Autonomic Nervous System

BIO 232

Anatomy and Physiology II

Autonomic Nervous System

• Sympathetic NS

• Parasympathetic NS

• Enteric NS

Neurons of Autonomic NS

• Preganglionic

• Cell bodies in spinal cord or brain stem

• Postganglionic

• Cell bodies in peripheral autonomic ganglia

• Visceral Afferents

Enteric NS

• Peripheral reflex network

• In wall of gastrointestinal tract

• Organizes gastrointestinal movements

• Afferent and efferents and interneurons

• Connects with sympathetic and parasympathetic NS

Peripheral Sympathetic NS

• Preganglionic - in thoracic & upper lumbar spinal cord

• Axons run in autonomic ganglia

• Postganglionic

• Axons in sympathetic chain or paravertebral ganglia

• Effector organs

• Blood vessels, hair, viscera, pupils, cardiac muscle, gland

Sympathetic NS

• Inhibitory effect on non sphincter muscle of viscera, digestive glands & SM of bronchi.

• All other effects excitatory!

Sympathetic Nervous System

Parasympathetic NS

• Preganglionic - in sacral cord and brain stem

• Vagus (CN X) - 75% of parasymp. outflow

• Postganglionic - short fibers

• All parasymp. innervated organs also sympathetically innervated (e.g. bladder, rectum,GI tract, heart, lungs, lacrimal & salivary glands)

Parasympathetic NS

• Not all Sympath. innervated structures are parasympath. innervated

• Exceptions

• Entire vascular system, adrenal medulla & pilomotor muscle - only sympath.

Parasympathetic Nervous System

Functions of the Autonomic NS

• Sympathetic and Parasympathetic systems are antagonistically organized

• Sympathetic - largely fight or flight response

• Works with adrenal medulla - epinephrine

• Parasympathetic - antagonizes sympathetic activity

• Not normally activated as a whole

Neurotransmitters of the Autonomic NS

• All preganglionic fibers - cholinergic

• Postganglionic parasympathetic fibers - cholinergic

• Postganglionic sympathetic fibers - NE - adrenergic

• Some sympathetic - cholinergic

• Sweat glands & blood vessels of skin and skeletal muscle

Autonomic Transmitters

Epinephrine & Norepinephrine

Adrenergic Stimulation Responses

• Receptors mediate both excitatory & inhibitory effects

• Alpha (a₁ & a₂) - constriction of smooth muscle (Vasoconstriction)

• Beta - promotes relaxation of smooth muscle

• Digestive system, bronchioles & uterus

• Some stimulation of contraction of cardiac muscle

Other Autonomic Transmitters

• Nonadrenergic - noncholinergic

• ATP

• VIP

• NO

Reflexes

• Somatic motor reflexes

• Autonomic (Visceral) Reflexes

Autonomic Reflexes

• Visceral functions primarily regulated by means of autonomic reflex

• Baroreceptor Reflex

• Baroreceptors (stretch receptors) in walls of aorta and carotid

• When stretched by high pressure

• Signals transmitted to brain stem

• These inhibit sympathetic outflow to heart and vessel

• Excite Vagus - parasympathetic system

• This allows arterial pressure to return toward normal

Baroreceptor Reflex

• Net effects

• Vasodilation

• Decreased heart rate & strength of contraction

• Conversely low pressure has opposite effect - causing pressure to rise back to normal

Control of Autonomic Functioning

• Several levels of control - in the spinal cord, brain stem, hypothalamus and cerebral cortex

• In general the cerebral cortex is the “head ganglion”

• Reflex control - spinal cord & brain stem

• Hypothalamic integration centers interact with both higher & lower centers to orchestrate autonomic, somatic and endocrine responses

• Cortical control influences autonomic functioning via the limbic system - rarely under conscious (e.g. biofeedback)

Referred Pain

• Pain stimuli arising in the viscera are perceived as somatic in origin

• Visceral pain afferents travel along the same pathways as somatic pain afferents

• Heart attack - pain radiates to the superior thoracic wall and the medial aspect of the left arm

• The same spinal segments (T1-T5) innervates the heart and the regions and the regions to which the pain signals from the heart tissue are referred

• Brain interprets most such signals as coming from the more common somatic pathway