ch15: neural intergration

Chapter 15: Neural Integration

Neural integration involves the processing and interpretation of sensory information and the execution of motor responses. Understanding neural pathways, receptors, and their functions is crucial in neurophysiology.

Terminology

Receptor

A receptor is defined as a specialized structure that detects a stimulus from the environment and initiates a neural response.

Key Characteristics:

  • Punctate distribution: This term refers to the uneven distribution of receptors throughout the body. Different areas have varying densities of receptors, impacting sensitivity to stimuli in those regions.

  • Adaptation: This is the process where the sensitivity of receptors decreases when exposed to constant stimulus over time.

Punctate Distribution

Punctate distribution shows how receptors are not uniformly spread but vary in density across different body parts. Areas with a high concentration of receptors include:

  • Foot

  • Toes

  • Genitals

  • Hip

  • Leg

  • Trunk

  • Neck

  • Head

  • Arm

  • Shoulder

  • Elbow

  • Forearm

  • Wrist

  • Hand (including fingers: little, ring, middle, index, thumb)

  • Eye

  • Nose

  • Face

  • Teeth, Lips, Gums, and Jaw

  • Tongue

  • Pharynx
    Each of these regions has varying sensitivity based on receptor distribution.

Adaptation

Adaptation refers to the progressive decline in sensitivity to a constant stimulus. It has various implications:

  • Mechanism: Over time, receptors become less responsive to a continuous stimulus, eventually failing to generate action potentials.

  • Examples: Subjects experience adaptation under prolonged scenarios such as sustained pressure on the skin or persistent odors.

  • Nociceptors (pain receptors) demonstrate minimal adaptation, continuously signaling painful stimuli, which is vital for protective responses.

Types of Receptors

Receptors can be classified based on location and the type of stimulus they detect.

By Location

  1. Exteroceptors: Located in the skin, they respond primarily to external stimuli such as touch or pressure.

  2. Interoceptors: Found in internal organs, they provide sensations such as pain and pressure from these organs, indicating dysfunction.

  3. Proprioceptors: These are responsible for sensing body position, including muscle spindles (detecting muscle stretch) and Golgi Tendon Organs (detecting muscle contraction).

By Stimulus

Different receptor types react to various stimuli:

  • Nociceptors: Pain receptors responding to damaged tissue.

  • Thermoreceptors: Detect changes in temperature.

  • Mechanoreceptors: Respond to mechanical changes, which can include tactile sensations.
      - Subtypes include tactile receptors such as:
        - Root hair plexus
        - Merkel discs
        - Meissner's corpuscles
        - Lamellated (Pacinian) corpuscles
        - Baroreceptors
        - Proprioceptors found in muscles and internal ear

  • Chemoreceptors: Detect chemical concentration changes.

  • Photoreceptors: Respond to light stimuli in the eyes, including rods and cones.

Nociceptors

Nociceptors are specialized receptors for detecting pain. They have the following characteristics:

  • Comprised of free nerve endings.

  • Function as tonic receptors, adapting slowly.

  • Sensitive to:
      - Extreme temperatures
      - Chemical changes (e.g. injury)
      - Mechanical damage (cuts).

Thermoreceptors

Thermoreceptors are responsible for detecting temperature variations and have:

  • Free nerve endings that operate as phasic receptors, adapting quickly.

  • Locations include:
      - Dermis
      - Skeletal muscles
      - Liver
      - Hypothalamus.

Mechanoreceptors

Mechanoreceptors respond to various mechanical stimuli:

Tactile Receptors

  • Hair root plexus: Fast adapting receptors.

  • Merkel discs: Tonic receptors that respond slowly.

  • Meissner disks: Located in dermal papillae, they are fast adapting and react to light touch and vibration.

  • Pacinian corpuscles: Situated near the hypodermis, responsive to deep pressure and high-frequency vibration, also fast adapting.

Baroreceptors

  • These receptors monitor changes in pressure, especially in major blood vessels, and are characterized as quickly adapting.

Proprioceptive Mechanoreceptors

  • Muscle Spindles: Detect stretching in muscles.

  • Golgi Tendon Organs: Monitor muscles’ tension levels.

  • Joint Capsule Receptors: Respond to tension and movement at joints.

  • Internal Ear Receptors: Involved in detecting head position.

Chemoreceptors and Photoreceptors

  • Chemoreceptors: Detect changes in chemical concentrations in surrounding fluids, such as:
      - Taste buds
      - Olfactory receptors for smell.

  • Photoreceptors: Detect light photons, primarily through rods and cones in the retina of the eyes.

Referred Pain

Referred pain occurs when the sensation of pain is felt at a site other than the source of the injury or damage.

  • Example: Pain from heart tissue may be felt in the arm.

  • Herniated discs can lead to nerve compression, producing pain in the leg, demonstrating the complex nature of pain perception and neural pathways.

Spinal Cord Tracts

Neural signals in the spinal cord travel via distinct tracts categorized into:

  • Motor tracts: Responsible for transmitting motor signals from the brain to muscles.

  • Sensory tracts: Conduct sensory signals from the body to the brain.

Sensory Pathways

Spinothalamic Tract

  • Comprises pathways carrying sensory information to the thalamus and includes:
      - Dorsal columns pathway: Involving discriminative touch and proprioception sensations.
      - Pathway includes crucial components:
        - Post-central gyrus
        - Third order neuron
        - Second order neuron
        - Medial lemniscus
        - Nucleus gracilis and nucleus cuneatus in the medulla
        - Fasciculus gracilis and fasciculus cuneatus
        - Dorsal root ganglion.
      - Lateral spinothalamic tract: Sends pain and temperature sensations from the right side of the body.

Anterior Spinothalamic Tract (Crude Touch and Pressure)

  • Receptor: Touch receptor that transforms stimulus into an action potential.

  • 1st neuron: Afferent neuron with its cell body in the dorsal root ganglion (DRG) which synapses in the posterior gray horn.

  • 2nd neuron: Cell body in posterior gray horn, crosses over to the anterior spinothalamic tract on the opposite side and projects its axon to the thalamus.

  • 3rd Neuron: Located in the thalamus, it extends to the postcentral gyrus.

Lateral Spinothalamic Tract (Pain & Temperature)

  • Receptor: Combines nociceptors and thermoreceptors to send signals about pain and temperature.

  • 1st Neuron: Afferent neuron with its cell body in DRG, synapsing in the posterior gray horn.

  • 2nd Neuron: Crosses to the lateral spinothalamic tract from the posterior gray horn.

  • 3rd Neuron: Cell body in the thalamus, projecting to the postcentral gyrus.

Posterior Columns (Discriminative Touch, Vibration & Proprioception)

  • Receptor: Meissner’s Corpuscles detect touch.

  • 1st Neuron: Afferent neuron begins in DRG; its axon extends into posterior gray horn without synapsing, continuing to the medulla oblongata.

  • 2nd Neuron: Cell body in the medulla, crossing over to the opposite side to ascend to the thalamus.

  • 3rd Neuron: Located in the thalamus, extending to the postcentral gyrus.

Spinocerebellar Tract

  • Primarily involves proprioceptors, including Golgi tendon organs, muscle spindles, and joint capsules, allowing feedback to the cerebellum about body position and movement.

Motor Pathways

Motor pathways are classified into two major categories:

  • Corticospinal (Pyramidal) tracts: Responsible for voluntary motor control.

  • Indirect Pathway (Extrapyramidal): Influences involuntary motor patterns

Direct Motor Pathway

The direct motor pathway includes:

  • Lateral Corticospinal Tract

  • Anterior Corticospinal Tract

  • Originates from the pre-central gyrus and travels down through the cerebral peduncle, midbrain, and pyramids in the medulla where decussation occurs (crossing over).

  • Key components include upper motor neurons and lower motor neurons.

Upper Motor Neuron (UMN)

  • The UMN originates in the precentral gyrus, with axons extending to the anterior gray horn.

  • About 80% of axons cross over at the decussation of the pyramids in the medulla, subsequently forming the lateral corticospinal tract. The remaining 20% continue along the anterior corticospinal tract and cross at the spinal cord itself.

Lower Motor Neuron (LMN)

  • The LMN consists of motor neurons residing in the anterior gray horn, whose axon extends to innervate skeletal muscles.

Indirect Motor Pathway

The indirect motor pathways, also known as extrapyramidal pathways, do not initiate in the precentral gyrus:

  • These pathways begin in the basal nuclei or reticular formation and influence the action of upper motor neurons (UMN).

  • Functions attributed to indirect pathways include starting or stopping movements and regulating muscle tone, ensuring that movements align with intended actions.