THE PERIPHERAL NERVOUS SYSTEM

Peripheral nervous system consists of:

• 31 pairs of spinal nerves• 12 pairs of cranial nerves• The autonomic nervous system• Peripheral nervous system consists of

Sensory nerve fibres

Motor nerve fibres

Each nerve consists of

• Numerous nerve fibres collected into bundles• Each bundle has coverings of protective

connective tissue :

1. Endoneurium - surrounds each individual fibre

2. Perineurium – surrounds each bundle

3. Epineurium – surrounds and covers a number of bundles of nerve fibres

Nerve Roots• Anterior nerve root consists of motor nerve

fibres from neurones in the anterior column of grey matter

• Posterior nerve root consists of sensory fibres

• Just outside the spinal cord there is a spinal ganglion(posterior root ganglion) consisting of nerve cells

• Sensory nerve fibres pass through these ganglia before entering the spinal cord.

• The area of skin supplied by each nerve is called a dermatome.

• For a short distance the roots have dura and arachnoid

• No pia• Anterior and posterior roots join to form mixed

spinal nerve

• Outside the intervertebral foramen the spinal nerve divides into :-

1. Ramus communicans2. a posterior ramus3. an anterior ramus

Rami and Plexuses• The ramus communicans is part of the autonomic nervous

system

• They join preganglionic sympthetic neurones

• The posterior ramus passes backwards to supply skin and muscles

• Tha anterior rami supply the anterior and lateral aspects of the neck, trunk and the upper and lower limbs.

• In the cervical, lumbar and sacral rgions the anterior rami unite near their origins to form nerwork of nerves or plexuses, where nerve fibres are regrouped and rearranged before proceeding to supply skin, bones muscles, and joints

The plexuses

• Five pairs of large plexuses• Cervical plexus• Brachial plexus• Lumbar plexus• Sacral plexus• Coccygeal plexus

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Cervical Plexus

• Formed by the anterior rami of the first four cervical nerves

• It lies opposite the 1st , 2nd , 3rd and 4th cervical vertebrae

• Supplies structures at the back and side of the head and the skin of the front of the neck

• Muscles of the neck• Phrenic nerve from this nerve supplies

diaphragm

Brachial Plexus• Formed by C5,6,7,8 and T1

• Situated above and behid the subclavian vessels and in the axilla

• The plexus can be recognised to have five parts – roots, trunks, divisions, cords and nerves

• The nerves which arise from the plexus are : musculocutaneous nerve, median nerve, ulnar nerve and axillary nerve

Formed by the anterior rami of L1, 2.3 and 4

Situated in front of the transverse processes of the lumbar vertebrae and behind the psoas muscle.

The main branches are : -

Iliohypogastric nerveIlioinguinal nerveGenitofemoral nerveLateral cutaneous nerve of thigh Femoral nerveObturator nerveLumbosacral trunk

Iliohypogastric, ilioinguinal nerves : supply muscles and the skin in the area of the lower abdomen, upper and medial aspects of the thigh and the inguinal region

Lateral cutaneous nerves of the thigh supplies the lateral skin of thigh

Femoral nerve : extensor muscles of the thigh – and the anterior; saphenous branch : medial skin of the leg, ankle and foot

Obturator nerve : adductor muscles

Lumbosacral trunk : goes to sacral plexus

The Sacral Plexus• Formed by by the anterior rami of the lumbosacral

trunk (L4,5) and sacral nerves (S1,2,3)• Lies on the posterior wall of the pelvic cavity• Branches supply the muscles and skin of the

pelvis, muscles around the hip joint• It gives off sciatic nerve (L 4, 5 S1, 2, 3)• Sciatic nerve : largest nerve in the body

passes through greater sciatic forameninto the buttock then descends through the back of thigh supplies the hamstring musclesat the level of the middle of the femur divides into two: - the tibial nerve and the common peroneal nerve

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• The tibial nerve descends through the popliteal fossa

• Supplies the skin and muscles of the leg, sole of the foot and toes

• Common peroneal nerve winds round the neck of the fibula

• Divides into deep peroneal (anterior tibial) and superficial peroneal nerves

• These two supply the skin and muscles of the leg

Pudendal nerveSupplies the external anal sphincter, the external urethral sphincter and adjacent skin

The Autonomic Nervous System

Visceral sensory

Visceral motor&

Autonomic nervous system

• The autonomic nervous system is the subdivision of the peripheral nervous system that regulates body activities that are generally not under conscious control

• Visceral motor innervates non-skeletal (non-somatic) muscles

• Visceral sensory

The autonomic nervous system, or ANS, is responsible for controlling

several body responses which are under the conscious level and which

are mostly involuntary, like breathing, digestion, sexual arousal, beating

of the heart, and many other biochemical processes.

There are two distinguishable divisions:

the parasympathetic and the sympathetic nervous systems.

ANS innervation is divided into sympathetic nervous system and parasympathetic nervous system divisions. The sympathetic division has thoracolumbar “outflow”, meaning that the neurons begin at the thoracic and lumbar (T1-L2) portions of the spinal cord. The parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the cranial nerves (CN 3, CN7, CN 9, CN10) and sacral (S2-S4) spinal cord.

The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will begin at the “outflow” and will synapse at the postganglionic, or second, neuron’s cell body. The post ganglionic neuron will then synapse at the target organ.

Divisions of the autonomic nervous system• Parasympathetic division• Sympathetic division

Serve most of the same organs but cause opposing or antagonistic effects

Parasysmpathetic: routine maintenanceThe hypothalamus is the center of the parasympathetic nervous system. It operates through various interconnecting systems and organs. This system is responsible in the “thinking” phase in stressful situations and is called the “Rest and Digest” phase because your body goes back to its normal responses.  

“rest &digest”

Sympathetic: mobilization & increased metabolism This type of the autonomic nervous system is responsible in the distribution of essential nutrients and oxygen to the various parts of the body.  The ANS is responsible for “Flight or Fight.” The body has to respond immediately because of the urgency of some situations. “fight, flight or fright” or “fight, flight or freeze”

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Where they come from

Parasympathetic:craniosacral

Sympathetic:thoracolumbar

Parasympathetic nervous system“rest & digest”

• Also called the craniosacral system because all its preganglionic neurons are in the brain stem or sacral levels of the spinal cord– Cranial nerves III,VII, IX and X– In lateral horn of gray matter from S2-S4

• Only innervate internal organs (not skin)• Acetylcholine is neurotransmitter at end organ

as well as at preganglionic synapse: “cholinergic”

Cranial outflowIII - pupils constrictVII - tears, nasal mucus, salivaIX – parotid salivary glandX (Vagus n) – visceral organs of thorax & abdomen:

Stimulates digestive glandsIncreases motility of smooth muscle of digestive tractDecreases heart rateCauses bronchial constriction

Sacral outflow (S2-4): form pelvic splanchnic nervesSupply 2nd half of large intestineSupply all the pelvic (genitourinary) organs

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Parasympathetic

Sympathetic nervous system“fight, flight or fright”

• Also called thoracolumbar system: all its neurons are in lateral horn of gray matter from T1-L2

• Lead to every part of the body (unlike parasymp.) – Easy to remember that when nervous, you sweat; when afraid,

hair stands on end; when excited blood pressure rises (vasoconstriction): these sympathetic only

– Also causes: dry mouth, pupils to dilate, increased heart & respiratory rates to increase O2 to skeletal muscles, and liver to release glucose

• Norepinephrine (aka noradrenaline) is neurotransmitter released by most postganglionic fibers (acetylcholine in preganglionic): “adrenergic”

Sympathetic nervous system continued• Regardless of target, all begin

same• Preganglionic axons exit

spinal cord through ventral root and enter spinal nerve

• Exit spinal nerve via communicating ramus

• Enter sympathetic trunk/chain where postganglionic neurons are

• Has three options…

Options of preganglionic axons in sympathetic trunk

1. Synapse on postganglionic neuron in chain ganglion then return to spinal nerve and follow its branch to the skin

2. Ascend or descend within sympathetic trunk, synapse with a posganglionic neuron within a chain ganglion, and return to spinal nerve at that level and follow branches to skin

3. Enter sympathetic chain, pass through without synapsing, form a splanchnic nerve that passes toward thoracic or abdominal organs– These synapse in prevertebral ganglion in front of

aorta– Postganglionic axons follow arteries to organs

Sympathetic

Visceral sensory neurons• Monitor temperature, pain, irritation, chemical changes and

stretch in the visceral organs– Brain interprets as hunger, fullness, pain, nausea, well-being

• Receptors widely scattered – localization poor (e.g. which part is giving you the gas pain?)

• Visceral sensory fibers run within autonomic nerves, especially vagus and sympathetic nerves– Sympathetic nerves carry most pain fibers from visceral organs of

body trunk

• Simplified pathway: sensory neurons to spinothalamic tract to thalamus to cerebral cortex

• Visceral pain is induced by stretching, infection and cramping of internal organs but seldom by cutting (e.g. cutting off a colon polyp) or scraping them

Visceral sensory and autonomic neurons participate in visceral reflex arcs

• Many are spinal reflexes such as defecation and micturitionreflexes

• Some only involve peripheralneurons: spinal cord not involved(not shown)*

*e.g. “enteric” nervous system: 3 neuron reflex arcs entirely within the wall of the gut

Central control of the Autonomic NS

Amygdala: main limbic region for emotions

-Stimulates sympathetic activity, especially previously learned fear-related behavior-Can be voluntary when decide to recall frightful experience - cerebral cortex acts through amygdala-Some people can regulate some autonomic activities by gaining extraordinary control over their emotions

Hypothalamus: main integration center

Reticular formation: most direct influence over autonomic function

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SCHEMATICS OF

ANS GANGLIA

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• Ganglia are located close to the spinal column (short pre- and long post-ganglionic fibers)

• Ganglia are often interconnected• Large ratio of post- to pre-ganglionic fiber

number

Sympathetic Nervous system

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• The anatomy of sympathetic ans is structurally designed to produce diffuse systemic responses by simultaneous targetting of multiple tissues

• In addition to the routine homeostatic maintenance of vital functions, the sympathetic nervous sytem is designed to respond to physiological stress

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PARASYMPATHETIC NERVOUS SYSTEM

• Ganglia are located close to or within the target tissue (long pre- and short post-ganglionic fibers)

• Ganglia are not interconnected• Ratio of post- to pre-ganglionic fiber number =

1

PARASYMPATHETIC NERVOUS SYSTEM

• The anatomy of ans is structurally designed to produce precise pin-point responses by targetting of specific tissues

• In addition to the routine homeostatic maintenance of vital functions, the parasympathetic nervous sytem is designed to limit responses to physiological stress