The Spinal Cord & Peripheral Nervous System

Peripheral Nerves

• There are 12 pairs of cranial nerves and

31 pairs of spinal nerves.

Spinal Nerves

• There are 31 pairs of spinal nerves.

• All are mixed nerves (motor & sensory).

Spinal Nerves

• There are 31 pairs of spinal nerves.

• All are mixed nerves.

• There are:– 8 cervical spinal nerves (C8 comes out

between C7 and T1)– 12 pairs of thoracic nerves,– 5 pairs of lumbar spinal nerves – and 5 pairs of sacral nerves.

CervicalnervesC1 – C8

ThoracicnervesT1 – T12

LumbarnervesL1 – L5

Sacral nervesS1 – S5

Coccygeal nerve Co1

Cervical plexus

Intercostalnerves

Cervicalenlargement

Lumbarenlargement

Cauda equina

Brachial plexus

Lumbar plexus

Sacral plexus

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Cranialdura mater

Sectionedpedicles ofcervicalvertebrae

Terminus ofmedullaoblongataof brain

Spinal nerverootlets

Dorsalmedian sulcusof spinal cord

(b) Cervical spinal cord.

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Anatomy of the spinal cord.

(a) Cross section of spinal cord and vertebra

Epidural space(contains fat)

Pia mater

Spinalmeninges

Arachnoidmater Dura mater

Bone ofvertebra

Subdural space

Subarachnoidspace(contains CSF)

Dorsal rootganglion

Bodyof vertebra

Spinal Nerves

The spinal nerve consists of a dorsal root and a ventral root.

The ventral root contains efferent, motor neurons (voluntary and involuntary).

The dorsal root is, afferent, sensory fibers.

Spinal Nerves

The spinal nerve is only 1-2 cm long coming out from the spine and the branches into a dorsal ramus and a ventral ramus.

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Somaticsensoryneuron

Dorsal root (sensory)

Dorsal root ganglion

Visceralsensory neuron

Somaticmotor neuron

Spinal nerve

Ventral root(motor)

Ventral horn(motor neurons)

Dorsal horn (interneurons)

Visceralmotorneuron

Interneurons receiving input from somatic sensory neurons

Interneurons receiving input from visceral sensory neurons

Visceral motor (autonomic) neurons

Somatic motor neurons

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Ascending tracts Descending tracts

Fasciculus gracilisDorsalwhitecolumn

Fasciculus cuneatus

Dorsalspinocerebellar tract

Lateralspinothalamic tract

Ventral spinothalamictract

Ventral whitecommissure

Lateralcorticospinal tract

Lateralreticulospinal tract

Ventral corticospinaltract

Medialreticulospinal tract

Rubrospinaltract

Vestibulospinal tractTectospinal tract

Ventralspinocerebellartract

Innervations of Specific Body Regions

Except for the spinal nerves T2-T12, all other spinal nerves join into inter lacing networks called nerve plexuses.

They are formed only by the ventral rami.

.

Dorsal ramus

Ventral ramus

Intercostal nerve

Spinal nerve

Rami communicantes

Dorsal rootganglion Dorsal rootVentral root

Sympathetic trunkganglion

Sternum

(b) Cross section of thorax showing the main roots and branches of a spinal nerve.

Branches of intercostalnerve

• Lateral cutaneous• Anterior cutaneous

Cervical Plexus

This is formed from the first 4 cervical spinal nerves.

Cervical Plexus

This is formed from the first 4 cervical spinal nerves.

The most important nerve coming from this plexus is the phrenic nerve.

Cervical Plexus

This is formed from the first 4 cervical spinal nerves.

The most important nerve coming from this plexus is the phrenic nerve.

This is the chief motor nerve for the diaphragm.

Hypoglossalnerve (XII)

C1

C2

C3

C4

C5

Segmentalbranches

Lesser occipitalnerveGreater auricularnerve

Ansa cervicalis

Phrenic nerve

Supraclavicularnerves

Accessory nerve (XI)

Transversecervical nerve

Ventralrami:

Ventral rami

Cervical Plexus

Damage to the spinal cord at C3- C5 could lead to respiratory arrest.

Brachial Plexus

It is formed from C5-C8 and T1 with 5 major roots supplying the muscles of the muscles of the shoulder, thorax and upper limb.

Upper

Middle Trunks

Lower

Roots (ventral rami):

Upper subscapular

Lower subscapular

Thoracodorsal

Medial cutaneousnerves of the armand forearm

Long thoracic

Medial pectoral

Lateral pectoral

Nerve tosubclaviusSuprascapular

Dorsal scapular

Posteriordivisions

Anteriordivisions

Lateral

PosteriorCords

Medial

AxillaryMusculo-cutaneous

Radial

Median

Ulnar

Posteriordivisions

Trunks Roots

C4

C5

C6

C7

C8

T1

(a) Roots (rami C5 – T1), trunks, divisions, and cords

Musculocutaneousnerve Lateral

cord

Posteriorcord

Medial cord

Radial nerve

Ulnar nerve

Axillary nerveBiceps brachii

CoracobrachialisMedian nerve

Radial nervebranches to triceps

(b) Cadaver photo

Lumbrosacral Plexus

It is formed from L1-L4 and lies within the psoas major muscle.

The largest nerve of this plexus is the femoral nerve.

The sacral plexus is formed from L4-S4 and lies just below the Lumbar Plexus

(a) Ventral rami and major branches of the lumbar plexus

Iliohypogastric

L1

L2

L3

L4

L5

Ilioinguinal

Genitofemoral

Lateral femoralcutaneous

Obturator

Femoral

Lumbosacraltrunk

Lateral femoralcutaneous

Anterior femoralcutaneousSaphenous

Obturator

IliohypogastricIlioinguinalFemoral

Ventral rami Ventralrami:

(b) Distribution of the major nerves from the lumbar plexus to the lower limb

Superiorgluteal

LumbosacraltrunkInferiorgluteal

CommonfibularTibialPosteriorfemoralcutaneous

Pudendal

Sciatic

(a) Ventral rami and major branches of the sacral plexus

L4

L5

S1

S2

S3

S4

S5

Co1

Ventral rami Ventral rami:

The largest nerve from this is the sciatic nerve.

It is the largest and thickest nerve in the body.

It supplies the entire lower limb.

Irritation to this nerve gives rise to Sciatica.

Dermatomes

`Dermatomes are areas of the skin innervated by the sensory nerves. These are fairly uniform and follow the nerve segments.

C2C3

C4

C5T1

T2

T2T3T4T5

C6

C8C7 C7

C6

T6T7T8T9

T10

T11

T12L1

S2S3

L1

L2

L3

L4

L5

L2

L3

L4

L5

S1

C5

C6

C8

T2

C5

C6

S1

(a) Anterior view

C2

C3

C4C5C6C7C8

C8 C8

C7 C7

T1T2T3T4T5T6T7T8T9

T10

T11T12

L1L2 L3

S1(b) Posterior view

L5S2

S1

S1

S3

S2 S1S2

S4S5

L5L5

L4L5L5

L4

C6 C6

C5

L4

L3

L2

L1

L4

Reflex Activity

• Reflexes can be inborn or learned. – An inborn reflex is a predictable, rapid

response to a stimulus. – Acquired reflexes result from repetition.

Reflex Arc

Stretch and Golgi Tendon Reflexes

The muscle spindles found in skeletal muscles provide information on the length of the muscle and the amount of tension on it.

A common example of this is the patellar reflex. This reflex causes the muscle to contract in response to stretching.

The patellar (knee-jerk) reflex – a specific example of a stretch reflex

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2 – L4)

3a

2

1

3b 3b

1 Tapping the patellar ligament excitesmuscle spindles in the quadriceps muscle.2

3a The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

3b The interneurons (green) makeinhibitory synapses with ventral hornneurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse–

+

Golgi tendon reflexes

Golgi tendon reflexes are the opposite and respond by causing muscle relaxation. This helps to prevent the muscle from over contracting.

+ Excitatory synapse– Inhibitory synapse

Quadriceps stronglycontracts. Golgi tendon organs are activated.

Afferent fibers synapsewith interneurons in the spinal cord.

Efferent impulsesto muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferentimpulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord

1 2

3a 3b

Quadriceps(extensors)

Golgitendon

organHamstrings

(flexors)

Flexor and Crossed-Extensor Reflexes

• These are initiated by painful stimuli and cause an automatic withdrawal of the threatened body part. Placing your hand on a hot stove is an example of this type of reflex.

  Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

Interneurons

Efferentfibers

FlexorinhibitedExtensorstimulated

+ Excitatory synapse– Inhibitory synapse

Superficial Reflexes

These are the result of gentle cutaneous stimulation. Common examples are the plantar reflex that tests the integrity of the corticospinal tract.

Babinski's sign

This plantar reflex occurs when the corticospinal tract is damaged.

In it the large toe dorsi flexes and the smaller toes fan laterally.

Spinal Cord Injuries

Damage to the spinal cord results in paralysis (loss of motor function) or

Paresthesia (sensory loss).

These are devastating injuries since the cord will not heal.

Spinal Cord Injuries

The Autonomic Nervous System

The motor portion of the peripheral nervous system is divided into – the somatic (voluntary) motor system (SMS)

and – the autonomic motor system (ANS).

Figure 14.1 Place of the ANS in the structural organization

of the nervous system.

Central nervous system (CNS) Peripheral nervous system (PNS)

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

Comparison

The SMS stimulates skeletal muscle whereas the ANS innervates cardiac and smooth muscle as well as glands.

Comparison

In the SMS the motor neuron cell bodies are located in the CNS and their axons extend to the muscles they innervate.

It is a single neuron chain.

Comparison

The ANS uses a two neuron chain.

The first is the preganglionic neuron; its cell body resides in the CNS or spinal cord.

Comparison

The ANS uses a two neuron chain.

The first is the preganglionic neuron; its cell body resides in the CNS or spinal cord.

The second motor neuron outside of the CNS.

This second neuron is called the postganglionic neuron.

Figure 14.2 Comparison of somatic and autonomic

nervous systems.

Skeletal muscle

Cell bodies in centralnervous system Peripheral nervous system Effect

+

+

Effectororgans

ACh

AChSmooth muscle(e.g., in gut),glands, cardiacmuscle

Ganglion

Adrenal medulla Blood vessel

ACh

ACh

ACh

NE

Epinephrine andnorepinephrine

Acetylcholine (ACh) Norepinephrine (NE)

Ganglion

Heavily myelinated axon

Lightly myelinatedpreganglionic axon

Lightly myelinatedpreganglionic axons

Neuro-transmitterat effector

Unmyelinatedpostganglionicaxon

Unmyelinatedpostganglionic axon

Stimulatory

Stimulatoryor inhibitory,dependingon neuro-transmitterandreceptorson effectororgans

Single neuron from CNS to effector organs

Two-neuron chain from CNS to effector organs

SO

MA

TIC

NE

RV

OU

SS

YSTEM

AU

TO

NO

MIC

NER

VO

US

SYSTEM

PA

RA

SYM

PA

TH

ETIC

SYM

PA

TH

ETIC

ANS Divisions

The ANS is divided up into the parasympathetic and sympathetic divisions.

They innervate the same organs and typically have opposite effects.

Parasympathetic Division

This system is associated with “resting and digesting”.

When this system is active, the body’s energy use is low, the body is relaxed and the body can focus on digestive processes.

Sympathetic Division

This is often referred to as the “fight or flight” system.

When this system is active, the body is excited and the body is in a high energy state.

Figure 14.3 Overview of the subdivisions of the ANS.

Salivaryglands

Eye

Skin*

Heart

Lungs

Liverand gall-bladder

Genitals

Pancreas

Eye

Lungs

Bladder

Liver andgall-bladder

Pancreas

Stomach

Cervical

Sympatheticganglia

Cranial

Lumbar

Thoracic

Genitals

Heart

Salivaryglands

Stomach

Bladder

Adrenalgland

Parasympathetic Sympathetic

Sacral

Brainstem

L1

T1

Parasympathetic (Craniosacral) Division

• The cranial outflow includes Cranial nerves III, VII, IX, and X.

• The sacral outflow innervates the colon and pelvic organs.

Sympathetic (Thoracolumbar) Division

It supplies the visceral organs but also structures are the body such as the sweat glands and peripheral arteries.

All cell bodies for the preganglionic neurons arise in the spinal cord between T1 and L12.

Visceral reflexes

The visceral reflex arc is similar to the somatic reflex arc and contains the same components except the motor neuron is from the ANS.

Visceral reflexes

The visceral pain afferent fibers travel along the same pathways as the somatic pain receptors and gives rise to the so called referred pain.

Heart

Lungs anddiaphragmLiver

Stomach

Kidneys

OvariesSmall intestine

Ureters

Urinarybladder

Colon

Pancreas

Liver

Heart

Appendix

Gallbladder

ANS Physiology

• The major neurotransmitters released by the ANS are acetylcholine (Ach) and norepinephrine (NE).

ANS Physiology

Ac is released by:– All ANS preganglionic axons– All parasympathetic postganglionic axons at

the synapses with their effectors

Ach releasing fibers are called cholinergic fibers

ANS Physiology

NE is released by postganglionic sympathetic fibers at the synapses with their effectors.

NE releasing fibers are called adrenergic fibers.

Receptor Type

To further complicate the matter, the effects of these neurotransmitters depend on the type of receptor to which they attach.

Cholinergic Receptors

Two types of receptors bind Ach and they are named for the drugs that can bind to them.

These are the nicotinic receptors (preganglionic sympathetic)and the muscarinic receptors (postganglionic parasympathetic).

Effects of Drugs

The existence of various types of receptors has led to the design of specific drugs that can either inhibit or stimulate a selected organ.

Atropine is an anticholinergic drug that causes the pupil to dilate, dry up respiratory secretions or prevent salivation.

Disorders

• POTS– Postural Orthostatic Tachycardia Syndrome