Question

1. Be able to draw and describe a monosynaptic reflex, a polysynaptic reflex, and a cross-extensor reflex. What is different about each of these compared to the others?

2. Describe the regions of the spinal cord and how the neurons enter and leave the cord. Be able to describe the neurons in incoming sensory pathways (first, second and third-order neurons) and the neurons used in outgoing motor pathways (upper and lower motor neurons). Anatomically where do you find each of these

Answer 1

1.

Monosynaptic reflexes:-

Reflexes in which only one synapse is present between the afferent and efferent neurons.eg, All the stretch reflexes(biceps,triceps, or knee jerks).

Dorsal root Interneuron releasing inhibitory mediator IPSP Ib fiber from Motor Golgi tendon neuron la fiber organ from muscle Ventral root Motor end-plate on extrafusal fiber Fig. 10.4.5 Pathway responsible for the stretch reflex () and inverse stretch reflex ()

polysynaptic reflexes:-

Reflexes in which one or more interneurons are present between the afferent and efferent neurons.eg, Withdrawal reflexes,gross flexor reflex, gross extensor reflex.

Reverberating circuits Afferent (sensory) neuron Interneurons Efferent (motor) neuron Fig. 10.4.6 Polysynaptic reflex pathway showing connections between afferent and efferent neurons in the spinal cord

cross-extensor reflex:-

• withdrawal reflex:-it is a polysynaptic reflex that occurs in response to a noxious or painfull stimulus.Therefore ,when a noxious stimulus is applied to a limb it leads to:
  • contraction of flexor group of muscles and inhibition of extensor muscles,therefore limb stimulated is flexed and withdrawn from the stimulus.This is called flexor response or flexor reflex.
  • with a stronger stimulus,in addition to flexor reflex there occurs extension of the opposite limb also.This is called crossed extensor reflex /response.

Stimulus Skin Extensor (+) Inhibitory Flexor interneurons (+) Flexor Extensor (-) (-) (A) Ipsilateral limb (Flexor reflex) (B) Contralateral limb Extensor reflex) Fig. 10.4.7 Pathway for flexor reflex (A) and crossed extensor reflex (B) {(+): Excitation; (-): Inhibition)

2.

A cross section of the spinal cord shows the following structures:-

DORSAL (POSTERIOR) Dorsal (posterior) white Posterior median column septum Dorsal (posterior) horn Dorsal grey Dorsal (posterior) root column Dorsal root Lateral white ganglion column Intermediolateral Mixed Flexors Trunk horn nerve Extremities Posterior grey commissure Extensors Central canal Ventral (anterior) root Ventral (anterior) horn White matter Anterior grey Anterior white column commissure Anterior median fissure VENTRAL (ANTERIOR) Fig. 10.1.3 Cross-section of the spinal cord

• A prominent fissure anteriorly,called anterior median fissure and a less prominent fissure(or septum) posteriorly,called posterior median fissure(or septum);between these fissures lies the central canal.
• The grey matter consisting of nerve cells from an H-shaped figure long and narrow dorsal(posterior)horns; and thick,broad ventral (anterior) horns.The horizontal stem of the H constitutes the grey commissure(anterior and posterior),which surrounds the central canal.
• The ventral or anterior horn contains the cell bodies of $\alpha and\gamma$ motor neurons;their axons (nerve fibers) pass out in the ventral(or anterior) roots.These nerve fibers are purely motor in function.
• The dorsal or posterior horn recieves the fibers of the posterior roots which are purely sensory in function.Their cell bodies are found in the dorsal (posterior) root ganglion which is a swelling on the posterior root.These ganglion cells are bipolar,their axons connect both with peripheral structures and with structures in the dorsal horn.
• surrounding the grey matter is the white matter,consisting of large numbers of ascending and descending axons cut across.The white matter is divided into three white columns:
  • anterior white column
  • lateral white column
  • dorsal white column
• The sensory and motor fibers join to form a mixed nerve which comes out from the vertebral canal as a peripheral spinal nerve via the intervertebral foramen.

Ascending sensory pathway are organised in three neuronal chain.i.e,

-First order neuron

-secons order neuron

-Thirs order neuron.

• first order neuron
  • cell body in posterior root ganglion
  • peripheral process connects with sensory receptor ending
  • central process enter the spinal cord through the posterior root
  • synapse with second order neuron in spinal gray matter.
• second order neuron
  • ​​​​​​​cell body in posterior gray column of spinal cord
  • axon crosses the midline (decussate)
  • ascend &synapse with third order neuron in VPL nucleus of thalamus
• Third order neuron
  • ​​​​​​​cell body in the thalamus
  • give rise to projection fibres to the cerebral cortex,postcentral gyrus(sensory area)

Descending tract

• lower motor neuron

​​​​​​​motorr neurons that innervate the voluntary muscles

• in anterior gray column of spinal cord/
• motor nuclei of brainstem

-innervates skeletal muscles

form final common pathway

• Upper motor neurons(UMN)
  • ​​​​​​​The descending supraspinal pathways that influence the activity of the LMN
  • contol voluntary motor activity
  • maintenance of posture&equilibrium
  • control of muscle tone and reflex activity

generally exerts their effect

• on groups of muscles(not on one specific muscle)
• reciprocally on agonist and antagonist muscle group.

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