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

• The somatic nervous system (SNS) is responsible for voluntary control of skeletal muscles, enabling conscious movement and motor responses to sensory stimuli
• In contrast, the autonomic nervous system (ANS) controls involuntary functions of internal organs, glands, and smooth muscles, maintaining homeostasis 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 voluntary movement
• 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 ganglia 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 norepinephrine (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 acetylcholine 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