The Peripheral Nervous System (PNS) Part 1

Peripheral Nervous System (PNS) Neural structures outside CNS, although soma may be w/in CNS sensory receptors, peripheral nerves, ganglia, & motor endings Divisions of PNS Afferent division (sensory) Efferent division (motor) Somatic – innervate skeletal muscle Autonomic – innervate smooth, cardiac, glands, visceral organs Sympathetic – soma in spinal cord Parasympathetic – soma in midbrain or sacrum

Motor neurons of somatic division Efferent Division Motor neurons of somatic division Terminate at motor end plate neuromuscular junction Motor neurons of autonomic division Varicosities in smooth & cardiac muscle & glands

Afferent Division - Sensory Neurons Sensory neuron termini (dendritic processes) specialized to respond to stimuli Activation triggers impulses to CNS Perception in brain

Receptor Classification by Stimulus Type Mechanoreceptors – change in neuron shape Thermoreceptors - temperature Photoreceptors - light Chemoreceptors - chemicals Nociceptors – pain-causing stimuli (chemicals)

Receptor Classification by Location: Exteroceptors Near body surface Respond to stimuli arising outside body Include special sense organs Interoceptors Respond to stimuli arising w/in body Found in internal viscera & blood vessels Proprioceptors Respond to stretch In skeletal muscles, tendons, joints, ligaments, & connective tissue

Receptor Classification by Structure Simple Dendritic process triggered directly by stimulus Encapsulated or unencapsulated nociceptors touch/pressure Complex Receptor cells w/in special sense organs Sensory neuron stimulated by bipolar neuron Photoreceptors Mechanoreceptors Olfactory receptors Gustatory receptors

Table 13.1.1

Table 13.1.2

Adaptation of Sensory Receptors sensory receptors subjected to unchanging stimulus Receptor membranes become less responsive Receptor potentials decline in frequency or stop Pressure, touch, & smell receptors adapt quickly Merkel’s discs, Ruffini’s corpuscles, & interoceptors for blood chemicals adapt slowly Pain receptors & proprioceptors do not adapt

Structure of a Nerve

Classification of Nerves by Directionality Sensory (afferent) – signals TO CNS Motor (efferent) – signals FROM CNS Mixed – both sensory & motor most common somatic & autonomic signals

Regeneration of Nerve Fibers Mature neurons are amitotic If soma remains intact, neuron can regenerate Figure 13.4

12 pairs of nerves directly from brain Sensory, motor, or mixed Cranial Nerves 12 pairs of nerves directly from brain Sensory, motor, or mixed I - XII according to anterior level of origin Named by to innervated organs/function Four cranial nerves carry parasympathetic fibers serving muscles & glands

Cranial Nerves

Summary of Cranial Nerves

Spinal Nerves Figure 13.6

Spinal Nerve Roots (ANS) Figure 13.7a

Interlacing nerve networks Nerve Plexuses Interlacing nerve networks cervical brachial lumbar sacral Branches of plexus contain fibers from several nerves Every muscle innervated by multiple spinal nerves

Cervical Plexus Figure 13.8

Brachial Plexus Figure 13.9a

Brachial Plexus: Distribution of Nerves Figure 13.9c

Spinal Nerve Innervation: Back, Anterolateral Thorax, & Abdominal Wall Figure 13.7b

Lumbar Plexus Figure 13.10

Sacral Plexus Figure 13.11

Rapid, predictable motor response to a stimulus Reflexes: Intrinsic or acquired Involve only PNS & spinal cord Can relay to higher brain centers Somatic reflexes – skeletal muscle Autonomic reflexes – smooth muscle, glands

Receptor Sensory neuron Integration center Motor neuron Effector Reflex Arc Components Receptor Sensory neuron Integration center Motor neuron Effector Receptor 1 2 3 4 Sensory neuron Integration center 5 Effector Motor neuron Stimulus Skin Spinal cord (in cross-section) Interneuron

Stretch & Deep Tendon Reflexes Proprioceptors in tendons & muscle continually maintain postural contractions & muscle tone These effects are via spinal reflex arcs

Muscle Spindles – Stretch Receptors Intrafusal muscle fibers lacking myofilaments in central regions Wrapped by type Ia & type II fibers afferent fibers Innervated by  efferent fibers

Operation of Muscle Spindles Stimulates action potential in postsynaptic neuron Does not stimulates action potential in postsynaptic neuron

Stretch Reflex Figure 13.17

Flexor & Crossed Extensor Reflex Figure 13.19

The Autonomic Nervous System

Autonomic Nervous System (ANS) Motor neurons that: Innervate smooth & cardiac muscle & glands Subconscious control

ANS differs from the SNS Effectors SNS – skeletal muscle ANS – non-skeletal muscle & gland cells Efferent pathways SNS – single PNS neuron ANS – 2 PNS neurons Target organ responses SNS – contraction of muscle ANS – contraction or relaxation, excretion Neurotransmitters used SNS – acetylcholine ANS – acetylcholine, norepinephrine & epinephrine

Distinctions of Efferent Pathways SNS motor neurons Single neuron extends from CNS to effector Heavily myelinated axons ANS motor neurons Two-neuron PNS chain Preganglionic neuron & postganglionic neuron Lightly myelinated preganglionic axon from CNS to ganglion Unmyelinated postganglionic axon extends to effector

Neurotransmitter Differences SNS neurons release acetylcholine (ACh), which has an excitatory effect In the ANS: Preganglionic fibers release ACh Postganglionic fibers release norepinephrine or ACh effect is stimulatory or inhibitory effect depends on neurotransmitter receptor in cells of effector tissue

Comparison of Somatic & Autonomic Systems Figure 14.2

Anatomy of ANS Short preganglionic Long preganglionic Long postganglionic Ganglia close to spinal cord Long preganglionic Short postganglionic Ganglia on/in target organ Figure 14.3

Parasympathetic (PANS) Divisions of the ANS Sympathetic (SANS) mobilizes the body during stressful situations Parasympathetic (PANS) stimulates maintenance activities & conserves body energy SANS & PANS counterbalance each other’s activity SANS signals usually override PANS

Examples of ANS Effects PANS Lowers BP, heart & respiratory rates Increases gastrointestinal tract activity Superficial arterioles open (smooth muscle relaxed) Pupils are constricted/dilated by light level only SANS Blood flow to organs/skin reduced, flow to muscles increased Heart & respiratory rates increased Iris contracts - Pupils dilate

Parasympathetic Division Outflow Cranial Outflow Cranial Nerve Ganglion Effector Organ(s) Occulomotor (III) Ciliary Eye Facial (VII) Pterygopalatin Submandibular Salivary, nasal, & lacrimal glands Glossopharyngeal (IX) Otic Parotid salivary glands Vagus (X) Located within the walls of target organs Heart, lungs, & most visceral organs Sacral Outflow S2-S4 Located within the walls of the target organs Large intestine, urinary bladder, ureters, & reproductive organs

Parasympathetic Division Outflow Longer preganglionic axons Ganglion near/on target organ Short postganglionic axons Vagus nerve (CN X) innervates all visceral organs

Postganglionic fibers innervate the numerous organs of the body Sympathetic Outflow Sympathetic neurons in lateral horns of spinal cord segments T1 through L2 T1-T4 preganglionic fibers pass through the white rami communicantes & synapse in sympathetic chain ganglia T5-L2 preganglionic fibers pass through the gray rami communicantes & chain ganglia to form splanchnic nerves & synapse in collateral ganglia around abdominal aorta Postganglionic fibers innervate the numerous organs of the body

Sympathetic Outflow Sympathetic neurons in lateral horns of spinal cord segments T1- L2 T1-T4 preganglionic fibers synapse in sympathetic chain ganglia T5-L2 preganglionic fibers form splanchnic nerves & synapse in collateral ganglia on abdominal aorta

Sympathetic Trunks & Pathways

T1-T4 preganglionic axons synapse in the superior cervical ganglion Pathways to the Head T1-T4 preganglionic axons synapse in the superior cervical ganglion Serve skin & blood vessels of the head Stimulate dilator muscles of the iris Inhibit nasal & salivary gland secretions

Pathways to the Thorax T1-T6 preganglionic axons synapse in cervical chain ganglia Postganglionic axons from middle & inferior cervical ganglia enter spinal nerves C4-C8 to innervate the heart, thyroid & skin of neck Other T1-T6 preganglionic axons synapse in nearest chain ganglia to directly serve the heart, aorta, lungs, & esophagus

Pathways with Synapses in Collateral Ganglia T5-L2 preganglionic axons exit sympathetic chain ganglia & form splanchnic nerves Splanchnic nerves form aortic plexus & numerous ganglia Postganglionic axons from abdominal ganglia innervate viscera

Pathways with Synapses in the Adrenal Medulla Axons of the thoracic splanchnic nerve go directly to the adrenal medulla Upon stimulation, medullary cells secrete norepinephrine & epinephrine into the blood greater thoracic splanchnic nerve

Visceral Reflexes Visceral reflexes have the same elements as somatic reflexes Afferent fibers are found in spinal & autonomic nerves

ANS Neurotransmitters PANS Acetylcholine (ACh) released by pre- & postganglionic axons SANS ACh released by preganglionic axons ACh or norepinephrine (NE) released by postganglionic axons Cholinergic fibers – ACh-releasing axons Adrenergic fibers –NE-releasing postganglionic SANS axons Excitatory or inhibitory effects depend upon the receptor type

Cholinergic Receptors Bind ACh Nicotinic receptors Muscarinic receptors Named & distinguished by interaction w/ agonists Nicotine Muscarine Agonist – stimulates effect Antagonist – blocks effect

Cholinergic Receptors Nicotinic Receptors Locations: Skeletal muscle motor end plates, CNS neurons SANS & PANS ganglionic neurons Adrenal medulla cells Ion channels ACh always stimulatory Muscarinic Receptors Locations Cells stimulated by postganglionic PANS fibers, CNS ACh inhibition or excitation depends on receptor subtype subtypes – M1, M2, M3

Receptors that bind to norepinephrin & epinephrine Adrenergic Receptors Receptors that bind to norepinephrin & epinephrine In cells innervated by SANS postganglionic axons Alpha subclasses - 1, 2, NE is stimulatory Beta Subclasses - 1, 2 , 3 NE is generally inhibitory Exception – NE binding to  receptors of the heart is stimulatory

Drugs that Influence the ANS Table 14.4.1

Levels of ANS Control Figure 14.9

Interactions of the Autonomic Divisions Most visceral organs innervated by both sympathetic & parasympathetic fibers results in dynamic antagonisms that precisely control visceral activity Sympathetic fibers increase heart & respiratory rates, & inhibit digestion & elimination Parasympathetic fibers decrease heart & respiratory rates, & allow for digestion & the discarding of wastes