13 Somatic Nervous System

Somatic Nervous System Overview

Chapter 13

Page 1: Definitions

  • Somatic Nervous System: Responsible for general sensations and skeletal muscles.

  • Autonomic Nervous System: Involves smooth muscle, cardiac tissue, and glands.

Page 2: Signal Transmission

  • Diagrams depict neural pathways illustrating signal transmission paths for simple actions.

  • Afferent (Sensory): Nerves that carry sensory information to the central nervous system (CNS).

  • Efferent (Motor): Nerves that carry motor commands from the CNS to effectors.

  • Different categories of receptors:

    • Special receptors: Specialized in detecting specific stimuli.

    • Somatic receptors: Detect external stimuli affecting the body surface (e.g. touch).

    • Visceral receptors: Respond to stimuli within the body (e.g. hunger).

  • Data flow: Receptors → effectors (Skeletal muscle, smooth muscle, glands) → information processing (Somatic and Autonomic pathways)

Page 3: Summary

  • Overview of two systems:

    • Somatic Nervous System: Handles general sensations and skeletal muscle control.

    • Autonomic Nervous System: Manages involuntary responses like those of smooth muscles and glands.

Page 7-9: Anatomy of the Spinal Nerves

  • General Anatomy:

    • Root (Bundle of Axons): Connects spinal cord to spinal nerve.

    • Ventral Root: Contains motor fibers.

    • Dorsal Root: Contains sensory fibers.

    • Dorsal Root Ganglion: Cluster of sensory cell bodies located in the dorsal root.

Page 11: Sensation vs. Perception

  • Sensation: Arriving information from receptors.

  • Perception: Conscious awareness of sensations; the brain's interpretation of sensory information.

  • Importance: The distinction between sensation and perception underlines the complexity of how stimuli are processed and understood by the brain.

Page 12-13: Sensory Receptors

  • Receptors Defined:

    • Structures that detect stimuli; can be simple (neuron dendrites) or complex (sense organs).

    • Transducers: Convert various stimuli (e.g. mechanical, thermal) into nerve signals.

  • General Senses vs Special Senses:

    • General: Pain, temperature, pressure, chemicals; found throughout the body and are generally simpler in structure.

    • Special: Located in the head; more complex structures (e.g. eyes, ears).

Page 14-15: Kinds of General Sensory Receptors

  • Free Nerve Endings: Detect pain, temperature.

  • Tactile Cells: Linked to light touch.

  • Tactile Discs: Nerve endings sensitive to touch.

  • Hair Receptors: Detect movement of hair.

  • Encapsulated Receptors: Include Tactile Corpuscles, End Bulbs, Bulbous Corpuscles, Lamellar Corpuscles, Muscle Spindles, and Tendon Organs.

Page 16: Classification of Receptors

  1. By Modality (type of stimulus):

    • Chemoreceptors: Detect chemical stimuli.

    • Thermoreceptors: Detect temperature.

    • Mechanoreceptors: Detect mechanical deformation.

    • Photoreceptors: Detect light.

    • Nociceptors: Detect pain.

  2. By Origin:

    • Interoceptors: Detect internal stimuli.

    • Proprioceptors: Sense body position and movement.

    • Exteroceptors: Detect external stimuli.

Page 17-20: Properties of Receptors

  1. Specificity: Individual receptors are sensitive to specific types of stimuli.

  2. Adaptation: The process where sensory receptors become less sensitive with constant stimulation.

  3. Receptive Fields: The area monitored by a single receptor cell; influences the ability to discriminate among stimuli based on area size.

  4. Phasic vs. Tonic Receptors:

    • Phasic Receptors: Normally inactive; activate and quickly adapt.

    • Tonic Receptors: Always active; adapt slowly to stimuli, providing continuous information.

Page 21-30: Types of General Senses

  • Nociceptors (Pain):

    • Free nerve endings with large receptive fields sensitive to extreme temperatures, mechanical damage, and chemicals.

    • Many nociceptors are tonic and show no adaptation; CNS can modulate pain perception through endorphins.

  • Thermoreceptors:

    • Free nerve endings sensitive to temperature changes, with cold and hot receptors being structurally similar.

  • Mechanoreceptors:

    • Sensitive to mechanical distortion (e.g. touch, pressure).

    • Types include:

    • Tactile: Involved in touch and vibration.

    • Baroreceptors: Detect pressure changes internally (e.g. in the circulatory system).

    • Proprioceptors: Monitor tension and joint position, primarily processed subconsciously.

  • Chemoreceptors:

    • Autonomic responses related to respiration and cardiovascular functions, detecting changes in pH, CO2, and O2.

Page 33-36: Spinal Cord Anatomy

  • Location: Extends from the brain stem and exits at the foramen magnum in the occipital bone, ending at L1.

  • Spinal Nerves: 31 pairs, all emerging from the spinal cord, terminating in a structure called the cauda equina.

  • Meninges: Protective layers surrounding the spinal cord:

    • Dura mater

    • Arachnoid mater

    • Pia mater

Page 37: White and Gray Matter in Spinal Cord

  • Gray Matter: Contains cell bodies of neurons and unmyelinated axons, organized into horns:

    • Anterior (Ventral) Horn

    • Posterior (Dorsal) Horn

  • White Matter: Composed of myelinated axons; functions in sensory (ascending) and motor (descending) signal transmission.

Pages 38-44: Ascending Sensory Tracts

  • General Organization:

    • A series of neurons relay sensory information:

      • 1st Order Neuron: Sensory neuron with cell body in dorsal root ganglia.

      • 2nd Order Neuron: Located in the CNS, may decussate (cross over).

      • 3rd Order Neuron: Synapses in the thalamus and projects to the cerebral cortex for awareness.

  • Major Sensory Tracts:

    • Cuneate Fasciculus: Relays information from the upper trunk and limbs, transmitting deep touch, visceral pain, and proprioception.

    • Spinothalamic Tract: Carries sensations of light touch, pain, temperature, tickle, itch, and pressure to the cerebral cortex.

    • Spinocerebellar Pathway: Proprioceptive information sent to the cerebellum regarding muscle and joint position.

Pages 45-54: Descending Motor Tracts

  • Primary Motor Cortex (PMC):

    • Site of conscious movement control, with size correlated to the amount of motor control dedicated to specific muscles.

  • Motor Neurons:

    • Upper Motor Neuron (UMN): Cell body in higher centers, facilitates/inhibits movement.

    • Lower Motor Neuron (LMN): Cell body in brain stem or spinal cord, directly innervates muscles.

  • Major Motor Tracts:

    • Corticospinal Tracts: Facilitate rapid and conscious control of limb movement.

    • Tectospinal Tract: Involved in reflexive head movement in response to auditory stimuli.

    • Reticulospinal Tracts: Control posture and balance, modulating pain transmission to the brain.

    • Vestibulospinal Tract: Associated with equilibrium and coordination across different muscle groups.

Page 55-59: Anatomy of a Spinal Nerve

  • Structure of a Nerve: Composed of axon bundles, surrounded by connective tissue (epineurium, perineurium, endoneurium).

  • Nerve Plexuses: Formed by the anterior and posterior rami, innervating specific muscle groups in the body.

    • Plexuses present: Cervical, Brachial, Lumbar, Sacral, Coccygeal.

  • Reflex Action: Contrasted with voluntary control, highlighting innate vs. learned responses.