The Somatic Nervous System

Expected Learning Outcomes

Understand the properties of various receptors involved in sensation.
Identify different somatosensory tracts and their functions.
Provide an overview of the concept of pain.
Identify somatomotor tracts responsible for movement.
Understand the mechanisms and processes involved in voluntary movement.

Components of the Somatic Nervous System

Somatic Sensory Systems: Handle sensory information from the body.
Somatic Motor Systems: Control voluntary muscle movements.
Special Senses: Include more complex sensory experiences such as vision and hearing.

Classification of Sensations

Sensory modalities can be broadly classified into the following categories:

Normal Somatic Sensation: Touch, pressure, temperature, pain, and vibration.
Special Senses: Taste, smell, vision, hearing, and equilibrium.
Chemical Sensations: Hunger, thirst, and fatigue.

Understanding the Somatic Sensory System

Somatic sensory information reaches the Central Nervous System (CNS) and travels via ascending pathways to the brain.
Visceral reflex sensory information, such as blood pressure, is integrated in the brainstem or spinal cord and does not always reach conscious perception.
Four major groups of receptors in the somatic sensory system:

Chemoreceptors: Respond to chemical ligands, such as those involved in taste and smell.
Mechanoreceptors: Detect mechanical energy, such as in hearing and touch.
Thermoreceptors: Sense changes in temperature.
Photoreceptors: Detect light, forming the basis of vision.

Components of Sensation

The components of sensation include:

Receptors: Monitor stimuli relevant to sensory perception.
Organ/System Involved: Specific bodily structures associated with the sensation.
Type of Stimulus Detected: Different stimuli like light, sound, pressure etc.
Afferent and Efferent Pathways: Pathways that carry information to and from the brain.
Cortical Regions Involved: Specific areas of brain responsible for processing different sensations.

Receptor Mechanisms

Receptors have specific mechanisms for receiving stimuli:

Every receptor features a low threshold for detection, meaning they are highly sensitive to their respective stimuli (e.g., light for photoreceptors).
Receptors do not interpret stimuli; they only receive environmental information and convert it into action potentials for processing in the brain.
Types of receptors in greater detail:

Chemoreceptors: Example - receptors for taste and olfactory (smell).
Photoreceptors: Example - rods and cones in the retina that respond to light intensity.
Mechanoreceptors: Example - receptors in the skin that respond to different types of pressure and vibration.

Thresholds in Sensation

Understanding stimulus and threshold is critical for sensory perception:

Stimulative conditions must reach a certain threshold level for sensory nerves to activate.
This threshold varies depending on the type of sensory receptor and environmental conditions.

Somatosensory Areas in the Cerebrum

The sensory areas of the brain's cerebrum have specific cortical representations:

The size of the representation area in the brain correlates with the sensitivity of the peripheral structure.

Somatic Senses

Major somatic sensations include:

Touch
Temperature
Pain
Itch
Proprioception

Somatosensory Pathways

Key pathways in the somatosensory system include:

Posterior Column System: Also known as spinocortical or Goll and Burdach tracts, it conveys fine touch, pressure, and proprioception; crossover occurs in the medulla oblongata.
Anterolateral System: Also known as spinothalamic tracts, it transmits temperature and pain sensations; crosses the midline immediately upon entering the spinal cord.
Trigeminal Pathway: Manages sensory information from the head region; crosses over in the brainstem.

General Pain Overview

Somatic Pain: Resulting from superficial injuries leads to vasodilation, increased blood flow (rubor), and heightened sensitivity (hyperalgesia).
Deep Pain: Described as dull and diffuse, distinct from somatic pain.
Visceral Pain: Diffuse and dull; often accompanied by autonomic responses and can be referred to other areas of the body.
Projected Pain: Pain experienced in other areas due to a pinched nerve where the sensory receptor is located.

Somatic Sensory Fibers

There are two primary sensory fiber types:

A-delta Fibers: Responsible for transmission of sharp, localized pain.
C Fibers: Associated with dull pain and itch sensation activated by histamine. Associated with skin conditions that contribute to sensation perceptions.

Referred Pain

Referred pain occurs when:

Pain from a specific organ, like the heart in case of cardiac ischemia, is felt in areas like the neck and left shoulder. This happens due to convergence of multiple primary sensory neurons on a single ascending tract.

Chronic Pain

Chronic pain is defined as:

Long-term pain or damage to the nervous system, often exceeding nociceptor activation thresholds.

Phantom Pain

Phantom pain is a phenomenon observed when:

A limb has been lost, leading to sensations experienced in the previously occupied space. This occurs due to reorganization of the somatosensory cortex and may lead to atypical sensory experiences.

Somatomotor System

The somatomotor system is crucial for:

Central Nervous System (CNS) regulation of muscle activity, both for posture and movement.
Representation of motor areas in the cortex, demonstrated through the concept of the motor homunculus.
The cerebellum’s role in coordinating and timing movements, receiving information from peripheral proprioceptors (spinocerebellar tracts) and the vestibular system.
Dysfunction in this system can lead to impairments such as ataxia, tremors, or hypotonia.

Corticospinal System

The corticospinal system is described as:

Direct Pathways: Responsible for fine voluntary and skilled movements primarily affecting cranial nerves' muscles; includes the corticospinal tracts where axons from upper motor neurons cross in the medulla to form pyramids, synapsing with lower motor neurons.
Corticobulbar Tracts: Connect motor areas in the head, with some fibers crossing midline while others remain ipsilateral, synapsing at the cranial motor nuclei.
Extracorticospinal System: Comprises indirect pathways that facilitate gross movements via multisynaptic connections.

Summary: Voluntary Movement

Understanding voluntary movement encompasses recognizing the integration of sensory and motor pathways to facilitate conscious control of muscle activity. This involves associations across various systems in the body, underscoring the complexity of human motor function.

Understanding the Somatic Nervous System

Understanding the Somatic Nervous System