Brainstem Control of Autonomic Function

Autonomic Nervous System

Definition: a motor system that innervates the heart, smooth muscle, and glands. Sensory is NOT part of the system

Autonomic pathway organization: primary visceral sensory neuron in peripheral ganglia Þ secondary order visceral sensory relay that projects locally and to the thalamus Þ visceral motor neurons. All autonomic cell groups are regulated by neurons above the level of the spinal cord and brainstem

Basic unit: two-neuron chain that consists of:

Preganglionic – cell body is located in the gray matter of the spinal cord or brainstem. Axon projects out of the CNS onto the postganglionic neuron
Postganglionic – cell body is located in a ganglion outside the CNS. Axon projects onto an end organ (heart, smooth muscle, or gland).

Central and reflex control: the basic two neuron autonomic effector does not operate autonomously. Activity of the preganglionic neuron is modulated reflexively and/or by specific centers in the brain (i.e. medulla, pons, hypothalamus, cerebral cortex)

3 divisions: Sympathetic, Parasympathetic, and Enteric

Anatomy

Sympathetic (thorcolumbar) nervous system

Preganlionic neuron – cell body in the intermediolateral cell column of spinal cord segments T1 to L2,3. Axons are myelinated (termed white rami), pass out of the spinal ventral root, enter chain root ganglia and synapse with postganglionic neurons (connections converge and diverge) in ganglia of peripheral nervous system

Postganglionic neuron – unmyelinated; innervate heart, smooth muscle, and glands. Emerge as gray rami innervates blood vessels (vasomotor), hair (pilomotor), and sweat glands (sudomotor). Cell bodies located in 2 types of ganglia:

(1) Chain ganglia (vertebral paravertebral ganglia) – 2 chains located next to each vertebrae on both sides. Each chain has 1 ganglion except cervical region which supply symp innervation to the heart and head (cervical ganglia fusion of superior, middle, and inferior cervical ganglion)

Stellate ganglion – fusion of inferior and 1^st thoracic ganglion

Sympathetic trunk – each chain ganglion and connecting axons (there are 2 – one on either side of spine)

(2) Prevertebral ganglia (collateral ganglia): irregular ganglionic mases that surround visceral branches of aorta. Innervate abdominal and pelvic viscera (stomach, intestine, adrenal medulla, bladder). Paired or unpaired. Names based on arteries with which they are associated (celiac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion)

Parasympathetic (craniosacral) nervous system

Preganglionic: Associated with cranial nerves: III (oculomotor), VII (facial), IX (glossopharyngeal), X (Vagus)

Innervates smooth muscle and glands of head, heart, and abdomen

X (Vagus) is most important, since it supplies all thoracic and abdominal viscera; motorneurons are found in dorsal motor nucleus (contains parasympathetic only) and nucleus ambiguus.

Unmyelinated and long (synapse with ganglia close to end organ, in contrast to sympathetic, which have chain ganglia)

Sacral location: intermediolateral gray segments S2-S4 Þ form the pelvic nerve innervates pelvic viscera

Postganglionic: short and unmyelinated (cell body in ganglia next to end organ), Cranial ganglia (ciliary, otic, submaxillary)

Sensory Inputs:

Afferent innervation from primary sensory neurons in cranial and dorsal root ganglia (DRG)

Cranial ganglia: Trigeminal (V), geniculate (VII), petrosal (IX), nodose (X); 10% of DRG innervate viscera

Nucleus Tractus Solitarius (nTS) – receives primary afferent projections from visceral sensory neurons in cranial ganglia

Control of Respiration and Blood Pressure: primary sensory neuron Þ nTS (interneuron) Þ spinal motor neuron

Deep Breathing Test Þ parasympathetic

Axon Reflex Sweat Test Þ post-ganglionic sympathetic

Respiratory Control

Respiratory rhythm generation: generation of breathing patterns is Autonomous; NO sensory input is required

Brainstem neurons involved:

(1) Nucleus Tract Solitarius – contains (1) Afferent chemo and pulmonary stretch receptors (2) dorsal respiratory neurons (fire during inspiration) that synapse in phrenic motor nucleus Þ innervates diaphragm

(2) Ventrolateral Medulla (VLM): contain pacemaker cells of respiration Þ generate bursts of spikes that are the source of the respiratory rhythm

vagal motorneurons in dorsal nucleus ambiguus (innervate laryngeal and pharyngeal muscles)
ventral lateral reticular formation (RF) neurons – innervate intercostal and phrenic motorneurons in spinal cord

Dorsolateral pons (pontine respiratory group, PRG) – modulate activity of the respiratory rhythm generator;

PRG has reciprocal connections with ventrolateral reticular formation, nTS, limbic system (amygdala), hypothalamus

Cortex – influences breathing via peripheral sensory neurons (most powerful influences)

Inspiration – active process, neurons fire in phase

Nucleus Tractus Solitarius (dorsal) drive phrenic motorneurons (moves diaphragm)

VLM (ventral) neurons drive phrenic and intercostal motorneurons – from reticular formation neurons

Both influenced by:

Chemoreceptor (carotid body) – low O₂Þ petrosal ganglion (IX) Þ tractus solitarius Þ Ý inspiratory activity

Pulmonary stretch receptor Þ nodose ganglion Þ inhibits inspiration (Hering-Breuer reflex)

Expiration – passive process (may become active during Ý ventilatory drive (i.e. exercise), neurons fire out of phase

VLM (medullary expiratory neurons) – inhibits inspiratory neurons. Ý firing in response to ß CO₂

PCO₂ – sensed within the CNS (ventral surface of medulla) connected to nucleus solitarius

Lesions

Cortex Þ Cheynes-Stokes breathing (hyperinflation then apnea) Abnormal but rhythmic why? Brainstem is intact medullary regulator is still operating

Brainstem (medulla) Þ affects solitarius Þ rhythmicity is lost

Pons Þ still rhythmic, inputs determine details of rhythmicity NOT rhythmicity itself

Blood Pressure Regulation

Determinants of BP: Cardiac output, peripheral resistance (vascular tone)

Cardiac output: regulated by sympathetic and parasympathetic (vagal)

Vascular tone: maintained by tonic vasomotor activity of sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (ILC) of thoracic and lumbar spineal cord.

Ventrolateral medullary reticular formation (pre-motor neurons) – tonic input from this formation project to interomedial column of sympathetic chain and maintains SPN fibers (clonadine stimulates this area; close to ventral respiratory group)

Brain stem neurons involved:

(1) nTS – site of afferent baro- and chemoreceptors, secondary neurons: homeostasis primarily attributable to baroreceptor reflex

sensory fibers from aortic arch (via nodose ganglion X) and carotid sinus (via petrosal ganglion IX) baroreceptors terminate in specific nTS subnulei

afferent stimulation causes hypotension and bradycardea; lesion of nTS Þ hypertension and abolition of baroreflexes

nTS neurons excite cardiac vagal motorneurons Þ ß HR and CO

baroreflex has inhibitory component; nTS neurons make inhibitory connections with symp pre-motor neurons in VLM

(2) VLM – pre-motor neurons to symp neurons in the ILC

Supramedullary structures (hypothalamus, cerebellum, cortex) – mediate cardiovascular adjustments to changing behavioral demands (i.e. stress, etc), but structures above medulla cannot maintain BP

Vasodilation pathways: arise in motor cortex (used during exercise) and amygdala (defensive behaviors)
Vasoconstriction pathways: cerebellum – mediates homeostasis of BP during postural changes (e.g. orthostatic hypotension)

Review

Homeostatic maintenance of BP: primary sensory Þ nucleus solitarius secondary neurons

Sympathetic limb: ventromedullar reticular formation Þ preganglionic Þ post ganglionic Þ BV or heart;

Parasympathetic limb (vagus): primary neuron Þ dorsal vagal motor nucleus Þ para control (e.g. BP Ý Þ Ý vagus (because inhibitory))

Control of Respiration