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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
Copyright © 2010 Pearson Education, Inc.
Cranialdura mater
Sectionedpedicles ofcervicalvertebrae
Terminus ofmedullaoblongataof brain
Spinal nerverootlets
Dorsalmedian sulcusof spinal cord
(b) Cervical spinal cord.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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.
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
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.
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.
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