs without conscious effort Components of reflex arc are: Sensory Receptor Afferent pathway Center Efferent pathway Effector organ

Receptor responds to the stimulus and produces action potential (AP) REFLEX Receptor responds to the stimulus and produces action potential (AP) ↓ AP is taken by afferent pathway to the integrating center (usually CNS) Integrating center processes all the information and makes the decision about the response Instruction from integrated center are sent via efferent pathway to the Effector organ (muscle or gland)

Classification of reflexes Monosynaptic reflex Also called as stretch reflex or tendon jerk e.g. Bicep jerk ,triceps jerk, knee jerk, ankle jerk Polysynaptic reflex e.g. Withdrawal reflex, Abdominal reflex, Plantar reflex Visceral reflex e.g. Micturation, defecation reflex Jendressik Phenomenon

Stretch Reflex [monosynaptic] Sudden stretch to a muscle leads to contraction of that muscle is known as stretch reflex. Stretch Reflex is a basic spinal reflex. Example Knee jerk The 5 components of Stretch Reflex Sensory receptor – Muscle Spindle in skeletal muscle Afferent pathway – 1a fibers Center – spinal cord Efferent fibers – α-motor neuron Effector organ – skeletal muscle contraction

Types of muscle fibers Extrafusal muscle fibers Takes part in muscle contraction Supplied by α - motor neuron Intrafusal muscle fibers Also called as Muscle Spindle Receptors for stretch reflex supplied by γ - motor neuron

Muscle spindle These are the receptors of the muscles Consist of two types of fibers Nuclear bag fiber Nuclear chain fiber Sensory innervations of muscle spindle Ia – nuclear bag & nuclear chain fibers II – nuclear chain fibers Motor supply – γ motor neuron Detect both dynamic and static changes in muscles

Muscle spindle

Types of Nerve Fibers

α - motor neuron γ - motor neuron Supply Extrafusal fibers Types of motor neuron α - motor neuron Supply Extrafusal fibers γ - motor neuron Supply Intrafusal fibers

Stretch Reflex (Knee Jerk) KNEE JERK – MONOSYNAPTIC REFLEX

Stretch Reflex (Knee Jerk)

Polysynaptic Reflex In polysynaptic reflex, there are many synapses between inter-neurons in the reflex pathway Eg. Withdrawal reflex: When a person touches a hot stove or pin prick, a withdrawal reflex occurs from the painful stimulus Receptor- pain and Temperature Receptors stimulated Afferent impulse travels via Sensory nerves Efferent fibers to Biceps to contract ( flexion of arm ) and inhibitory neuron stimulated to inhibit contraction of Triceps. This type of connection causing stimulation of nerve supply to one muscle and simultaneous inhibition of the nerve to its Antagonistic muscle is known as RECIPROCAL INNERVATION.

Withdrawal reflex – Polysynaptic reflex

Poly synaptic reflex Important Afferent neuron stimulate also interneuron that carry signals to the Brain ( Ascending tracts in the spinal cord ) Therefore Brain can modify the with drawl reflex. How? By sending impulses via descending pathways to the efferent motor neuron supplying the involved muscles and prevent contracting of biceps inspite of painful stimulus e.g. pin prick

REFLEXES Jendressik Phenomenon (Reinforcement)

Jendressik Phenomenon (Reinforcement) Superficial Reflexes

Plantar Reflex

Dermatomes of Spinal roots and divisions of trigeminal nerve (V1 ,V2, V3)

Cranial Nerve Spinal Cord Reflexes

Polysynaptic Reflexes Golgi tendon Reflex Crossed Extensor Reflex

Golgi Tendon Organs Golgi tendon organ is present in the tendon and is Composed of Nerve fiber endings that wind between collagen fibers inside connective tissue capsule that make up the tendon.

Golgi Tendon Reflex If muscle is stretched by striking the tendon Free nerve endings are pinched in the tendon and they fire Activation of Golgi tendon organs Inhibits alpha motor neurons and decreases muscle contraction

Fig 13.6 – Muscle reflexes Silverthorn 2nd Ed

Crossed Extensor Reflex Extension of opposite limb Flexion & withdrawal of stimulated limb Painful Stimuli

Crossed Extensor Reflex

Crossed Extensor Reflex

Clinical Importance of reflexes To test the integrity of reflex arc. Localization of neurological lesion. Deep tendon reflexes are absent in lower motor neuron lesion. They become exaggerated in upper motor neuron lesions. Pendular jerks are observed in cerebellar lesions. Identifying the type of lesion. Monitoring the progress of neurological deficit.

References Human physiology by Lauralee Sherwood, 8th edition Text Book Of Physiology by Guyton & Hall, 11th edition

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