CH 14 Autonomic and Somatic Nervous Systems
AUTONOMIC NERVOUS SYSTEM & SOMATIC NERVOUS SYSTEM
Learning Goals
Describe the components of the autonomic nervous system (ANS) and the somatic nervous system (SNS)
Differentiate between the structures of the sympathetic and parasympathetic divisions of the ANS
Name the components of a visceral reflex specific to the autonomic division to which it belongs
Predict the response of a target effector to autonomic input on the basis of the released signaling molecule
Describe how the central nervous system coordinates and contributes to autonomic functions
Distinguish between general and special senses
Describe regions of the central nervous system that contribute to somatic functions
Explain the stimulus-response motor pathway
Layout of the Nervous System
Nervous System Categories:
Central Nervous System (CNS): Composed of the brain and spinal cord.
Peripheral Nervous System (PNS): Divided into two main parts:
Autonomic Nervous System: Communicates with internal organs and glands, further divided into:
Enteric Division: Focused on digestion.
Sympathetic Division: Often referred to as the "arousing" system.
Parasympathetic Division: Known as the "calming" system.
Somatic Nervous System: Communicates with sense organs and voluntary muscles.
Sensory (Afferent) Nervous System: Carries sensory input to the CNS.
Motor (Efferent) Nervous System: Responsible for motor outputs to the muscles.
Important NS Terminology to Know
Nerves: Groups of axons in PNS
Tracts: Groups of axons in CNS
Nuclei: Groups of cell bodies in CNS
Ganglia: Groups of cell bodies in PNS
Somatic vs. Autonomic Nervous Systems
Autonomic Nervous System (ANS):
Controls involuntary functions.
Maintains homeostasis.
Contains afferent and efferent neurons.
Efferent neurons innervate visceral organs (smooth muscle cells, cardiac muscle cells, glands).
Initiation is not consciously controlled.
Somatic Nervous System (SNS):
Controls voluntary functions.
Contains afferent and efferent neurons.
Efferent neurons innervate skeletal muscles.
Produces voluntary muscle contractions.
Initiation is consciously controlled.
Includes sensory neurons for tactile (touch, pressure, vibration, tickle, itch), thermal, pain, proprioception, sight, hearing, taste, smell, and equilibrium.
Autonomic Motor Pathways
Main Differences Between Somatic and Autonomic Systems:
Autonomic motor neurons do not directly innervate their target; they require a two-neuron circuit.
Most autonomic motor pathways consist of two motor neurons in series:
Preganglionic Neuron:
The initial efferent neuron.
Cell body resides within the CNS.
Axon terminals release acetylcholine (ACh).
Myelinated axon extends to an autonomic ganglion.
Postganglionic Neuron:
Cell body resides in the autonomic ganglion in the PNS.
Axons travel to target cells, releasing either ACh or norepinephrine (NorE).
Triggers specific changes leading to either inhibitory or excitatory responses.
Unmyelinated axon extends from the ganglion to the effector (target).
Divisions of the ANS and Their Responses
Parasympathetic Nervous System:
Known as the craniosacral division (preganglionic neurons from cranial nerves and sacral spinal cord).
Most active during resting conditions; promotes the “rest and digest” response.
Actions: Decreases heart rate, constricts pupils, increases digestive activity, decreases breathing rate.
Sympathetic Nervous System:
Known as the thoracolumbar division (preganglionic neurons from thoracic and lumbar spinal cord).
Most active during stress or exertion; promotes the “fight or flight” response.
Actions: Increases heart rate, dilates pupils, decreases digestive activity, increases breathing rate.
ANS Neurotransmitters and Receptors
Cholinergic Neurons: Release acetylcholine (ACh).
Cholinergic Receptors:
Nicotinic Receptors: Found in synapses between preganglionic and postganglionic neurons in both sympathetic and parasympathetic divisions. Causes excitation.
Muscarinic Receptors: Located on target organs innervated by the parasympathetic nervous system (and some sympathetic effectors like sweat glands). Responses can be excitatory or inhibitory.
Adrenergic Neurons: Release norepinephrine (NorE). Primarily associated with sympathetic postganglionic neurons (except sweat glands).
Adrenergic Receptors: Contains two major types:
Alpha Adrenergic:
Alpha-1: Found on walls of blood vessels; causes vasoconstriction.
Alpha-2: Primarily found on pre-synaptic neurons.
Beta Adrenergic:
Beta-1: Found in cardiac muscle cells; increases heart activity.
Beta-2: Found in smooth muscle cells lining respiratory tract; promotes bronchodilation.
Beta-3: Primarily found in adipose cells; regulates fat burning.
Sensation, Receptors, and Somatic Sensory Pathways
Components of Sensation:
Stimulation of sensory receptors (must be close enough to detect changes).
Transduction of stimulus into graded potentials (varying amplitude based on stimulus strength; not action potentials).
Generation of nerve impulses to the brain or spinal cord (when graded potentials summate to a threshold).
Integration of sensory input occurs in the CNS by a specific brain region to produce conscious sensation.
Modalities of Sensation: Unique characteristics that distinguish one sensation from another (e.g., pain, touch, hearing). Each sensory neuron carries only one type of message.
Sensory Receptor Classification:
Categories of Senses:
General Senses: Distributed widely throughout the body.
Somatic sensations (tactile, thermal, pain, proprioception): Arise from skin, subcutaneous tissue, mucous membranes, muscles, tendons, joints.
Visceral sensations: Related to internal organs (e.g., stretch, nausea, hunger, pressure).
Special Senses: Localized within specific, complex sense organs primarily in the head (smell, taste, vision, hearing, equilibrium).
Structural Classification:
Free nerve endings: Lack specialization; detect temperature, pain, and tickle sensations.
Encapsulated nerve endings: Enclosed in connective tissues; detect pressure and vibration (e.g., Pacinian corpuscles, Meissner’s corpuscles).
Separate cells: Specialized cells like hair cells (hearing), photoreceptors (vision), and gustatory receptor cells (taste).
Functional Classification:
Exteroceptors: Near the surface, detect external stimuli.
Interoceptors (Visceroceptors): Located internally, detect internal stimuli (mostly involuntary), monitoring internal conditions (e.g., baroreceptors for blood pressure).
Proprioceptors: Detect body position and muscle tension, critical for coordination and balance. Found within muscles, joints, tendons, and inner ear. Three main types: muscle spindles, tendon organs, and joint kinesthetic receptors.
Receptor Modes (Stimulus Type):
Mechanoreceptors: Detect stretching and pressure.
Thermoreceptors: Detect temperature changes.
Nociceptors: Detect painful stimuli.
Photoreceptors: Activated by light.
Chemoreceptors: Detect chemicals.
Osmoreceptors: Detect osmotic pressure.
Cutaneous Sensations: Arise from receptors in skin, subcutaneous tissue, and mucous membranes. Sensitivity varies by body region.
Somatic Sensory Pathways (Neural Pathway of Sensations):
Convey sensory input from peripheral receptors to the CNS.
Pathways consist of three types of neurons:
First-order Neurons: Carry impulses from receptors to the CNS (brainstem or spinal cord).
Second-order Neurons: Transmit impulses from the CNS to the thalamus.
Third-order Neurons: Carry impulses from the thalamus to the primary somatosensory area of the cortex.
Ascend to the cerebral cortex via three main pathways:
Posterior Column-Medial Lemniscus Pathway
Anterolateral (Spinothalamic) Pathway
Trigeminal Pathway
Sensation vs. Perception
Sensation: Awareness of external or internal stimuli, can be conscious or subconscious. It involves the detection of a stimulus by a receptor.
Perception: The interpretation of sensations, which is a conscious process. People interpret sensations differently due to various factors.
Sensory Adaptation
Most sensory receptors are adaptable; their sensitivity decreases with prolonged exposure to a stimulus.
Fast-adapting receptors include touch and smell (e.g., no longer sensing clothing or strong colognes).
Some receptors adapt more slowly or not at all (e.g., pain receptors remain sensitive to alert about potential harm).
Pain Sensation
Nociceptors: Free nerve endings responsible for pain, present in most body tissues. Highly significant for survival since they alert the brain of potential harm.
Types of Pain:
Acute Pain: Quick and sharp, carried by myelinated neurons (faster).
Chronic Pain: Slow, throbbing pain, transmitted by unmyelinated neurons (slower).
Location-based Pain:
Superficial pain: Occurs at the skin level.
Deep somatic pain: Occurs in muscles, tendons, and joints.
Referred Pain: Pain felt in a location distant from the source of the stimulus (e.g., pain during a heart attack felt in the left arm) due to shared nerve pathways.
Phantom Pain: Sensation following a limb amputation, generated by nerve impulses from residual nerves, often resulting in pain, itching, or pressure where the limb was removed.
Pain Relief Mechanisms
Anesthesia: Blocks pain sensations from reaching the brain.
General anesthesia: Induces unconsciousness and loss of all sensations.
Spinal anesthesia: Injected into the subarachnoid space, blocking sensations below it.
Analgesics: Reduce pain perception and can include medications like aspirin which block pain signal transmission.
Introduction to Special Senses
Special Senses: Smell, taste, vision, hearing, and equilibrium are distinct receptor types, predominantly localized in the head.
Olfaction (smell) and Gustation (taste) work closely together, transmitting chemical impulses to the brain.
Emotional responses are often triggered in the limbic system associated with olfactory cues, impacting memories (e.g., smell of apple pie).
Gustation: Taste Sensation
Five Primary Tastes: Sweet, sour, bitter, salty, umami (savory).
Approximately 10,000 taste buds located on the tongue, soft palate, pharynx, and larynx, diminishing with age.
Structure of Taste Buds: Composed of gustatory receptor cells, supporting cells, and basal cells for continual renewal.
Tongue Papillae Types:
Circumvallate Papillae: Large, at back of the tongue (100-300 taste buds each).
Fungiform Papillae: Scattered, with about 5 taste buds each.
Foliate Papillae: Located in trenches on tongue margins (most degenerate in childhood).
Filiform Papillae: Lacking taste buds; provide traction for food.
For tasting, substances must dissolve in saliva; receptor potential triggers nerve impulses transmitted to cranial nerves (VII, IX, and X) to the brain.
Olfaction: Smell Sensation
Olfactory receptors are located in the nasal epithelium, responding to odorants.
Basal cells renew receptors monthly; olfactory glands produce mucus essential for odorant detection.
Neural Pathway: Olfactory signals travel to the olfactory bulbs, then to limbic and cerebral cortex regions for perception.
Hearing
Regions of the ear include:
External Ear: Collects sound waves (pinna, auditory canal).
Middle Ear: Amplifies sound via ossicles (malleus, incus, stapes).
Inner Ear: Contains structures (cochlea, semicircular canals) critical for hearing and equilibrium.
Sound vibrations travel through the tympanic membrane into the ossicles and eventually to the cochlea, where auditory signals are transformed into nerve impulses sent to the cerebral cortex for perception.
Balance: Maintained through vestibular apparatus detecting head position and body movement (static and dynamic equilibrium).
AUTONOMIC NERVOUS SYSTEM & SOMATIC NERVOUS SYSTEM
Learning Goals
Describe the components of the autonomic nervous system (ANS) and the somatic nervous system (SNS)
Autonomic Nervous System (ANS): This system communicates with internal organs and glands, controlling involuntary functions and maintaining homeostasis. It contains both afferent (sensory) and efferent (motor) neurons. Its efferent neurons innervate visceral organs. The ANS is further divided into:
Enteric Division: Primarily focused on digestion.
Sympathetic Division: Known as the "arousing" system, promoting the “fight or flight” response during stress or exertion.
Parasympathetic Division: Known as the "calming" system, promoting the “rest and digest” response during resting conditions.
Somatic Nervous System (SNS): This system communicates with sense organs and voluntary muscles, controlling voluntary functions. It also contains both afferent (sensory) and efferent (motor) neurons. Its efferent neurons innervate skeletal muscles, producing voluntary contractions. It includes sensory neurons responsible for touch, pain, temperature, proprioception, sight, hearing, taste, smell, and equilibrium.
Differentiate between the structures of the sympathetic and parasympathetic divisions of the ANS
Both sympathetic and parasympathetic motor pathways consist of two motor neurons in series that do not directly innervate their target:
Preganglionic Neuron: The initial efferent neuron whose cell body resides within the CNS (brain or spinal cord). Its myelinated axon extends to an autonomic ganglion and releases acetylcholine (ACh) at its terminals.
Postganglionic Neuron: The second neuron whose cell body resides in an autonomic ganglion in the PNS. Its unmyelinated axon extends from the ganglion to the effector (target cells), releasing either ACh or norepinephrine (NorE), triggering specific excitatory or inhibitory responses.
Structural Differences by Division:
Parasympathetic Nervous System: Referred to as the craniosacral division due to its preganglionic neurons originating from cranial nerves and the sacral spinal cord. It is most active during resting conditions.
Sympathetic Nervous System: Referred to as the thoracolumbar division due to its preganglionic neurons originating from the thoracic and lumbar regions of the spinal cord. It is most active during stress or exertion.
Name the components of a visceral reflex specific to the autonomic division to which it belongs
A visceral reflex arc, which mediates involuntary actions, involves:
Sensory Receptor: Detects changes in internal organs or conditions (e.g., stretch, chemical changes).
Afferent Neuron: Carries sensory input from the receptor to the Central Nervous System (CNS).
Integrating Center (CNS): Processes the sensory information, often in the brainstem, hypothalamus, or spinal cord.
Efferent Pathway (Autonomic Motor Pathway): Consists of two neurons in series:
Preganglionic Neuron: Originates in the CNS.
Postganglionic Neuron: Extends from an autonomic ganglion to the target effector.
Effector: A smooth muscle, cardiac muscle, or gland that responds to the autonomic output.
The specificity to the autonomic division (sympathetic or parasympathetic) comes from the origin of the preganglionic neuron (craniosacral for parasympathetic; thoracolumbar for sympathetic), the location of the ganglion, and the neurotransmitter/receptor combination at the effector.
Predict the response of a target effector to autonomic input on the basis of the released signaling molecule
The response of a target effector depends on the neurotransmitter released by the postganglionic neuron and the type of receptor on the effector cell:
Cholinergic Neurons (release Acetylcholine - ACh):
Nicotinic Receptors: Found in synapses between preganglionic and postganglionic neurons in both sympathetic and parasympathetic divisions. Stimulation always causes excitation of the postganglionic neuron.
Muscarinic Receptors: Located on target organs innervated by the parasympathetic nervous system (and some sympathetic effectors like sweat glands). Responses can be excitatory or inhibitory depending on the target cell, for example:
Parasympathetic Actions: Decreases heart rate, constricts pupils, increases digestive activity, decreases breathing rate.
Adrenergic Neurons (release Norepinephrine - NorE): Primarily associated with the sympathetic nervous system's postganglionic neurons (except for sweat glands which are cholinergic).
Alpha Adrenergic Receptors:
Alpha-1: Found on walls of blood vessels, causing vasoconstriction.
Alpha-2: Primarily found on pre-synaptic neurons, often inhibiting NorE release.
Beta Adrenergic Receptors:
Beta-1: Found in cardiac muscle cells, increasing heart rate and contractility.
Beta-2: Found in smooth muscle cells lining the respiratory tract, promoting bronchodilation.
Beta-3: Primarily found in adipose cells, regulating fat burning.
Sympathetic Actions: Increases heart rate, dilates pupils, decreases digestive activity, increases breathing rate.
Describe how the central nervous system coordinates and contributes to autonomic functions
The Central Nervous System (CNS), composed of the brain and spinal cord, plays a crucial role in coordinating autonomic functions:
Origin of Preganglionic Neurons: The cell bodies of the initial efferent (preganglionic) neurons for both sympathetic and parasympathetic divisions are housed within the CNS. Sympathetic preganglionic neurons originate in the thoracolumbar spinal cord, while parasympathetic preganglionic neurons originate from the brainstem (cranial nerves) and sacral spinal cord.
Integration of Input: Sensory information from visceral receptors (via afferent neurons) is conveyed to the CNS, where it is integrated at various levels, including the brainstem, hypothalamus, and spinal cord. These centers then initiate appropriate autonomic efferent responses.
Higher-Level Control: The hypothalamus serves as a major integrative center for the ANS, regulating functions like body temperature, hunger, thirst, and emotional responses through its control over the sympathetic and parasympathetic systems. The cerebral cortex can also influence autonomic responses, particularly in emotional situations.
Distinguish between general and special senses
General Senses: These are distributed widely throughout the body and include:
Somatic Sensations: Arise from receptors in the skin, subcutaneous tissue, mucous membranes, muscles, tendons, and joints. They include tactile sensations (touch, pressure, vibration, tickle, itch), thermal sensations (warmth, cold), pain, and proprioception (awareness of body position and movement).
Visceral Sensations: Arise from receptors in internal organs, conveying information about conditions like stretch, nausea, hunger, and pressure.
Special Senses: These are localized within specific, complex sense organs primarily in the head. They include:
Smell (Olfaction)
Taste (Gustation)
Vision
Hearing
Equilibrium (Balance)
Describe regions of the central nervous system that contribute to somatic functions
The CNS extensively contributes to somatic functions, processing sensory input and initiating motor output:
Spinal Cord: Serves as the initial relay for impulses from somatic sensory receptors to the brain and contains motor neurons that directly innervate skeletal muscles for reflexes and voluntary movements.
Brainstem: Contains nuclei involved in regulating some somatic motor functions and serves as a pathway for sensory and motor tracts.
Thalamus: Acts as a major relay station for almost all somatic sensory input (except olfaction) to the cerebral cortex. It filters and processes sensory information before transmitting it.
Cerebral Cortex:
Primary Somatosensory Area (Parietal Lobe): Located in the postcentral gyrus, this region receives and interprets somatic sensations (touch, pain, temperature, proprioception), allowing for conscious perception.
Primary Motor Area (Frontal Lobe): Located in the precentral gyrus, this area initiates voluntary movements by sending signals to lower motor neurons.
Association Areas: Other cortical regions integrate sensory and motor information, planning and coordinating complex somatic movements.
Explain the stimulus-response motor pathway
A stimulus-response motor pathway describes the sequence of events from detecting a stimulus to executing a motor response. This can be understood differently for somatic and autonomic systems:
General Requirements for Sensation (leading to a response):
Stimulation of a Sensory Receptor: A stimulus (e.g., touch, light, temperature change) activates a specialized receptor.
Transduction of Stimulus: The receptor converts the stimulus energy into a graded potential (a local electrical signal).
Generation of Nerve Impulse: If the graded potential reaches a threshold, it generates an action potential (nerve impulse) that propagates along a sensory neuron to the CNS.
Integration of Sensory Input: The CNS processes the sensory information in specific brain regions, leading to perception and decision-making for a motor response.
Somatic Motor Pathway (Voluntary Control):
Sensory Input: Sensory neurons (first, second, and third-order) convey impulses from receptors (e.g., cutaneous receptors, proprioceptors) to the cerebral cortex (primary somatosensory area via the thalamus) for conscious perception.
CNS Processing & Decision: The cerebral cortex (e.g., primary motor area, association areas) processes the sensory information and initiates a voluntary motor command based on conscious thought.
Motor Output: This command travels down descending motor pathways (e.g., corticospinal tracts) to activate lower motor neurons in the spinal cord.
Effector Response: The lower motor neuron's axon directly innervates specific skeletal muscle fibers, releasing ACh at the neuromuscular junction, causing the muscle to contract and produce a voluntary movement.
Autonomic Motor Pathway (Involuntary Control/Visceral Reflex):
Sensory Input: Visceral receptors detect internal stimuli (e.g., changes in blood pressure, chemical levels) and send impulses via afferent neurons to the CNS.
CNS Integration: The CNS (e.g., brainstem, hypothalamus, spinal cord) integrates this involuntary sensory input and initiates an appropriate autonomic response without conscious control.
Motor Output (Two-Neuron Chain):
A preganglionic neuron (originating in CNS) releases ACh onto a postganglionic neuron in an autonomic ganglion.
The postganglionic neuron (in PNS) extends to the effector (smooth muscle, cardiac muscle, or gland), releasing either ACh (parasympathetic) or NorE (sympathetic).
Effector Response: The effector responds involuntarily, leading to changes such as altered heart rate, digestive activity, or glandular secretion.