Autonomic Nervous System in a Nutshell

Autonomic Nervous System

Nervous system: Functional divisions

• Somatic: Innervation of skeletal muscles (innervated by component of spinal nerve = efferent fiber i.e. motor neuron).  Also sensory component for skin = afferent, is connected to primary sensory neuron in periphery→carries sensory information to spinal cord.  From work “somite”→thus these sites of innervation arose from the somites (skin, muscles).
• Visceral: organs.  Splanchnic / autonomic.  Has a motor component = visceral efferents – ends on smooth muscle, cardiac muscle, exocrine glands that secrete substances (sweat, saliva, mucus/ not hormones).  Sensory component – information that comes back from these structures.  Very diffuse information = distension, stretch, the condition of the organ- may also lead to pain.  Pain sensed by organs is not the same type of pain sensed by skin.  Heart attacks are sensed by visceral afferents.

Somatic efferent: motor neuron to skeletal muscle

Somatic afferent: sensory neuron from skin

Visceral efferents: motor neuron to smooth muscle, cardiac muscle, exocrine glands

Visceral afferent: sensory information from viscera (organs + other sources).

Autonomic Nervous System: Three divisions supply SM/ cardiac M / exocrine glands.

• Sympathetic: fight or flight. Increased heart rate / respiratory rate / vasodilation in periphery / papillary dilation / vasoconstriction in abdomen.
• Parasympathetic: rest and digest.  Decrease heart rate / RR / increased peristalsis / increased salivary gland activity / urination / defecation.
• Enteric: found only in wall of gastrointestinal tract.  Self sustaining – has both afferents and efferents in the wall.  If cut al connection to GI tract, still have peristalsis.

Common themes for both sympathetic and parasympathetic divisions:

• They both have visceral efferents to same set of effectors (SM, CM, EG)
• They both use a 2 neuron chain / link between the CNS and the effector. Use neuron in the CNS→synapse with periphery autonomic ganglion (contains a motor neuron and has a synapse in it) as opposed to dorsal root ganglion (which does not have synapse and contains a sensory neuron). CNS→ preganglionic neuron(body in CNS) →(leaves CNS) → autonomic ganglion synapseà post ganglionic neuron→ effector.

The somatic efferent uses only one neuron from CNS to skeletal muscle.

The Autonomic efferent uses the 2 neuron system (pre / post ganglionic neurons) to aunomic effectors.

Differences between divisions- have completely different locations for pre / post ganglionic neurons.

Sympathetic nervous system:

Preganglionic neuron in lateral horn of spinal cord in segments T1-L2.

The postganglionic neuron is in either:

• Paravertebral ganglia– also called paravertebral chain, hangs off of spinal nerve. Aka sympathetic trunk / sympathetic chain. Are adjacent to the vertebral body.  There are 22 pairs, thus do not have a ganglion for every spinal nerve (only have 3 ganglia in C region).  Usually 1:1 in T and L region.  Works with muscle, vessels, skin.
• postvertebral ganglia – used for organs.

Paravertebral ganglia:

• White ramus contains the preganglionic axon, is myelinated.  Allows entry into paravertebral ganglion in order to synapse with postganglionic neuron.
• Gray ramus contains the postganglionic axonà allows reentry into dorsal and ventral ramus and off to skin. Is not myelinated.
• For spinal nerve segments that do not contain lateral horns: Upper thoracic segments control the upper portions and lower T segments control lower portions.  They utilize a chain in which more preGN arise from lateral hornsà travel through white ramusà through its own paravertebral ganglion (not synapse yet)à through chain via interganglionic segmentà to another paravertebral ganglion in which it will synpaseà postganglionic neuron then travels through gray ramus to D and V ramus.
• Highest part of chain = superior cervicle ganglion
• Lowst part of chain = ganglion impar on internal aspect of coccyx.

Intercostal nerves are ventral rami.

Prevertebral Ganglia:

• Still have origin of PreGN→through white rami→through paravertebral G→down splanchnic nerve to synapse within prevertebral ganglion→post GN follow blood vessels to the organ where post GN acts on organ in abdomen / pelvis.
• Pairs of prevertebral Ganglia are located along the descending aorta in the abdomen.
• Important nerve branches: celiac, superior mesenteric, inferior mesenteric, aorticorenal ganglia.
• T5-L2 segments supply the abdomen / pelvis through prevertebral ganglia method.
• Special case for adrenal medulla (suprarenal gland) aka adrenal gland. This is a unique situation for the ANS. Innervated by sympathetic division only.
• Adrenal gland: 2 parts
  • Outer portion = cortex
  • Inner portion = medulla. Cells here are specialized neurosecretory cells.  They release norepinephrine

Need way to reach medulla region. The Pre GN travels through normal path through paravertebral ganglion AND prevertebral ganglionà all the way to medulla where it “synapses” (stimulates) with neurosecretory cells themselves, which thus are considered the post GN’s.

The Pre GN’s for adrenal gland come from T9-T12 (approximately).

Parasympathetic:

• The Cranial nerves serve as the preganglionic neurons (3,7,9,10).  These all end in named gangliaà synapse →post GN→go out to structures.
• Cranial nerve 10 = Vagus. It is the wandererà wanders into thorax to supply heart and respiratory tract / esophagusà goes onto supply abdomen (liver, pancreas, kidney etc.).
• The post ganglionic fibers off of Vagus are short and are within organ (come off of ganglia near organ called terminal ganglia)
• Vagus nerve has 100K’s of branchesà preganglionic parasympathetic fibers.

Terminal Ganglia for rectum / sex organs = comes from PSN from Sacral parasympathetic nucleus within S2,3,4 (sacral outflow).

S2-S4 have lateral horns: Use the same 2 cell chain. PreGN travesl from lateral hornà ventral root→avoid paravertebral ganglion→enter ventral ramus→leave ventral ramus as independent branch where it becomes a pelvic splanchnic nerve→goes to organ→terminal ganglia→postGN→organ.

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