#13335 - Organisation Of The Autonomic Nervous System - Organisation of the Body

Organisation of the autonomic nervous system

-all of the peripherial autonomic nervous system derives from the neural crest cels

-as neural plate folds meet, the neural crest forms at the interface between neuroectoderm and ectoderm.

-first cells of the neural crest migrate away from neural tube and ectoderm to form the enteric nervous system

-next wave of neural crest cells migrate through the somitic mesoderm to form the segmental sympathetic chain ganglia

-outflow from the sympathetic chain ganglia is arranged segmentally, neural crest migrates through the rostral half of each somite. The caudal half has inhibitory molecules ephrins that prevent neural crest migration- forms segmental sympathetic chain ganglia

-parasympathetic ganglia and the prevertebral sympathetic ganglia are neural crest derived.

-preganglionic fibres of the ANS are CNS neurons lie in the intermediate part of the spinal cord and are patterned by SHH signalling notochord.

-motor neurones come from CNS not from neural crest.

Function of the ANS

-involuntary control of body functions maintains the body’s homeostasis

Symapthetic- fight/flight response

Parasympathetic – rest/digest

Enteric- Intrinisic nerves in the gut wall

-output from the CNS travels along 2 anatomically functionally distinct pathways

-somatic motor neurones that innervate striated skeletal muscle

-autonomic motor neurones- innervate smooth muscle, cardiac muscle, secretory epithelia and glands

Organisation of the ANS

• Arise in the CNS and innervate target tissue by a 2 synapse pathway.

• Preganglionic neurones have cell bodies that lie within the CNS and neurones found in coloumns of cells in the brainstem and spinal cord. Axons come out from the CNS and make synapses with post ganglionic neurones

• Post ganglionic neurones- found in the peripheral ganglia and the axons from these neurones project to their targets.

Both the sympathetic and parasympathetic are independent to each other and work synergistically to regulate visceral activity

-increase output from the sympathetic nervous system occurs under stress/anxiety/fear/excitement/physical

-increase output from the parasympathetic nervous system occurs during sedentary activity such as eating/ vegetative behaviour

-the enteric nervous system- system of afferent neurons, interneurones, motor neurones that forms network of neurones called plexuses. These surround the GI tract and are controlled by the CNS through sympathetic and parasympathetic fibres

Organisation of the sympathetic chain ganglia

-Preganglionic neurones-

-cell bodies of preganglionic sympathetic motor neurones are in the thoracic and upper lumbar spinal cord. T1-L3.

-Axons leave through the ventral roots along with the axons of the somatic motor neurones. After they enter the spinal nerve, Sympathetic motor neurones diverge from the somatic motor neurones and enter the white rami communicantes (preganglionic motor neurones are myelinated)

-paravertebral ganglia

-axons from the preganglionic neurones enter the sympathetic paravertebral ganglion through a white ramus. This ganglia lies adjacent to the vertebral coloumn and extend from the the upper part of the neck to the coccyx. One ganglion is positioned at level of each spinal root, some adjacent ganglia are fused

-superior cervical ganglion (fusion of C1-C4) supplies the head, neck- dilates the pupil, elevates the eyelid, innervates cilary body, stimulates salivation

-middle cervical ganglion (fusion of C5-C6), Stellate ganglia (C7,C8,T1)- innervate heart, lungs, bronchi

-throacic chain ganglia innervates all other organs in a segmental fashion

-after entering the paravertebral ganglia the preganglionic sympathetic axon has three fates

-synapse within that segmental paravertebral ganglion

-travel up/ down and synapse with a neighbouring paravertebral ganglion

-enter the greater/less splanchnic nere to synapse within one of the ganglia of the prevertebral plexuses

prevertebral ganglia

-prevertebral plexus lies in front of the aorta and along its major arterial branches, the major ganglia are named after arteries they are adjacent to

-plexus contains the prevertebral ganglia and inter conneted fibres

-main prevertebral ganglia innervate the gut and are supplied by the greater, lesser, least splanchnic nerves

-coeliac ganglion supplies the foregut- innervations of the coleiac ganglion inhibits digestion in the stomach and stimulates the release of glucose from liver

-superior mesenteric ganglion supplies the midgut

-inferior mesnetricganglion supplies the hind gut

each preganglionic sympathetic fibre synpapses with many post ganglionic sympathetic neurones that are located within one/ several nearby paravertebral/prevertebral ganglia. so sympathetic outpur has widespread effects.

Post ganglionic neurons of the sympathetic nervous system

-unmyelinated

-cell bodies are located within the paravertebral ganglia send axons through grey rami communicantes (grey as the axons are unmyelinated) which rejoin the mixed spinal nerves

-the preganglionic neurons located only in T1-L3 segments as white rami are only found at these levels but each ganglion sends out post ganglionic axons and these grey rami are present at all levels (C2-C3 to coccyx.)

-the axons from parvertebral/prevertebral ganglia travel to target organs within other nerves/along blood vessels

-the post ganglionic nerverse are very long as the paravertebral/ prevertebral are very close to the spinal cord.

-the terminals of sympathetic neurons are formed by varicosities which don’t make synapses. The varicosities secrete neurotransmitter (noradrenaline) which acts locally on target cells

Effects of sympathetic nervous system- Flight and fight response

-when sympathetic nervous system is stimulated noradrenalin and adrenalin are released from terminals and adrenal glands

Exception: the post ganglionic nerves of innervating the sweat glands release ACH acting on muscarinic receptors

-the adrenalin or noradrenalin acts on two types of adreno receptors (alpha, beta), a1- stimulates contraction of the muscles of the blood vessels, a2 (autoreceptors), b1 (cardiac contraction), b2 (relaxation of the bronchial muscles)

-stimulation of adrenal medulla by (ACH)-nACHr by greater splanchic nerve (preganglionic neurone) results in secretion of adrenaline which leads to a global response

-causes dilation of pupil optimising light passage to retina (with fright)- alpha receptors on iris

-Beta 1 receptors on the heart: heart rate increases, force of contraction increases, conduction increases- major arteries to the limb muscles dilate to allow for increased flow but peripheral arteries and veins constrict. Peripheral vessles in the skin constrict (white as a sheet)

-Beta 2 receptors act on lungs: The airways open to increase ventilation of lungs by relaxation of the bronchial smooth muscle. respiratory changes- rate and depth of ventilation increases- innervation of the diagphragm muscles and intercostals muscles are somatic

-glucose release from the liver is increased- action of a, b receptors

-increase in sweat

-GI tract- decreased motility and decrease peristalsic movement- action of alpha, and beta 2 receptors

-liver- gluconeogenesis and glycogenolysis- increase in energy supplies

-kidney- rennin secretion

-salivary glands- viscous salivary secretions- action of ACH on M3 receptors

Parasympathetic division of the ANS

-cell bodies of the preganglionic parasympathetic neurons are located in the medulla, pons and midbraine and S2-S4 segments of the spinal cord

-no segmentally organised ganglia

-the preganglionic parasympathetic fibres that originate in the brain distribute with 4 cranial nerves

-oculomotor nerve (CN 3)- synapses onto post ganglionic neurons in the cilary ganglion and controls the smooth muscles of the eye. The constrictor muscles of the pupil and the cilary muscles which controls the shape of the lens

-facial nerve (CN 7)- post ganglionic fibres in the pterygopalatine ganglion and supply the lacrimal glands which produce tears and innervate the nasal muscosa. Another branch of the facial nerve carries preganglionic fibres to two salivary glands- submandibular and sublinguinal

-glossopharyngeal nerve (CN 9)- synapses with the postganglionic fibre in the...