PNS Efferent Division Somatic & Autonomic

PNS Efferent Division Somatic & Autonomic

Efferent Pathways Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector (lacks ganglia) Pathways in the ANS are a two-neuron chain –The preganglionic (first) neuron has a lightly myelinated axon. The ganglionic (second) unmyelinated neuron extends to an effector organ via the postganglionic axon

Comparison of Somatic and Autonomic Systems

Efferent Reflex Pathways

Synapses in Autonomic Nerves Varicosities NT released to ECF No cleft Impact Large area Slow acting Long duration Figure 11-8: Varicosities of autonomic neurons

Efferent Pathways: Motor & Autonomic Figure 11-11: Summary of efferent pathways

Autonomic Nervous System Sympathetic & Parasympathetic Regulation of the “internal” environment generally outside of our conscious control: “autonomous” Innervates organs that are not usually under voluntary control - glands, smooth/cardiac muscle Efferent (motor) systems “visceromotor” - effectors are part of visceral organs and blood vessels Involve 2 neurons that synapse in a peripheral ganglion Presynaptic neuron is myelinated and postsynaptic neuron is unmyelinated Autonomic nerves release NT that may be stimulatory or inhibitory

ANS Autonomic nerve pathway Extends from CNS to an innervated organ Two-neuron chain Preganglionic fiber (synapses with cell body of second neuron) Postganglionic fiber (innervates effector organ)

Divisions of the ANS Both have preganglionic neurons that originate in CNS. Both have postganglionic neurons that originate outside of the CNS in ganglia. Figure 9-6

Anatomical Differences between the Sympathetic and Parasympathetic Divisions PNS Fibers originate from cranial and sacral areas of CNS Preganglionic fibers are longer Very short postganglionic fibers Preganglionic fibers release acetylcholine (Ach) Postganglionic fibers release acetylcholine SNS Fibers originate in thoracic and lumbar regions of spinal cord Most preganglionic fibers are short Long postganglionic fibers Preganglionic fibers release acetylcholine (Ach) Most postganglionic fibers release noradrenaline (norepinephrine)

Neurochemistry of the ANS All preganglionic fibers release acetylcholine (=cholinergic) Postganglionic PARASYMPATHETIC fibers release acetylcholine(=cholinergic) Postganglionic SYMPATHETIC fibers release norepinephrine(=adrenergic) Exceptions: Adrenal medullary chromaffin cells secrete epinephrine Sympathetic nerves innervating sweat glands secrete acetylcholine Sympathetic nerves innervating blood vessels in skeletal muscle secrete acetylcholine Sympathetic nerves innervating renal blood vessels secrete dopamine

Functional Differences Sympathetic - “fight or flight” Catabolic (expend energy) Release of norepinephrine (NT) from postganglionic fibers and epinephrine (NT) from adrenal medulla. Mass activation prepares for intense activity. Heart rate (HR) increases. Bronchioles dilate. Blood [glucose] increases. Parasympathetic - “feed & breed”, “rest & digest” Maintain homeostasis Normally not activated as a whole, stimulation of separate parasympathetic nerves. Release ACh as NT. Relaxing effects: Decreases HR. Dilates visceral blood vessels. Increases digestive activity. Dual innervation of many organs — having a brake and an accelerator provides more control

Autonomic Pathways

Synaptic Organization

Adrenal Glands Adrenal medulla secretes epinephrine (Epi) and norepinephrine (NE) when stimulated by the sympathetic nervous system. Modified sympathetic ganglion, derived from same embryonic tissue that forms postganglionic sympathetic neurons. Sympathoadrenal system: mass activation of the sympathetic nervous system. Innervated by preganglionic sympathetic fibers. Stimulation of preganglionic fiber prompts secretion of hormones into blood About 20% of hormone release is norepinephrine About 80% of hormone released is epinephrine (adrenaline)

Adrenergic and Cholinergic NTs ACh is NT for all preganglionic fibers of both sympathetic and parasympathetic nervous systems. Transmission at these synapses is termed cholinergic: ACh is NT released by most postganglionic parasympathetic fibers at synapse with effector. Axons of postganglionic neurons have numerous varicosities along the axon that contain NT.

Adrenergic and Cholinergic NTs (continued) NT released by most postganglionic sympathetic nerve fibers is NE. Epi, released by the adrenal medulla is synthesized from the same precursor as NE. Transmission at these synapses is called adrenergic Collectively called catecholamines.

Responses to Cholinergic Stimulation All somatic motor neurons, all preganglionic and most postganglionic parasympathetic neurons are cholinergic. Release ACh as NT. Somatic motor neurons and all preganglionic autonomic neurons are excitatory. Postganglionic axons, may be excitatory or inhibitory. Muscarinic receptors: ACh binds to receptor. Requires the mediation of G-proteins. Nicotinic receptors (ligand-gated): ACh binds to 2 nicotinic receptor binding sites. Opens a Na+/K+ channel. Always excitatory.

Responses to Cholinergic Stimulation (continued) Figure 9-1

Responses to Adrenergic Stimulation Has both excitatory and inhibitory effects All act through G-proteins Alpha adrenergic responses due to Ca2+ A1 : excitatory constricts smooth muscles A2 : inhibitory decreases contraction of smooth muscle Beta adrenergic responses due to cAMP B1 : excitatory increases HR and force of contraction B2 : inhibitory relaxes bronchial smooth muscles

Responses to Adrenergic Stimulation

Organs With Dual Innervation Most visceral organs receive dual innervation (innervation by both sympathetic and parasympathetic fibers). Antagonistic effects: Sympathetic and parasympathetic fibers innervate the same cells. Actions counteract each other ex. Heart rate Complementary - sympathetic and parasympathetic stimulation produces similar effects ex. salivary gland secretion Cooperative - Sympathetic and parasympathetic stimulation produce different effects that work together to produce desired effect ex. Parasympathetic fibers penile erection Sympathetic fibers ejaculation

Dual Antagonistic Innervation

Organs Without Dual Innervation Regulation achieved by increasing or decreasing firing rate Adrenal medulla, arrector pili muscle, sweat glands, and most blood vessels receive only sympathetic innervation

Sympathetic vs Parasympathetic

Levels of ANS Control The hypothalamus is the main integration center of ANS activity Subconscious cerebral input via limbic lobe connections influences hypothalamic function Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

Levels of Autonomic Control Figure 16.12

Regulation of the ANS by CNS Prefontal association cortex and limbic system -Responsible for visceral responses that are characteristic of emotional states and behavior Hypothalamus – sympathetic response to anger or fear is brought on by hypothalamus through medulla Medulla: Most directly controls activity of autonomic system Location of centers for control of cardiovascular pulmonary, urinary, reproductive and digestive systems. Some autonomic reflexes integrated at spinal cord (urination, erection)

Somatic Motor Controls Skeletal Muscles Body movement Appendages Locomotion Single neuron CNS origin Myelinated Terminus Branches Neuromuscular junction

Somatic Efferent Consists of the axons of motor neurons which originate in the spinal cord and terminate on skeletal muscle Acetylcholine released from a motor neuron stimulates muscle contraction Motor neurons are the final common pathway by which various regions of the CNS exert control over skeletal muscle activity The areas of the CNS that influence skeletal muscle activity by acting through the motor neurons are the spinal cord, motor regions of the cortex, basal nuclei, cerebellum, and brain stem

Nerve Stimulus Skeletal muscles are stimulated by motor neurons of the somatic nervous system Axons of these neurons travel in nerves to muscle cells Axons of motor neurons branch repeatedly as they enter muscles Each axonal ending forms a neuromuscular junction with a muscle fiber

Neuromuscular Junction

Neuromuscular Junction When a nerve impulse reaches the neuromuscular junction: Voltage-regulated calcium channels in the axon membrane open and allow Ca2+ to enter the axon Ca2+ inside the axon terminal causes some of the synaptic vesicles to fuse with the axon membrane and release ACh into the synaptic cleft (exocytosis) ACh diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma Binding of ACh to receptors on the sarcolemma initiates an action potential in the muscle ACh is quickly destroyed by acetylcholinesterase

Figure 7.6: Events at a neuromuscular junction. Fig. 7-6, p. 245

Motor Unit: Neuromuscular Functional Unit A motor neuron and all the muscle fibers it supplies is called a Motor Unit Each muscle has at least one motor nerve that may contain hundreds of motor neuron axons. Axons branch into terminals, each forming a neuromuscular junction with a single muscle fiber

Motor Unit The number of muscle fibers per motor unit can vary from a few to several hundred Muscles that control fine movements (fingers, eyes) have small motor units Large weight-bearing muscles (thighs, hips) have large motor units Muscle fibers in a single motor unit are spread throughout the muscle. As a result, stimulation of a single motor unit causes weak contraction of the entire muscle

Summary Autonomic branches: sympathetic and parasympathetic Regulate glands, smooth & cardiac muscles Team with endocrine to regulate homeostasis Are regulated by hypothalamus, pons & medulla Have pathways with 2 neurons and a ganglion Use varicosities to release NTs Have diverse receptors: tonic & antagonistic regulation

Summary Efferent motor neurons control skeletal muscles Single long myelinated neuron from CNS Neuromuscular junction structure & mechanism