Neurophysiology of Sensation and Reflex Arcs

The Nervous System

• The nervous system comprises the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves).

• It controls intelligence, learning, memory, movement, sensations, and basic body functions via the autonomic nervous system.

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Functional Divisions of the Nervous System

• The nervous system has three main divisions:

  • Sensory division.

  • Center (brain and spinal cord).

  • Motor division: voluntary (somatic) and involuntary (autonomic).

• The neuron is the nervous system's building block, composed of the soma, dendrites, and axons.

Reflex Arc

• The reflex arc is a basic structural and functional unit that allows involuntary and automatic reactions to stimuli.

• Components of the reflex arc:

  • Receptor: A specialized structure at the beginning of a sensory neuron that receives the stimulus.

  • Afferent Neuron: The sensory neuron that transmits sensory information from the receptor to the spinal cord.

  • Center: Located in the brain or spinal cord; relays information from the sensory afferent neuron to the motor efferent neuron.

  • Efferent Neuron: The 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.

• Classification of receptors based on the type of energy needed to stimulate them:

  • Mechanoreceptors: Respond to mechanical energy.

    • Examples: Pacinian corpuscles, touch receptors, pressure receptors, proprioceptors (detect movement and position of joints), and vibration receptors.

  • 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 or injurious stimuli.

  • Chemoreceptors: Respond to chemical changes.

    • Examples: chemosensitive cells in the olfactory mucous membrane, taste buds, osmoreceptors and glucoreceptors in the hypothalamus, and receptors sensitive to blood gases.

Somatic Sensory Afferents

• Each receptor is typically a peripheral beginning of an afferent nerve, which carries information from receptors to the CNS (brain and spinal cord) as a nerve impulse.

• These nerves (somatic sensory nerves) are either peripheral branches of the posterior root ganglion or sensory fibers of the cranial nerves.

Classification of Somatic Sensory Nerve Fibers

• Based on diameter, they are classified into four types (I, II, III, and IV) from largest to smallest.

• Based on velocity of conduction, they are classified into three types (A, B, and C). Group A is further subdivided into A$\alpha$, A$\beta$, and A$\delta$.

• Each type of sensation has its own importance; sensations are carried to the brain at different speeds.

  • Proprioceptive sensations are carried by A$\alpha$ fibers.

  • Fine touch, pressure, stereognosis, and vibration are carried by A$\beta$ fibers.

  • Fast pain is carried by A$\delta$ fibers.

  • Slow pain is carried by C fibers.

Sensory Pathways

• Sensory signals are transmitted to the brain via two major pathways: the dorsal column and the spinothalamic (ventrolateral) tract.

• Each pathway consists of 3 order neurons, from the afferent fiber to the destination in the brain, and carries specific sensations.

1. The Dorsal Column

• Unimodal, carrying mechanoreceptive sensations.

  • Fine touch and pressure.

  • Vibration.

  • Stereognosis.

  • Position.

2. The Spinothalamic (Ventrolateral) Tract

• Polymodal, carrying different types of sensations via two divisions.

  • Lateral Spinothalamic Tract:

    • Pain.

    • Temperature.

  • Ventral Spinothalamic Tract:

    • Crude touch and pressure.

    • Tickle and itch.

Details of the Dorsal Column Pathway

• Fibers are thick, myelinated type A$\alpha$ and A$\beta$ fibers from specialized mechanoreceptors.

• Paths of the three order neurons:

  • First Order Neurons:

    • Located in dorsal root ganglia (DRG).

    • Peripheral branches are thick, myelinated (A$\alpha$ and A$\beta$) afferent fibers.

    • Central branches enter the spinal cord via the dorsal root and ascend 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:

    • Located in gracile and cuneate nuclei in the medulla.

    • Axons cross to the opposite side and ascend to synapse with 3rd order neurons in the thalamus.

  • Third Order Neurons:

    • Located in nuclei of the thalamus.

    • Axons ascend in the sensory radiation to reach the somatic sensory cortex.