pathways

Nervous System Pathways and Function Overview

Sensory Pathways

  • Signal Receiving:
    • Sensory information is processed when an activation signal is sent.
    • Key structure: Dorsal Root
    • Dorsal Root Ganglion:
    • A cluster of neurons in the peripheral nervous system where sensory neurons send their axons.
    • Information travels from the sensory receptor (e.g., touching something sharp) to the dorsal root, proceeding to the Posterior Horn of the spinal cord.
Pathway of Sensory Information
  • Dorsal Root Ganglion: House sensory neurons.
  • Posterior Horn: Entry point into the spinal cord for sensory information.
  • Tracks: Describes the pathways that ascend to the brain.
  • Exiting the Spinal Cord:
    • Travels via the Anterior Horn to effectors (muscle or gland).
    • Types of pathways: could be gray or white matter; could utilize different neurotransfer processes.

Organization of the Spinal Cord

  • Columns and Tracts:
    • White Matter Tracts: Carry specific types of information.
    • Ascending Tracts: Carry sensory information from the spinal cord to the brain.
    • Descending Tracts: Carry motor commands from the brain back through the spinal cord.
Relationship of Types of Information to Pathways
  • Touch, Pressure, Proprioception, Vibration: Typically processed ipsilaterally in spinal cord; enter via the Dorsal Horn.
  • Pain and Temperature: Cross to the opposite side (decussation) shortly after entering the spinal cord.

Sensory Pathways Classification

  • First-order Neurons: Carry impulses from sensory receptors to the spinal cord.
  • Second-order Neurons: Carries impulses to the thalamus in the brain.
  • Third-order Neurons: Sends signals from the thalamus to the appropriate cortical area for perception.
  • Decussation: Most tracts cross over from one side of the body to the other at some point—typically at or before reaching the thalamus.
Types of Sensory Pathways

  • Posterior Column Pathway:

    • Function: Fine touch, pressure, and proprioception.
    • Pathway:
    1. First-order neuron synapses at the Dorsal Root Ganglion.
    2. Ascends to the medulla oblongata.
    3. Second-order neuron synapses at the medulla; information then relayed to the thalamus.
    4. Third-order neuron directs to the Primary Somatosensory Cortex.

  • Spinothalamic Pathways:

    • Anterior Spinothalamic:
    • Function: Crude touch and general pressure.
    • Pathway: First order neuron to dorsal horn, synapses, crosses over, and then ascends to the thalamus.
    • Lateral Spinothalamic:
    • Function: Pain and temperature sensations.
    • Pathway: Analogous to the anterior pathway but with specifics for pain and temperature detection.

  • Spinocerebellar Pathway:

    • Function: Proprioception (balance, coordination).
    • Does NOT go to the cerebrum; it goes directly to the cerebellum.

Motor Pathways

  • Define Effector: Typically refers to muscles or glands that carry out responses.
  • Two Main Types of Motor Pathways:
    • Somatic Motor Pathways: Voluntary control of skeletal muscle; uses one neuron from the CNS to the effector.
    • Autonomic Motor Pathways: Involuntary control primarily targets smooth muscles, cardiac muscles, and glands. Considered to use two neurons: preganglionic and postganglionic.
Structure of Somatic vs. Autonomic Pathways
  • Somatic Nervous System: One upper motor neuron synapsing directly onto the lower motor neuron which extends to skeletal muscle.
  • Autonomic Nervous System:
    • Two Types of Neurons:
    1. Preganglionic Neuron: Originates in the CNS, synapses in an autonomic ganglion.
    2. Postganglionic Neuron: Extends from the ganglion to the targeted tissue, which can either be sympathetic or parasympathetic.
Sympathetic vs. Parasympathetic Pathways
  • Sympathetic Pathways: Short preganglionic nerves (thoracolumbar origin); synapse closer to the spinal cord (sympathetic chain).
  • Parasympathetic Pathways: Long preganglionic nerves; synapse closer to target organs. Known as craniosacral division.
Neurotransmitters in Autonomic Division

  • Sympathetic:

    • Preganglionic neurons release acetylcholine at the ganglion (nicotinic receptors).
    • Postganglionic neurons primarily release norepinephrine at the effector (adrenergic receptors).

  • Parasympathetic:

    • Preganglionic neurons also release acetylcholine (nicotinic receptors).
    • Postganglionic neurons release acetylcholine as well (muscarinic receptors).

Higher Order Brain Functions

  • Experiences, behaviors and motor control derive from cerebral cortex interactions and communication across brain hemispheres.
  • Functional Specialization:
    • Left Hemisphere: Typically associated with language processing (Broca’s Area for speech production; Wernicke’s Area for comprehension).
    • Right Hemisphere: Spatial awareness, emotional connections, nonverbal processing, and artistic endeavors.
Reticular Activating System (RAS)
  • Component of the reticular formation regulating consciousness and alertness. Vital for responding to stimuli.
  • Impacts:
    • Levels of sleep and awareness (e.g. somnolence, stupor, coma).

Aging and the Nervous System

  • General aging effects can lead to slower cognitive processing, memory loss, and a reduction in neuronal number and function.
  • Notable changes:
    • Decreased blood flow to the brain, which correlates to reduced cognitive function.
  • Increased risk of accumulation of waste products in the brain, potentially leading to disorders such as dementia.

Summary

  • Emphasis on understanding pathways is vital as it underpins how the nervous system operates in response to sensory information and regulates motor functions.
  • Key Vocabulary:
    • Ascend and Descend in reference to within the nervous systems.
    • Understanding Decussation and its implications on sensory processing and motor command execution.
    • The distinction between the Autonomic Nervous System’s sympathetic and parasympathetic responses and their receptors.

Reflexes

  • Monosynaptic vs Polysynaptic Reflexes: Degree of complexity involving CNS in coordination.
  • Example of Enteric Nervous System: Functions of the digestive organs without direct CNS involvement.

Additional Notes

  • Expect pathway questions on assessments; grasp the general flow of sensory input and motor output through the mentioned tracts and structures.