Physiology of Pheripheral Nervous SystemSomatic Nervous SystemDept. PhysiologyMedical School, University of Sumatera Utara

Peripheral NervesMixed nerves carry somatic and autonomic (visceral) impulsesThe four types of mixed nerves are:Somatic afferent and somatic efferentVisceral afferent and visceral efferentPeripheral nerves originate from the brain or spinal column

Sensory/Motor + Somatic/VisceralSomaticNervousSystemAutonomicNervousSystem

SomaticVisceralSensory(Afferent)somatic sensory[General Somatic Afferent (GSA)]visceral sensory[General Visceral Afferent (GVA)]Motor(Efferent)somatic motor[General Somatic Efferent (GSE)]visceral motor[General Visceral Efferent (GVE)]

Somatic Nervous SystemControls voluntary actionsMade up of the cranial and spinal nerves that go from the central nervous system to your skeletal muscles

Originate from the brain : Cranial NervesTwelve pairs of cranial nerves arise from the brain They have sensory, motor, or both sensory and motor functionsEach nerve is identified by a number (I through XII) and a nameFour cranial nerves carry parasympathetic fibers that serve muscles and glands

Cranial Nerves

Summary of Function of Cranial Nerves

PNS: Cranial nerves(12 pairs)

Cranial Nerve I: Olfactoryolfactory epithelium through the cribriform plate of the ethmoid bone the olfactory bulb and terminate in the primary olfactory cortex

Functions solely by carrying afferent impulses for the sense of smell

Cranial Nerve II: Opticthe retina of the eye the optic canals and converge at the optic chiasm continue to the thalamus where they synapseFrom there, the optic radiation fibers run to the visual cortexFunctions solely by carrying afferent impulses for vision

Cranial Nerve III: OculomotorFibers from the ventral midbrain pass through the superior orbital fissure to the extrinsic eye musclesFunctions : raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shapeParasympathetic cell bodies are in the ciliary ganglia

Cranial Nerve IV: TrochlearFibers emerge from the dorsal midbrain enter the orbits via the superior orbital fissures innervate the superior oblique musclePrimarily a motor nerve that directs the eyeball

Cranial Nerve V: TrigeminalComposed of three divisions Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

Cranial Nerve VI: AbdcuensPrimarily a motor nerve innervating the lateral rectus muscle

Cranial Nerve VII: FacialMotor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glandsSensory function is taste from the anterior two-thirds of the tongue

Cranial Nerve VIII: VestibulocochlearTwo divisions cochlear (hearing) and vestibular (balance)Functions are solely sensory equilibrium and hearing

Cranial Nerve IX: GlossopharyngealMotor innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary glandSensory fibers conduct taste and general sensory impulses from the tongue and pharynx

Cranial Nerve X: VagusThe only cranial nerve that extends beyond the head and neckThe vagus is a mixed nerveMost motor fibers are parasympathetic fibers to the heart, lungs, and visceral organsIts sensory function is in taste

Cranial Nerve XI: AccessoryPrimarily a motor nerve Supplies fibers to the larynx, pharynx, and soft palateInnervates the trapezius and sternocleidomastoid, which move the head and neck

Cranial Nerve XII: HypoglossalInnervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech

Originate from the spinal column: Spinal NervesFigure 13.6

Spinal Nerves: RootsEach spinal nerve connects to the spinal cord via two medial rootsEach root forms a series of rootlets that attach to the spinal cord Ventral roots arise from the anterior horn and contain motor (efferent) fibersDorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Spinal Nerves: Roots

DermatomesA dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerveAll spinal nerves except C1 participate in dermatomes

Dermatomes

Motor EndingsPNS elements that activate effectors by releasing neurotransmitters at:Neuromuscular junctions Varicosities at smooth muscle and glands

Innervation of Skeletal MuscleTakes place at a neuromusclular junctionAcetylcholine is the neurotransmitter that diffuses across the synaptic cleftACh binds to receptors resulting in:Movement of Na+ and K+ across the membraneDepolarization of the interior of the muscle cellAn end-plate potential that triggers an action potential

The somatic nervous system signals skeletal muscles. A somatic (motor) neuron has its cell body in the CNS and its axon terminates at a skeletal muscle.Somatic motor neurons are the final common pathway. Skeletal muscle activity can be affected only by acting on these motor neurons.Note: a single peripheral axon, no peripheral ganglia or post synapitic fibers

The neuromuscular junctionThis junction is where the axon of a somatic efferent neuron signals a skeletal muscle fiber (cell)neuromuscular junction or terminal button. The motor end plate is the specialized part of the muscle cell membrane facing the terminal button.

Signalling is mediate by AChAn action potential at the axon terminal opens voltage-gated calcium channels in the terminal button. Calcium ions diffuse into the terminal button from the ECF. By this calcium influx, acetylcholine is released by exocytosis from vesicles in the terminal button. Acetylcholine diffuses through the space between the nerve cell and muscle cell.This neurotransmitter binds to receptor sites on the motor end plate of the muscle cell membrane.This binding opens channels for sodium influx into the ICF of the muscle cell.This produces a local current flow that opens adjacent sodium channels in the motor end plate.An action potential is initiated through the muscle fiber.

Axon of motor neuronMyelin sheathAxon terminalTerminal buttonVesicle of acetylcholineAcetylcholine receptor siteAcetycholinesterasePlasma membraneof muscle fiberVoltage-gatedNa+ channelChemically gatedcation channelMotor end plateContractile elements within muscle fiberVoltage-gatedcalcium channelAction potentialpropagationin motor neuronAction potentialpropagationin muscle fiber

Acetylcholinesterase destroys acetylcholine. Breakdown of ACh by Acetylcholinesterase terminates signalling. This terminates the induction of action potentials of the muscle fiber and allows it to relax.

Neuromuscular toxins and agonists. The neuromuscular junction is vulnerable to several chemical agents and diseases.Examples are:black widow spider venomcauses release of ACh from all storage vesicles botulinum toxin, prevents the release of acetylcholine. Curare blocks the acetylcholine receptorNeostigmine limits acetylcholinesterase activityOrganophosphates (nerve gas agent) permanently inhibit acetylcholinesterase activity