3.2 Peripheral nervous system: somatic and autonomic divisions
4 min read•august 14, 2024
The peripheral nervous system connects your brain to the rest of your body. It's split into two main parts: the somatic system, which controls your voluntary movements, and the autonomic system, which handles automatic functions like breathing and digestion.
The autonomic system is further divided into the sympathetic ("fight-or-flight") and parasympathetic ("rest-and-digest") systems. These work together to keep your body balanced, responding to stress and relaxation as needed.
Somatic vs Autonomic Nervous Systems
Functional Differences and Voluntary Control
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- The (SNS) is responsible for voluntary control of skeletal muscles, enabling conscious movement and motor responses to sensory stimuli
- In contrast, the (ANS) controls involuntary functions of internal organs, glands, and smooth muscles, maintaining without conscious effort (heart rate, digestion, respiration)
Anatomical Organization and Neuron Types
- The SNS consists of sensory neurons that carry information from sensory receptors (touch, pain, proprioception) to the central nervous system (CNS) and motor neurons that carry signals from the CNS to skeletal muscles for
- Somatic nerves are typically unipolar neurons with cell bodies in the spinal cord or brainstem, forming reflex arcs and allowing for rapid motor responses
- The ANS is divided into the sympathetic nervous system (fight-or-flight response) and the parasympathetic nervous system (rest-and-digest functions), which work in opposition to maintain homeostasis
- Autonomic nerves are multipolar neurons with cell bodies in outside the CNS, allowing for diffuse and sustained control of organ functions
Organization and Functions of the Sympathetic Nervous System
Anatomical Origin and Ganglia
- The sympathetic nervous system (SNS) originates from the thoracic and lumbar regions of the spinal cord (T1-L2), with preganglionic neurons synapsing in paravertebral ganglia (chain ganglia) or prevertebral ganglia near the target organs
- Postganglionic neurons in these ganglia then innervate various organs and tissues throughout the body, allowing for widespread activation during stress or emergencies
Neurotransmitters and Receptor Types
- The SNS releases primarily (noradrenaline) as a neurotransmitter from postganglionic nerve endings, which binds to adrenergic receptors (alpha and beta) on target tissues
- Adrenal medulla, stimulated by preganglionic SNS fibers, releases epinephrine (adrenaline) and norepinephrine into the bloodstream for systemic effects
Physiological Effects and Fight-or-Flight Response
- SNS activation prepares the body for "fight-or-flight" responses during stress or emergencies, prioritizing energy distribution to vital organs and muscles
- Increased heart rate and contractility, blood pressure, and blood glucose levels (glycogenolysis and gluconeogenesis) to support enhanced physical performance
- Dilation of bronchioles and pupils to improve oxygenation and visual acuity, while decreasing digestive (peristalsis, secretion) and urinary functions (bladder relaxation) to conserve energy
Organization and Functions of the Parasympathetic Nervous System
Anatomical Origin and Ganglia
- The parasympathetic nervous system (PNS) originates from the cranial nerves (III, VII, IX, X) in the brainstem and the sacral region (S2-S4) of the spinal cord, promoting "rest-and-digest" functions during relaxed states
- Preganglionic neurons in the brainstem or sacral spinal cord synapse with postganglionic neurons in terminal ganglia near or within the target organs (ciliary, pterygopalatine, submandibular, otic, pelvic ganglia)
Neurotransmitters and Receptor Types
- The PNS releases primarily as a neurotransmitter from both pre- and postganglionic nerve endings, which binds to muscarinic receptors (M1-M5) on target tissues
- Nicotinic receptors are present in the ganglia and adrenal medulla, mediating fast synaptic transmission and release of catecholamines
Physiological Effects and Rest-and-Digest Functions
- PNS activation promotes energy conservation and maintains normal bodily functions during periods of rest or relaxation
- Decreased heart rate (bradycardia) and blood pressure to reduce cardiac workload and maintain stable circulation
- Increased digestive functions (peristalsis, secretion of enzymes and bile) and urinary functions (bladder contraction, sphincter relaxation) to support nutrient absorption and waste elimination
- Constriction of bronchioles and pupils to optimize respiration and protect the eyes from bright light
Enteric Nervous System in Gastrointestinal Function
Anatomical Organization and Plexuses
- The enteric nervous system (ENS) is a semi-autonomous division of the ANS that controls the gastrointestinal (GI) tract, consisting of the myenteric (Auerbach's) plexus and submucosal (Meissner's) plexus embedded in the GI wall
- The myenteric plexus lies between the circular and longitudinal smooth muscle layers, regulating motility, while the submucosal plexus controls secretion and local blood flow
Neuron Types and Local Reflex Circuits
- The ENS contains sensory neurons (intrinsic primary afferent neurons), interneurons, and motor neurons (excitatory and inhibitory) that can function independently of the CNS to regulate GI functions
- Sensory neurons detect mechanical (stretch, tension) and chemical (pH, nutrients) stimuli in the GI tract, relaying information to interneurons that integrate signals and coordinate motor neuron activity
- Local reflex circuits within the ENS mediate peristalsis, secretion, and absorption without input from the CNS, allowing for autonomous control of GI functions
Regulation of Gastrointestinal Motility and Secretion
- Motor neurons in the ENS control the contraction and relaxation of smooth muscles in the GI tract (circular and longitudinal), generating peristaltic waves that propel food and waste through the digestive system
- Excitatory motor neurons release acetylcholine and substance P, while inhibitory motor neurons release nitric oxide and vasoactive intestinal peptide (VIP) to modulate smooth muscle tone
- Secretomotor neurons stimulate the release of digestive enzymes, mucus, and hormones from the GI epithelium and glands (salivary, gastric, pancreatic, intestinal), facilitating digestion and absorption of nutrients
Communication with the CNS and Autonomic Modulation
- The ENS communicates with the CNS through the vagus nerve (cranial nerve X) and prevertebral ganglia, allowing for central modulation of GI functions in response to emotional states, circadian rhythms, and other factors
- Parasympathetic innervation (vagus nerve) stimulates GI motility, secretion, and relaxation of sphincters, while sympathetic innervation (prevertebral ganglia) inhibits these functions and constricts sphincters
- The ENS also interacts with the gut-brain axis, involving bidirectional communication between the GI tract, enteric microbiota, and the CNS, influencing mood, behavior, and overall health
Key Terms to Review (18)
Acetylcholine: Acetylcholine is a neurotransmitter that plays a vital role in communication between neurons and is involved in various physiological functions such as muscle contraction, memory, and attention. It is found both in the central nervous system and the peripheral nervous system, influencing numerous neural pathways and processes.
Afferent pathways: Afferent pathways are neural routes that carry sensory information from the peripheral nervous system to the central nervous system. These pathways are crucial for processing sensory input, enabling the brain to interpret stimuli from the environment, including touch, pain, temperature, and more, thereby influencing both somatic and autonomic responses.
Autonomic dysreflexia: Autonomic dysreflexia is a medical emergency characterized by an overreaction of the autonomic nervous system, typically occurring in individuals with spinal cord injuries at or above the T6 level. This condition can lead to dangerously high blood pressure and other severe symptoms due to excessive sympathetic nervous system activation, often triggered by a noxious stimulus below the injury site. Understanding this condition is crucial for recognizing the significance of the autonomic divisions of the nervous system and their role in maintaining homeostasis.
Autonomic Nervous System: The autonomic nervous system (ANS) is a crucial component of the peripheral nervous system that regulates involuntary physiological functions, including heart rate, blood pressure, respiration, digestion, and sexual arousal. It operates largely below the level of consciousness and is divided into two main branches: the sympathetic and parasympathetic systems, which work together to maintain homeostasis within the body.
Efferent pathways: Efferent pathways are neural routes that carry signals away from the central nervous system (CNS) to peripheral effectors, such as muscles and glands. These pathways are crucial for executing motor commands and autonomic responses, enabling the body to react to stimuli and maintain homeostasis. They play a key role in the somatic division, which controls voluntary movements, and the autonomic division, which regulates involuntary functions.
Ganglia: Ganglia are clusters of neuronal cell bodies located outside the central nervous system that act as relay stations for transmitting information in both the somatic and autonomic divisions of the peripheral nervous system. They play a crucial role in processing and integrating signals, allowing for effective communication between the central nervous system and various parts of the body.
Homeostasis: Homeostasis refers to the body's ability to maintain a stable internal environment despite external changes. It involves various physiological processes that regulate factors like temperature, pH, and ion concentrations, allowing organisms to function optimally. This balance is crucial for processes like sleep regulation, the functioning of the nervous system, and emotional responses.
Involuntary Control: Involuntary control refers to the automatic regulation of bodily functions and responses that occur without conscious thought or intention. This type of control is crucial for maintaining homeostasis and managing essential functions such as heart rate, digestion, and respiratory rate. It is primarily governed by the autonomic division of the peripheral nervous system, contrasting with voluntary control, which is mediated by the somatic division.
Nerve fibers: Nerve fibers are the long, slender projections of nerve cells (neurons) that transmit electrical signals throughout the nervous system. These fibers are essential for communication between different parts of the body and can be categorized into different types based on their diameter and conduction velocity, influencing their specific roles in the somatic and autonomic divisions of the peripheral nervous system.
Neuropathy: Neuropathy refers to a range of conditions that involve damage to the peripheral nerves, which can lead to symptoms such as pain, weakness, and numbness. This condition can arise from various causes, including diabetes, infections, and exposure to toxins, and it significantly affects how the somatic and autonomic divisions of the peripheral nervous system function, potentially leading to impaired sensation and motor control.
Norepinephrine: Norepinephrine is a neurotransmitter and hormone that plays a critical role in the body's response to stress and in regulating mood, attention, and arousal. It acts primarily in the brain and the peripheral nervous system, influencing various physiological functions, such as heart rate and blood pressure, while also impacting cognitive processes like focus and emotional responses.
Parasympathetic division: The parasympathetic division is a part of the autonomic nervous system that promotes a 'rest and digest' response, counteracting the effects of the sympathetic division. This system is responsible for stimulating activities that occur when the body is at rest, such as digestion, urination, and conserving energy. It plays a crucial role in maintaining homeostasis by regulating bodily functions when the body is not under stress.
Sensory information: Sensory information refers to the data received by the nervous system from sensory receptors that detect stimuli from the environment. This information is crucial for perceiving the world around us and is processed by the brain, leading to appropriate responses. It connects directly to the peripheral nervous system, which includes the somatic division responsible for voluntary movements and the autonomic division that manages involuntary bodily functions.
Somatic Nervous System: The somatic nervous system is a part of the peripheral nervous system that is responsible for voluntary control of body movements through the activation of skeletal muscles. It consists of motor neurons that transmit signals from the central nervous system to the muscles and sensory neurons that convey information from sensory organs to the central nervous system, allowing for conscious perception and response to stimuli.
Stretch reflex: The stretch reflex is an automatic response that occurs when a muscle is stretched, leading to a contraction of that muscle and a simultaneous relaxation of its antagonist. This mechanism helps maintain posture and balance, allowing for quick adjustments to muscle length and tension. It primarily involves sensory neurons, motor neurons, and interneurons in the spinal cord, highlighting its connection to the nervous system's reflex activities and motor control.
Sympathetic division: The sympathetic division is a part of the autonomic nervous system responsible for the body's 'fight or flight' response, activating during stressful situations to prepare the body for action. This division increases heart rate, dilates airways, and inhibits digestion to ensure that energy is directed toward immediate physical demands. It works in concert with the parasympathetic division, which promotes rest and recovery.
Voluntary movement: Voluntary movement refers to the intentional and controlled actions of the body that are initiated by conscious thought and facilitated by the nervous system. This type of movement is primarily governed by the somatic division of the peripheral nervous system, which connects the central nervous system to skeletal muscles, allowing individuals to engage in activities such as walking, speaking, or playing sports.
Withdrawal reflex: The withdrawal reflex is a rapid, automatic response that occurs when a body part is exposed to a painful stimulus, causing the affected limb to quickly withdraw from the source of pain. This reflex is essential for protecting the body from harm and involves both sensory and motor neurons working together in the spinal cord without requiring conscious thought.