Chapter 16: Somatic Nervous System

Sensation

  • Sensation: - Definition:

    • Conscious or subconscious awareness of changes in the external or internal environment.

    • Sensory impulses that reach the spinal cord (SC) may serve as input for spinal reflexes.

    • Sensory impulses that reach the brainstem elicit more complex reflexes, such as those from baroreceptors.

    • When sensory impulses reach the cortex, they allow us to become aware of the stimulus and precisely locate and identify sensations such as touch, pain, hearing, or taste.

  • Perception: - Definition:

    • Conscious interpretation of sensations.

    • Primarily a function of the cerebral cortex.

    • There is no perception of some sensory information because it never reaches the cortex.

Sensory Modalities

  • Sensory modality: - Definition:

    • Each unique type of sensation (e.g., touch, pain, vision, hearing) is referred to as a sensory modality.

    • A given sensory neuron conveys information for only ONE sensory modality.

  • General Senses: - Definition:

    • Refers to both somatic senses and visceral senses.

    • Somatic Senses:

      • Include:

        • Tactile sensations (e.g., touch, pressure, vibration, itch, tickle)

        • Thermal sensations (warm and cold)

        • Pain sensations

        • Proprioceptive sensations

    • Visceral Senses:

      • Provide information about conditions within internal organs (e.g., pressure, stretch, nausea, hunger, temperature).

  • Special Senses: - Include:

    • Smell, taste, hearing, and equilibrium/balance.

Process of Sensation

  • Step 1: Stimulation of a sensory receptor. - An appropriate stimulus must occur within the sensory receptor’s receptive field, which is the region of the body where stimulation activates the receptor and produces a response.

  • Step 2: Transduction of stimulus. - A sensory receptor transduces (converts) energy in a stimulus into a graded potential.

    • Graded potentials vary in amplitude depending on the strength of the stimulus and are not propagated.

  • Step 3: Generation of a nerve impulse. - When a graded potential in a sensory neuron reaches threshold, it triggers one or more nerve impulses that propagate toward the central nervous system (CNS).

    • Sensory neurons conducting impulses from the peripheral nervous system (PNS) to the CNS are called first-order neurons.

  • Step 4: Integration of sensory input. - A particular region of the CNS receives and integrates the sensory nerve impulses.

    • Conscious sensations or perceptions are integrated in the cerebral cortex.

Sensory Receptors

  • On a microscopic level, sensory receptors may be classified into:

    1. Free nerve endings of first-order sensory neurons.

    2. Encapsulated free nerve endings of first-order sensory neurons.

    3. Separate cells that synapse with first-order sensory neurons.

    • Receptors for:

      • Pain, temperature, tickle, itch, and some touch.

      • Other somatic and visceral sensations such as pressure, vibration, and certain touch modalities.

  • Sensory receptors produce two types of graded potentials:

    1. Generator potentials:

      • Produced when the dendrites of free nerve endings, encapsulated nerve endings, or the receptive part of olfactory receptors become stimulated.

      • When the generator potential is large enough, it reaches threshold and triggers one or more nerve impulses in the axon of a first-order sensory neuron.

      • Generator potentials lead to action potentials.

    2. Receptor potentials:

      • Produced by sensory receptors that are separate cells.

      • Triggered through neurotransmitter release via exocytosis.

      • Neurotransmitters diffuse across the synapse producing a postsynaptic potential, allowing the first-order sensory neuron to send a nerve impulse to the CNS.

Classification of Sensory Receptors

  • Based on location:

    1. Exteroceptors:

      • Located at or near the external surface of the body.

      • Sensitive to stimuli originating outside the body.

      • Provide information about the external environment (e.g., sensations of hearing, vision, smell, taste, touch, pressure, vibration, temperature, and pain).

    2. Interceptors:

      • Located in blood vessels, visceral organs, muscles, and the nervous system.

      • Monitor internal environmental conditions.

      • Nerve impulses produced by interceptors are usually not consciously perceived but may be felt as pain or pressure when activated by strong stimuli.

    3. Proprioceptors:

      • Located in muscles, tendons, joints, and the inner ear.

      • Provide information about body position, muscle length and tension, as well as joint movement.

  • Based on type of stimuli detected:

    1. Mechanoreceptors:

      • Sensitive to mechanical stimuli such as deformation, stretching, or bending of cells.

      • Provide sensations of touch, pressure, vibration, proprioception, hearing, and balance.

      • Also monitor the stretching of blood vessels and internal organs.

    2. Thermoreceptors:

      • Detect changes in temperature.

    3. Nociceptors:

      • Respond to painful stimuli from physical or chemical damage to tissue.

    4. Photoreceptors:

      • Detect light that strikes the retina of the eye.

    5. Chemoreceptors:

      • Detect chemicals in the mouth (taste), nose (smell), and body fluids.

    6. Osmoreceptors:

      • Detect osmotic pressure of body fluids.

Sensory Receptors: Adaptation

  • Adaptation: - Definition:

    • The generator potential or receptor potential decreases in amplitude during a maintained, constant stimulus.

    • This causes the frequency of nerve impulses in the first-order neuron to decrease.

    • Due to adaptation, the perception of a sensation may fade over time, even if the stimulus is still present.

    • Examples:

      • Getting acclimated to a hot tub.

    • Types of receptors based on adaptation:

      • Rapidly adapting receptors: - Adapt quickly; specialized for signaling changes in a stimulus.

        • Examples: Receptors associated with pressure, touch, and smell.

      • Slowly adapting receptors: - Adapt slowly; continue to trigger nerve impulses as long as the stimulus is present.

        • Monitor stimuli associated with pain, body position, and chemical composition of the blood.

Somatic Sensations

  • Tactile Sensations: - Touch:

    • Sensation of touch generally results from stimulation of tactile receptors in the skin or subcutaneous layer.

    • Two types of rapidly adapting touch receptors:

      1. Meissner Corpuscles:

        • Generate nerve impulses at the onset of touch.

        • Abundant in fingertips, hands, eyelids, tip of tongue, lips, nipples, soles, clitoris, and tip of penis.

      2. Hair Root Plexuses:

        • Located in hairy skin.

        • Consists of free nerve endings wrapped around hair follicles.

        • Detect movements on the skin surface that disturb the hair (e.g., an insect landing on the skin).

    • Two types of slowly adapting touch receptors:

      1. Type I Cutaneous Mechanoreceptors (Tactile Discs): - Saucer-shaped, flattened free nerve endings in contact with Merkel cells (tactile epithelial cells) of the stratum basale.

        • Abundant in fingertips, hands, lips, and external genitalia.

      2. Type II Cutaneous Mechanoreceptors (Ruffini Corpuscles): - Elongated encapsulated receptors located deep in the dermis and in ligaments and tendons.

        • Most sensitive to stretching that occurs as digits or limbs move.

    • Pressure and Vibration:

    • Pressure: - Sustained sensation felt over a larger area than touch; occurs with deformation of deeper tissues.

      • Receptors involved: tactile discs and lamellated corpuscles.

      • Lamellated Corpuscles (Pacinian Corpuscles):

      • Large, oval structures formed by a multi-layered connective tissue capsule surrounding a dendrite.

      • Rapidly adapting and distributed in the body (e.g., skin, mucous, serous membranes, joints).

    • Vibration: - Results from rapidly repetitive sensory signals from tactile receptors.

      • Receptors involved: Meissner corpuscles (detect low frequency) and lamellated corpuscles (detect higher frequency).

    • Itch and Tickle:

    • Itch: - Results from stimulation of free nerve endings by certain chemicals (e.g., bradykinin).

      • Often results from a local inflammatory response (bradykinin acts as a potent vasodilator).

    • Tickle: - Thought to be mediated by free nerve endings.

  • Phantom Limb Sensation: - Description:

    • Patients with an amputated limb may experience sensations like itching, pressure, tingling, or pain as if the limb were still present.

    • Explanations:

      1. Severed endings of sensory axons may still be activated, causing the cerebral cortex to interpret the sensation as coming from the non-existent limb.

      2. The cortical area that previously received sensory input from the missing limb may undergo extensive functional reorganization, responding to stimuli from another body part.

Somatic Sensations: Thermal Sensations

  • Thermoreceptors: - Free nerve endings with receptive fields about 11 mm in diameter on the skin surface.

    • Two distinct receptors for detecting coldness and warmth.

    • Both receptor types are rapidly adapting:

    • Cold receptors: - Located in the stratum basale of the epidermis.

      • Attached to medium diameter, myelinated A fibers.

      • Activated by temperatures between 10−40°C(50−105°F)10-40 \degree C (50-105 \degree F).

    • Warm receptors: - Less abundant than cold receptors.

      • Located in the dermis.

      • Attached to small diameter, unmyelinated C fibers.

      • Activated by temperatures between 32−48°C(90−118°F)32-48 \degree C (90-118 \degree F).

Somatic Sensations: Pain Sensations

  • Nociceptors: - Receptors for pain; free nerve endings found in every tissue of the body EXCEPT the brain.

    • Intense thermal, mechanical, or chemical stimuli can activate these receptors.

    • Tissue irritation/injury releases chemicals (e.g., prostaglandins, kinins, potassium) that stimulate nociceptors.

    • Pain may persist after a pain-producing stimulus is removed due to lingering pain-mediating chemicals and the fact that nociceptors exhibit very little adaptation.

  • Types of Pain: - Fast Pain (acute, sharp, pricking):

    • Occurs rapidly within 0.10.1 seconds after a stimulus is applied.

    • Nerve impulses propagate along medium diameter A fibers.

    • Example: pain from a needle puncture.

    • Slow Pain (chronic, dull, aching, throbbing):

    • Begins a second or more after the stimulus is applied.

    • Gradually increases in intensity over seconds to minutes.

    • Nerve impulses propagate along unmyelinated C fibers.

  • Referred Pain: - Description:

    • Visceral pain may be felt in or just deep to the skin that overlies the stimulated organ or in a surface area distant from the stimulated organ (referred pain).

Somatic Sensations: Proprioceptive Sensations

  • Proprioceptive sensations: - Allow us to recognize that parts of our body belong to us.

    • Enable us to know the position and movement of our head and limbs without looking.

    • Kinesthesia:

    • Perception of body movement.

    • Proprioceptors (located in postural muscles and tendons) inform us of:

      • The degree of muscle contraction, tension in tendons, and position of joints.

    • Weight Discrimination:

    • Ability to assess the weight of an object.

  • Types of Proprioceptors: - Muscle Spindles:

    • Proprioceptors in skeletal muscles that monitor changes in muscle length and participate in stretch reflexes.

    • Tendon Organs:

    • Located at the junction of a tendon and a muscle.

    • Initiate tendon reflexes protecting the tendon and associated muscles from damage due to excessive tension.

Somatic Sensory Pathways

  • Somatic sensory pathways: - Relay information from somatic sensory receptors to primary somatosensory areas in the cerebral cortex and cerebellum.

    • First-order neuron:

    • Conducts impulses from somatic receptors to the brainstem or spinal cord.

    • From the face, mouth, teeth, and eyes: impulses propagate via cranial nerves to the brainstem.

    • From neck, trunk, limbs, and posterior aspect of the head: impulses propagate along spinal nerves to the spinal cord.

    • Second-order neuron:

    • Conducts impulses from brainstem and spinal cord to thalamus.

    • Axons of second-order neurons decussate (cross to the opposite side) in the brainstem or spinal cord before ascending to the ventral posterior nucleus of the thalamus.

    • Thus, all somatic sensory information from one side of the body reaches the thalamus on the other side.

    • Third-order neuron:

    • Conducts impulses from the thalamus to the primary somatosensory area of the cortex on the same side (which is opposite of where it entered).

Specific Somatic Sensory Pathways

  • Posterior Column-Medial Lemniscus Pathway: - Conducts nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior aspect of the cerebral cortex.

    • Named after the two white matter tracts that the impulses travel through.

  • Anterolateral (Spinothalamic) Pathway: - Conducts nerve impulses for pain, temperature, itch, and tickle from the limbs, trunk, neck, and posterior head.

  • Trigeminothalamic Pathway: - Conducts nerve impulses for somatic sensations (tactile, thermal, and pain) from the face, nasal cavity, oral cavity, and teeth.

  • Corticospinal Pathway: - Conducts impulses for the control of muscles of the limbs and trunk.

  • Corticobulbar Pathway: - Conducts nerve impulses for control of skeletal muscles in the head.