Lecture 14: Visceral Motor System

Visceral (Autonomic) Nervous System

It acts on smooth muscle fibers, cardiac muscle fibers, and glands

It functions subconsciously and involuntarily

Two branches → sympathetic and parasympathetic → dual innervation

Sympathetic Nervous System

Mediates the “4 F responses” → fight, flight, fright, and sexual activity

Uses energy → increased peripheral motor activity

Uses Norepinephrine and Acetylcholine as neurotransmitters

Parasympathetic Nervous system

Most active during rest and stimulates digestive activities

Produces energy → increased internal motor activity

Uses Acetylcholine as a neurotransmitter

Autonomic Nervous System

Concerned with homeostasis

Governed by descending pathways from the hypothalamus and the reticular formation

Sends information to preganglionic neurons in the brainstem and spinal cord

Somatic vs Autonomic

Lower motor neurons of the autonomic nervous system are located outside the central nervous system → in “autonomic ganglia” (cell bodies)

Contacts between visceral motor neurons and target organs → volume transmission → dumping of neurotransmitters into a cellular space and receptors take it up (autonomic)

Hypothalamus and regions in the brainstem regulate the visceral motor system (vs. motor and pre-motor cortex)

Variability of neurotransmitters used and postsynaptic receptors is higher → more degrees of freedom in regulating homeostatic/autonomic functions

Preganglionic Neurons

Sympathetic → located in the thoracic and lumbar spinal cord

Parasympathetic → located in the brainstem and sacral

Fight or flight

• Pupils dilate → more light

• Blood vessels in skin and gut constrict → more blood to muscles

• Hairs stand on end → fearsome appearance

• Bronchi dilate → increase oxygenation

• Heart rate accelerates → maximal blood transport

• Digestion is stopped → preserves temporarily unnecessary energy

• Stimulation of epinephrine and norepinephrine and glucose → increase metabolism

Sympathetic Neurons

Preganglionic neurons in lateral horn of spinal cord

Neurons in sympathetic ganglia = primary/lower motor neurons → Directly innervate smooth muscles, cardiac muscles and glands

Parasympathetic division functions

The parasympathetic system is responsible for stimulation of "rest-and-digest" activities that occur when the body is at rest, including sexual arousal and SLUDD:

• Salivation

• Lacrimation (tears)

• Urination

• Digestion

• Defecation

Parasympathetic pathway needed

Enteric Component

Myenteric Plexus → Regulating the musculature of the gut

Submucous (Meeissner’s plexus) → Chemical monitoring & glandular secretion

ANS Neurotransmission

Sympathetic ganglilon cells → (mostly) release Norepinephrine on target cells

Parasympathetic ganglion cells → use only Acetylcholine

Neurotransmission onto target cells

Effects of Acetylcholine (parasympathetic) and Norepinephrine (sympathetic) are usually opposite

Effect also depends on postsynaptic receptor type.

→ Example: Alpha and beta-adrenergic (Norepineprhine = noradrenaline) receptors on smooth muscles of blood vessels (sympathetic regulation) → constricts vs dilates

Visceral Sensory Information

• Provides feedback to local reflexes that modulate visceral motor activity

• Informs higher integrative centers of threatening conditions and/or recruit coordination when needed

• Some synapses in dorsal horn are on anterolateral system neurons (pain) and intermediate gray regions near lateral horn (ANS) → referred pain

• Also synapse on central target = nucleus of the solitary tract (NTS) in medulla

Nucleus of the Solitary Tract

Central autonomic network → integration of visceral sensory information with input from other sensory areas → also integrate with emotional processing (ex. blushing)

The relay center for visceral sensory and motor integration

Provides input to:

• Visceral motor nuclei

• Reticular formation premotor centers

• Integrative centers in amygdala and hypothalamus

• Parabrachial nucleus

Hypothalamus Regulation

Plays an important role in the coordination and expression of visceral motor activity

It’s the central location in the brain → integrates info from the forebrain, brainstem, and spinal cord

Major outflow is to the reticular formation premotor circuits + direct projections to preganglionic neurons

Hypothalamus and Brainstem regulate ANS

The hypothalamus is involved in the regulation of:

• Blood flow (heart rate, vasomotor tone, blood osmolarity, drinking, salt intake)

• Energy metabolism (monitors blood glucose levels, feeding behavior, digestion, metabolic rate, temperature)

• Reproductive activity (influence gender identity, mating behavior, menstrual cycles, pregnancy, lactation)

• Response to threatening situations (release of stress hormones, balance between sympathetic vs. parasympathetic tone, regional blood flow)

• Sleeping/waking (circadian rhythms)

Autonomic cardiac function

Baroreceptors measure arterial blood pressure → via vagus nerve to nucleus of the solitary tract

Chemoreceptors measure oxygenation and CO2 → Via glossopharyngeal nerve to nucleus of the solitary tract

→ Rise in blood pressure → inhibition of sympathetic tone

→ Parallel increase in parasympathetic activity

Autonomic bladder function

Sympathetic → smooth muscle of bladder relaxes & internal urethral sphincter closes → lower motor neurons

Parasympathetic → smooth muscle of bladder contracts & bladder empties → mechanoreceptors

Horner’s Syndrome

• Damage to the pathway that controls the sympathetic division of the VMS to the head & neck

• Main features:

• Decreased diameter of the pupil on the side of the lesion

• Droopy eyelid

• Sunken appearance of the affected eye

• Decreased sweating, increased skin temperature, and flushing of the skin on the same side of the face & neck

• Damage along pathway from hypothalamus & reticular formation to sympathetic preganglionic neurons in the intermediolateral cell column of the thoracic spinal cord

Paraventricular nucleus

Receives inputs from most other nuclei of hypothalamus

Regulates release of hormones from pituitary gland

Projects to brainstem and preganglionic neurons to regulate ANS