Peripheral Nervous System Part 2 Flashcards
What are the spinal nerves?
Spinal Nerves • 31 pairs—contain thousands of nerve fibers • Connect to the spinal cord • Named for point of emergence from the spinal cord – 8 pairs of cervical nerves (C1–C8) – 12 pairs of thoracic nerves (T1–T12) – 5 pairs of lumbar nerves (L1–L5) – 5 pairs of sacral nerves (S1–S5) – 1 pair of coccygeal nerves (Co1)
Where are the spinal nerves?
Connect to the spinal cord by the dorsal root and ventral root – Dorsal root—contains sensory fibers • Cell bodies—located in the dorsal root ganglion – Ventral root—contains motor fibers arising from anterior (ventral) gray column
What are parts of spinal nerves?
Spinal nerves branch into dorsal ramus
(branch) and ventral ramus (branch)
• Dorsal and ventral rami contain both
sensory and motor fibers
What innervates the back muscles?
• Dorsal rami innervate back muscles • Follow a neat, segmented pattern • Innervate a horizontal strip of muscle and skin in line with emergence point from the vertebral column
What innervates the thoracic wall?
• Thoracic region innervated by ventral rami
arranged in simple, segmented pattern
• In the thorax, each ventral ramus continues as an
intercostal nerve
• Intercostal nerves run inferior to each rib, hence T12
intercostal nerve is called subcostal
• Each intercostal nerve gives off lateral and anterior
cutaneous branches
• Intercostal nerves—supply intercostal muscles,
skin, and abdominal wall
What is a nerve plexus?
NERVE PLEXUS—a network of nerves • VENTRAL RAMI from peripheral nerves (except T2–T12) form nerve plexuses • Fibers from multiple ventral rami crisscross, branch and join with one another to form nerve plexuses • Primarily serve the arms and legs
What are the different nerve plexuses?
- Cervical
- Brachial
- Lumbar
- Sacral
Describe the Cervical Plexus
• Buried deep in the neck under the sternocleidomastoid muscle and extends into posterior triangle of neck • Formed by ventral rami of first four cervical nerves (C1–C4) • Most are cutaneous sensory nerves innervating neck, posterior head, and superior shoulder skin • Some branches innervate muscles of the anterior neck
What is the Phrenic Nerve in the Cervical Plexus?
PHRENIC NERVE is the most important nerve of
the cervical plexus and innervates the
mediastinal pleura, pericardium, & diaphragm
• Supplies both motor and sensory innervation
to the diaphragm
• Hiccups result when the phrenic nerve
induces abrupt, rhythmic contractions of the
diaphragm
• The hiccup reflexive response commonly
originates from sensory irritation of the
diaphragm or stomach
Describe the Brachial Plexus
Brachial plexus lies in the neck and axilla
• Formed by ventral rami of C5–C8, T1
• Cords give rise to main nerves of the upper limb
Anterior Division:
Musculocutaneous, Median, and
Ulnar Nerves from Lateral and
Medial Cords of the Brachial Plexus
Innervate muscles of anterior compartments:
flexor muscles of arm (biceps), forearm, &
hand.
• Supply cutaneous sensory innervation to part
of forearm and hand.
Nerves from Lateral & Medial Cords of brachial plexus
Musculocutaneous nerve—main branch of the
lateral cord
– Innervates the anterior arm biceps brachii and
brachialis and sensory dermis of part of forearm
• Median nerve—originates from both lateral and
medial cords
– Innervates anterior forearm muscles, lumbrical
muscles to digits 2 & 3, and sensory dermis of
lateral palm
• Ulnar nerve—branches from the medial cord
– Innervates intrinsic hand muscles and sensory
dermis of the medial hand
Median nerve injury of brachial plexus:
elbow injury to nerve causes
diminished innervation of flexor muscles of forearm
which reduces ability to flex 2nd and 3rd fingers or
oppose the thumb to little finger
Ulnar Nerve injury of brachial plexus
intrinsic hand muscle
innervation is diminished which reduces finger
movement and “clawhand” can develop
Radial and Axillary Nerves from
the Posterior Cord of brachial plexus
• Radial
– Largest branch of the brachial plexus
– Innervates muscles of the posterior upper
limb including triceps as well as extensor
muscles of the forearm
• Axillary
– Innervates the deltoid and teres minor
Radial Nerve Injury of brachial plexus:
diminished innervation of
extensor muscles of forearm reduces ability to
extend arm at wrist
Posterior Division:
Radial Nerve and Axillary Nerve from
and Posterior Cord of Brachial Plexus
Supply cutaneous innervation to much of
posterior arm, forearm and hand
What is the lumbar plexus?
Innervation of the Lower Limb
• Lumbar plexus arises from L1– L4
• Innervate muscles in anterior and medial
compartments of the thigh
• Supply dermis sensory innervation to
anterior and medial portions of the thigh
and leg
Lumbar Plexus and innervation of lower limb
Main branches provide motor innervation of
anterior & medial thigh
• Femoral nerve—innervates anterior thigh
muscles that flex the hip (rectus femoris and
iliacus) and extend the leg at the knee
(quadriceps femoris group)
• Obturator nerve—innervates medial thigh
adductor muscles; adduction brings leg closer
to the midline of the body
– Smaller branches innervate the posterior
abdominal wall and psoas muscle
The Lumbar Plexus and
Innervation of the Lower Limb
sensory innervation
- Anterior-medial lower extremities
* Medial lower extremities
What is the sacral plexus?
Arises from spinal nerves L4–S4 • Caudal to the lumbar plexus • Sometimes considered collectively with the lumbar plexus and together they are called lumbosacral plexus
What does the sacral plexus innervate?
• Superior and inferior gluteal nerves – Innervate the gluteal muscles • Pudendal nerve – Innervates muscles and sensory dermis of the perineum
The Sacral Plexus
• Innervates muscles in the posterior compartment of the thigh and all of the muscles in the leg • Supply sensory dermis innervation to the lateral and posterior portions of the leg and to the foot
What is the sciatic nerve?
• Sciatic nerve—the largest nerve of the sacral plexus
and the thickest and longest nerve in body
• Supplies motor innervation to all of the lower limb
except for the anterior and medial thigh
• Innervates hamstring muscles (semitendinosus,
semimembranosus, and biceps femoris) in the
posterior compartment to the thigh which enables the
knee to flex
• Sciatic nerve is actually two nerves in one sheath:
• Tibial nerve—innervates most of the posterior
lower limb (thigh and leg) muscles
• Common fibular (peroneal) nerve—innervates
muscles of the anterolateral leg
What is a Foot drop
• Damage to the common fibular
(peroneal) nerve diminishes innervation
of muscles in the anterior-lateral leg
with reduces dorsiflexion
What is sciatica?
A herniated lumbar disc which presses on the sacral dorsal roots can result in aching, stabbing pain over the distribution of the sciatic nerve
Innervation of the Skin: Dermatomes
Innervation of the Skin: Dermatomes
• Dermatome—an area of skin innervated by
cutaneous branches of a single spinal nerve
• Upper limb: brachial plexus nerves
• Trunk: intercostal and subcostal nerves
• Lower limb anterior surface: lumbar plexus
nerves
• Lower limb posterior surface: sacral plexus
nerves
Innervation of the Skin: Dermatomes (continued)
• Adjacent dermatomes are typically not as
clearly demarcated from one another as the
dermatome map indicates
• On the trunk, adjacent dermatomes may
overlap with each other by a full 50%
• On the limbs, by contrast, the overlap of
adjacent dermatomes is less and some skin
patches are innervated by only one spinal
nerve
Shingles: herpes zoster
• When a person gets the chicken pox (Varicella)
virus lesion on the skin, the virus travels up the
sensory neurons to the cell bodies in the sensory
ganglia where it becomes dormant
• Reactivation of chicken pox (Varicella) virus is
called shingles and this causes a red painful itchy
skin rash within the dermatome of the sensory
nerve where the virus has been dormant
• Reactivation may be brought on by stress
• Shingles is mostly experienced by people over 50
years of age, but can occur at any age
Poliomyelitis infection by poliovirus causes inflammation of gray matter and destruction of motor neurons resulting in motor deficits
Spinal polio: asymetric muscle paralysis
most typically of the legs; most common form
• Bulbar polio: weakness of muscles
innervated by cranial nerves
• Polioencephalitis: infection of brainstem or
higher in brain
Leprosy = Hansen’s Disease
• Caused by infection from the bacteria
Mycobacterium leprae
• A granulomatous disease of the
peripheral nerves, eyes, skin, and upper
respiratory tract that can result in
sensory and motor deficits
• Sensory insensitivity to fine touch, pain,
and heat receptors
• Spinal nerve and cranial nerve palsies
Myasthenia gravis
Disorder of somatic motor system
Progressive weakening of the skeletal muscles • An autoimmune disorder • Antibodies diminish acetylcholine receptors at motor endplates
Neuronal Regeneration in PNS
• Neural injuries in post-fetal period may cause
permanent dysfunction in children and adults
• In the PNS, if axons alone are destroyed, cells
bodies often survive and the axons may
regenerate
• If nerves are severed in PNS, macrophages
invade and destroy axon distal to the injury
• Axon filaments grow peripherally from injured site
in PNS through a regeneration tube formed by
surviving Schwann cells
• As axon regenerates, a new myelin sheath is
formed
