8.3 Peripheral Nervous System

The peripheral nervous system connects your brain and spinal cord to the rest of your body. It's split into two parts: the somatic system for voluntary movements and sensations, and the autonomic system for involuntary functions like heart rate and digestion.

The autonomic system has two divisions: sympathetic (fight-or-flight) and parasympathetic (rest-and-digest). These work together to keep your body balanced. Spinal and cranial nerves carry signals between your central nervous system and body parts, controlling everything from muscle movements to organ functions.

Somatic vs Autonomic Nervous Systems

Peripheral Nervous System Divisions

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• The peripheral nervous system (PNS) is divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS), each with distinct functions and structures
• The SNS uses acetylcholine as its primary neurotransmitter at the neuromuscular junction, while the ANS uses acetylcholine and norepinephrine at various synapses
• The cell bodies of somatic motor neurons are located in the brainstem and spinal cord, while autonomic motor neurons have cell bodies in the brainstem, spinal cord, and peripheral ganglia

Somatic Nervous System Functions

• The somatic nervous system is responsible for voluntary control of skeletal muscles, allowing for conscious movement (walking, grasping objects) and sensory perception from the skin, muscles, and joints
• Somatic sensory receptors detect stimuli such as touch, pressure, temperature, and proprioception, relaying information to the central nervous system for processing
• Somatic motor neurons directly innervate skeletal muscle fibers, initiating muscle contraction in response to conscious commands from the brain

Autonomic Nervous System Functions

• The autonomic nervous system regulates involuntary functions of internal organs, glands, and smooth muscles, maintaining homeostasis without conscious control
• Autonomic functions include regulation of heart rate, blood pressure, digestion, respiration, metabolism, and body temperature
• The ANS consists of the sympathetic and parasympathetic divisions, which often have opposing effects on target organs to maintain balance (heart rate increase vs decrease)
• Autonomic sensory receptors monitor internal conditions (blood pressure, blood chemistry, organ distension) and relay information to autonomic control centers in the brain

Sympathetic and Parasympathetic Divisions

Sympathetic Division

• The sympathetic division is responsible for the "fight or flight" response, preparing the body for stressful or emergency situations
• Sympathetic activation leads to increased heart rate, blood pressure, and blood glucose levels while decreasing digestion and other non-essential functions
• Sympathetic preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord (T1-L2) and synapse with postganglionic neurons in paravertebral or prevertebral ganglia
• Sympathetic postganglionic neurons release norepinephrine at target organs, except for sweat glands which receive acetylcholine

Parasympathetic Division

• The parasympathetic division is responsible for the "rest and digest" response, conserving energy and promoting normal body functions such as digestion, urination, and defecation
• Parasympathetic activation leads to decreased heart rate, increased digestion and secretion, and relaxation of smooth muscles in the bladder and rectum
• Parasympathetic preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, and X) and the sacral region of the spinal cord (S2-S4), synapsing with postganglionic neurons near or within the target organs
• Parasympathetic postganglionic neurons release acetylcholine at target organs

Dual Innervation and Antagonistic Effects

• Most organs receive dual innervation from both sympathetic and parasympathetic divisions, which typically have opposing effects on the target tissues
• For example, sympathetic stimulation increases heart rate while parasympathetic stimulation decreases heart rate, allowing for precise regulation of cardiac function
• The balance between sympathetic and parasympathetic tone is crucial for maintaining homeostasis and adapting to changing environmental demands
• Some organs, like the adrenal medulla and most blood vessels, receive only sympathetic innervation, while others, like the ciliary muscle of the eye, receive only parasympathetic innervation

Spinal and Cranial Nerves

Spinal Nerves

• The peripheral nervous system includes 31 pairs of spinal nerves, which carry sensory and motor information between the central nervous system and the rest of the body
• Spinal nerves originate from the spinal cord and are named according to their corresponding vertebral level (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)
• Each spinal nerve is formed by the convergence of dorsal (sensory) and ventral (motor) roots, which then divide into smaller branches to innervate specific dermatomes and myotomes
• Spinal nerves contain both somatic and autonomic fibers, with autonomic fibers separating to form sympathetic chain ganglia and splanchnic nerves

Cranial Nerves

• The peripheral nervous system also includes 12 pairs of cranial nerves, which originate from the brainstem and are numbered I-XII based on their rostral-caudal position
• Each cranial nerve has specific sensory, motor, or mixed functions, innervating structures in the head, neck, and some thoracic and abdominal organs
• Some key cranial nerves include:
  • Olfactory (I): Sense of smell
  • Optic (II): Vision
  • Oculomotor (III): Eye movement and pupillary constriction
  • Trigeminal (V): Sensory innervation of the face and motor innervation of the jaw muscles
  • Facial (VII): Facial expression, taste, and salivation
  • Vestibulocochlear (VIII): Hearing and balance
  • Glossopharyngeal (IX): Taste, swallowing, and salivation
  • Vagus (X): Parasympathetic innervation of thoracic and abdominal organs, taste, and swallowing
  • Hypoglossal (XII): Tongue movement
• Damage to specific cranial nerves can lead to distinct sensory deficits, motor impairments, or other neurological symptoms depending on the nerve's function and distribution (facial palsy, hearing loss, dysphagia)

Reflexes in the Peripheral Nervous System

Reflex Arcs

• Reflexes are rapid, involuntary responses to specific stimuli that occur without conscious input from the brain, allowing for quick and protective reactions to potentially harmful situations
• Reflex arcs are the neural pathways that mediate reflexes, typically consisting of a sensory receptor, afferent neuron, integration center (in the spinal cord or brainstem), efferent neuron, and effector organ
• Sensory receptors detect the stimulus (touch, pressure, stretch) and generate an action potential in the afferent neuron
• The afferent neuron synapses with the efferent neuron directly (monosynaptic) or via interneurons (polysynaptic) in the integration center
• The efferent neuron transmits the signal to the effector organ (muscle or gland), causing a response (contraction or secretion)

Types of Reflexes

• Monosynaptic reflexes, such as the stretch reflex, involve a single synapse between the sensory and motor neurons, resulting in a rapid response like the knee-jerk reflex
• Polysynaptic reflexes, such as the withdrawal reflex, involve multiple synapses and interneurons, allowing for more complex and coordinated responses to stimuli like touching a hot surface
• Somatic reflexes involve skeletal muscle responses to stimuli, such as the patellar tendon reflex or the crossed extensor reflex
• Autonomic reflexes involve smooth muscle, cardiac muscle, or glandular responses to stimuli, such as the pupillary light reflex or the baroreflex

Significance of Reflexes

• Reflexes play a crucial role in maintaining posture, balance, and muscle tone, as well as protecting the body from potential injuries by triggering automatic responses
• The stretch reflex helps maintain proper muscle length and tension, preventing overstretching and contributing to smooth, coordinated movement
• Withdrawal reflexes protect the body from damaging stimuli by rapidly removing the affected limb or body part from the source of the stimulus (hot surface, sharp object)
• Some reflexes, like the pupillary light reflex and the gag reflex, are mediated by cranial nerves and serve specific protective or adaptive functions for the eyes and throat, respectively
• Abnormal reflex responses can indicate underlying neurological disorders or damage to the peripheral or central nervous systems, making reflex testing an important component of neurological examinations (Babinski sign, absent reflexes)