Somatic and Autonomic Nervous Systems

Anatomy

31 spinal nerves

• 8 cervical

• 12 thoracic

• 5 lumbar

• 5 sacral

• 1 coccygeal

Somatic motor vs Visceral motor

Somatic sensory vs Visceral sensory

Somites

will become body tube (peripheral skeleton, muscles, dermis)

What is responsible for body tube?

somatic nervous system

What is responsible for gut tube?

autonomic nervous system

CNS

• brain and spinal cord

• formed by neural tube

PNS

• spinal nerves and cranial nerves

• ganglia formed by neural crest cells

Somatic nervous system

somatic nerves innervate somites and their associated dermis

• somatic motor: (efferent) innervates skeletal muscle

• somatic sensory (afferent): touch, vibration, proprioception, pain

Somatic nerves

Sensory cell bodies are in a ganglia outside the CNS (DRG for spinal nerves, cranial nerve ganglia for cranial nerves)

• Derived from neural crest

• Somatic sensory neuron have distal axons that innervate a dermatome and proximal axons that synapse in the CNS

Motor neurons are located in the ventral horn of the spinal (brainstem nuclei for cranial nerves)

• Derived from neural tube

• Somatic motor neurons in the spinal cord (lower motor neurons) synapse directly on their targets

Somatic sensory

• Pseudo-unipolar neurons from in the Neural crest

• Cell bodies become the dorsal root ganglia

• Proximal axon enters the CNS

• Distal axon innervates a dermatome

Somatic motor

• Multipolar neuron

• Forms in the CNS (Ventral horn)

• Distal axon innervates a myotome

Dorsal and ventral root

• Dorsal root: sensory, innervates a dermatome

• Ventral root: motor, innervates a myotome

Dermatome

“real estate” on the skin that a particular spinal nerve is responsible for

Myotome

a muscle/muscle group/part of a muscle that a single spinal nerve innervates

• migrate during development, but maintain innervation

ANS

Sensory: (Visceral sensory)

• These are afferents that provide chemoreceptive, stretch receptive and mechanosensation for the internal body (Gut Tube)

Motor: (Visceral motor)

• These are efferents that supply smooth muscle, heart muscle and glands (Gut Tube)

  • (glands and blood vessels in the skin start as gut tube)

Visceral sensory

psuedounipolar and fibers enter the spinal cord along with their somatic counterparts

• pseudo-unipolar cell body in the DRG.

• Distal axon in the viscera (either gut tube or cardiovascular)

• Proximal axon enters the spinal cord but not necessarily that close to where the distal axon is innervating

• Visceral reflex afferent fibers provide input to the CNS for control of glands and smooth muscle

• Visceral pain afferent fibers detect distention and pain, however, the pain is generally dull and poorly localized

Visceral motor

controls smooth muscle cardiac muscle and glands are part of a two-neuron system to the gut tube

• Preganglionic neurons are in the brainstem or lateral horn of the spinal cord

• Postganglionic neurons in a ganglia outside the CNS

• Could be sympathetic or parasympathetic

Generalities of sympathetics

• Fight or Flight increase body’s “readiness”

• Preganglionic neuron in the lateral horn of spinal cord

• Postganglionic neuron in the sympathetic chain (we will learn exceptions here)

• Preganglionic neurotransmitter is Ach

• Postganglionic neurotransmitter is NE or ACh

Generalities of parasympathetis

• Relaxes most systems in body that sympathetics excite (“rest and digest”)

• Preganglionic neurons in the brainstem or sacral nucleus

• Postganglionic in ganglia closer to target

• Preganglionic neurotransmitter is Ach

• Postganglionic neurotransmitter is Ach

Specific sympathetic function

• Increases heart rate and blood pressure (increases stroke volume)

• Increases respiration

• Decreases blood flow to areas not essential for immediate survival (skin and digestive system)

• Increases blood flow the muscles, heart and brain

• Dilates pupils

Sympathetic Route 1: When traveling to a dermatome associated with a thoracic or upper lumber level

Preganglionic cell originates in lateral horn

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Leaves via ventral root of spinal cord

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Enters white ramus communicans

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Enters the sympathetic chain ganglia

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Synapses in the post ganglionic cell body

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Leaves by gray ramus

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To spinal nerve

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Travels to effector

Sympathetic Route 2: Going to the cervical or Sacral dermatomes

Preganglionic cell body originates in the lateral horn

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Leaves via ventral root of spinal cord

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Goes through white ramus communicans

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Enters sympathetic chain ganglia

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Travels up/down the chain to its preferred ganglia and synapses

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Exits through the cervical gray ramus communicans

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Into spinal nerve to effector

Sympathetic Route 3: Thoracic viscera

Preganglionic cell body originates in the lateral horn

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Leaves via ventral root of spinal cord

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Goes through white ramus communicans

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Enters sympathetic chain ganglia

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Travels up/down/lateral chain ganglia

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Synapses at desired ganglia

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Leaves ganglia and enters specialized (does not leave by gray ramus)

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Multiple cardiac plexi are created around the organ (forms own specialized cardiac nerves)

Sympathetic Route 4: Abdominal viscera

Preganglionic cell body originates in the lateral horn

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Exits via ventral root of spinal cord

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Goes through white ramus communicans

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Enters the sympathetic chain ganglia (does not synapse in chain ganglia)

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Exits into splanchnic nerve (without synapsing)

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Travels to vasculature (paraaortic ganglia) and synapses at ganglia (on aorta)

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Travels through postganglionic route (blood vessels) to the effector

Sympathetic Route 5: Adrenal gland

Preganglionic cell body originates in lateral horn

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Exits via ventral root of spinal cord

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Goes through white ramus communicans

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Enters the sympathetic chain ganglia

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Exits into splanchnic nerve

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Splanchnic nerve goes directly into adrenal medulla

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No synapse because there is no postganglionic neuron; preganglionic releases

achetylcholine which result in immediate release of norepinephrine and epinephrine from adrenal medulla (adrenal medulla releases directly into blood stream)

(*uses blood instead of fibers to get to its destination)

Parasympathetic Route: Cranial nerves except Vagus nerve

Preganglionic neurons are located the brainstem for cranial nerves

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Brainstem parasympathetics travel with cranial nerves and act primarily in the head and neck

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Postganglionic neurons are located in the walls of the viscera (intramural ganglia) they innervate

Parasympathetic Route: Vagus nerve

Preganglionic neurons are located the brainstem for cranial nerves

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Brainstem parasympathetics travels down head and neck, until end of hind gut (innervates thoracic and abdominal viscera), gives off branches to plexuses in the organs it innervates

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In the sacrum preganglionic parasympathetics can be found in the lateral regions of S2-S4 spinal segments which

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Preganglionic fibers exit the ventral horns and travel the ventral rami until they can break off as pelvic splanchnic nerves and follow blood vessels to pelvic viscera to get the rest of the abdomen, since the Vagus nerve stops at the end of the hind gut (Do not travel in the sympathetic trunks)

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Postganglionic neurons are located in the walls of the viscera (intramural ganglia) they innervate

Hypogastric nerve

• Sympathetic going to the pelvis need to descend

• Parasympathetics going up to the colon need to ascend

• Hypogastric nerve is the conduit in and out of the pelvis

Some sympathelics that are paraaortic that reed to make their way down into pelvis to supply pelvic structures, form plexus above pelvis, travel down hypogastric nerve to supply sympathetics to pelvic structures.

At same rate, parasympathetics coming in from S2,S3,S4 form plexus to supply pelvic structures, travel up the abdomen to supply sone of the abdomen viscera (hind gut)