Neurophysiology: Reflex Arc and Sensory Pathways

• By the end of this lecture, participants should be able to:
  • Explain the concept of reflex arc in neurophysiology.
  • Identify sensory receptors.
  • Define different sensory pathways.

The Nervous System

• The nervous system is made up of the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves).
• The nervous system controls intelligence, learning, memory, movement, sensations, and basic body functions via the autonomic nervous system.
• The nervous system can be functionally divided into three main divisions:
  • Sensory division
  • Center (brain and spinal cord)
  • Motor division (voluntary and involuntary)
• The neuron is the building block of the nervous system, formed of soma, dendrites, and axons.

The Reflex Arc

• Reflex occurs in response to a pain receptor stimulation.
• The reflex arc is the basic structural and functional unit of the nervous system.
• The reflex arc allows the body to react involuntarily and automatically to internal and external stimuli.
• The reflex arc consists of:
  • Receptor: Specialized structure present at the beginning of a sensory neuron.
  • Afferent neuron: Sensory neuron that transmits information from the receptor to the spinal cord.
  • Center: Present in the brain or spinal cord, relays information from the sensory afferent neuron to the motor efferent neuron.
  • Efferent neuron: Motor neuron that transmits instructions from the CNS to effector organs.
  • Effector organ: Smooth, skeletal, or cardiac muscles and glands.

Sensory Nervous System: Sensory Receptors

• Sensory receptors are specialized structures that produce action potentials and transmit them to sensory afferent fibers, which conduct them to the CNS.
• Receptors are classified according to the specific type of energy needed to stimulate them.
  • Mechanoreceptors: Respond to mechanical forms of energy.
    • Include touch receptors, pressure receptors, and proprioceptors in muscles, ligaments, and joints that detect movement, joint position, and vibration.
  • Thermoreceptors: Detect cold and warm changes in the environment.
  • Photoreceptors: Rods and cones in the retina.
  • Nociceptors (pain receptors): Free nerve endings that respond to noxious stimuli.
  • Chemoreceptors: Respond to chemical changes in the environment.
    • These are present as chemosensitive cells in the olfactory mucous membrane, taste buds, osmoreceptors in the hypothalamus, and glucoreceptors of the hypothalamus (sensitive to blood gases).

Somatic Sensory Afferents

• Each receptor is the peripheral beginning of an afferent nerve, carrying information from the receptors to CNS (brain & spinal cord) as a nerve impulse.
• Somatic sensory nerves are either peripheral branches of the posterior root ganglion or sensory fibers of the cranial nerves.

Classification and Characteristics of Somatic Sensory Nerve Fibers

• Classified into 4 types based on diameter, from largest to least thickness: I, II, III, and IV.
• Classified into 3 types based on velocity of conduction: A, B, and C.
  • Group A is further subdivided into Aα, Aβ, and Aδ.
• Each type of sensation has its own importance; some sensations must be carried to the brain very fast, for example, proprioceptive sensation carried by Aα fibers, while fine touch, pressure, stereognosis, and vibration carried by Aβ, while others may be carried more slowly.
  • Fast pain by Aδ fibers.
  • Slow pain by C fibers.

Sensory Pathways

• Sensory signals are transmitted to the brain via one of two major pathways:
  • The dorsal column.
  • The spinothalamic (ventrolateral) tract.
• Each pathway is composed of 3 order neurons that carries specific sensations from the afferent fiber to the brain.

The Dorsal Column

• The Dorsal Column shows unimodality as it carries mechanoreceptive sensations:
  • Fine touch and pressure
  • Vibration
  • Stereognosis
  • Position
• Fibers contributing to this pathway are thick myelinated type Aα and Aβ fibers, carrying sensations from specialized mechanoreceptors.
• The paths of the three order neurons are as follows:
  • FIRST ORDER NEURONS:
    • Present in dorsal root ganglia (DRG).
    • Peripheral branches of these neurons are thick myelinated (Αα and Aβ) afferent fibers.
    • Central branches enter the spinal cord via the dorsal root, ascend upwards in the ipsilateral (same) side dorsal column as the gracile and cuneate tracts to end in 2nd order neurons in the medulla.
  • SECOND ORDER NEURONS:
    • Present in gracile and cuneate nuclei in the medulla.
    • Axons of these nuclei cross to the opposite side, and then pass upwards to synapse with the 3rd order neurons in the thalamus.
  • THIRD ORDER NEURONS:
    • Present in nuclei of the thalamus.
    • Axons ascend in the sensory radiation to reach the somatic sensory cortex.

The Spinothalamic (Ventrolateral) Tract

• The Spinothalamic (Ventrolateral) Tract: shows polymodality as it carries different types of sensations in its two divisions:
  • The Lateral Spinothalamic Tract carries:
    • Pain
    • Temperature
  • The Ventral Spinothalamic Tract carries:
    • Crude touch and pressure
    • Tickle and itch