Ses 5 Dermatomes And Myotomes MSK Flashcards
Define the term dermatome and describe the embryonic
development of dermatomes
A dermatome is an area of skin that is supplied by a single spinal nerve.
By day 30, each embryo has approximately 34-35 pairs of somites.
4th week after fertilisation, each somite differentiates into: sclerotome - ventral and gives rise the vertebrae and ribs
dermatomyotome - dorsal and forms the dermis (via the dermatomes) and the muscle tissue (via the myotomes)
Learn forester map
T4/T5 = nipples, T10 = umbilicus, L1 = groin
Describe the composition and formation of a mixed spinal nerve
A spinal nerve is a mixed nerve that carries motor, sensory and autonomic signals between the body and the spinal cord.
Dorsal and ventral roots join together in the intervertebral foramina.
State the anatomical distribution of the dermatomes of the upper
and lower limbs
anterior rami of C5-T1 and L1 – S5 enter the limb buds instead supply the muscles and skin of the upper and lower limbs respectively.
Axial line
junction of two dermatomes supplied from
discontinuous spinal levels.
How are pre and post axial boundaries marked
Boundaries of the dorsal (posterior) and ventral (anterior) compartments are marked by superficial veins.
In the upper limb, the cephalic vein marks the pre-axial border and the basilic vein marks the post-axial border.
In the lower limb, the long saphenous vein marks the pre-axial border and the short saphenous vein marks the post- axial border.
Peripheral nerve distribution pic
Definition of myotome and motor unit and embryology of myotome.
A myotome is the group of muscle fibres supplied by a single spinal nerve.
In contrast, a motor unit is a single motor neuron (i.e. only one axon) and
the skeletal muscle fibres it innervates.
4th week after fertilisation, each somite differentiates into sclerotome and dermamyotome.
Myotomes lower limbs
L2: hip flexion(bold)
L3: knee extension(bold) and hip adduction
L4: ankle dorsiflexion(bold)
L5: great toe extension(bold) /ankle inversion / hip abduction
S1: ankle plantar-flexion(bold) / ankle eversion / hip extension
S2: knee flexion / great toe flexion(bold)
Neural level definition
The neural level is the lowest level of fully intact sensation and motor function
Mechanical back pain
Lumbar
It is characterised by pain when the spine is loaded,
that worsens with exercise and is relieved by rest.
Risk factors include obesity, poor posture, a sedentary lifestyle with deconditioning of the paraspinal (core)
muscles, poorly-designed seating and incorrect manual handling.
disc degeneration and marginal osteophytosis.
The nucleus pulposus of the intervertebral discs dehydrates with age. This leads
to a decrease in the height of the discs, bulging of the discs and alteration of the
load stresses on the joints.
Syndesmophytes therefore develop adjacent to the end plates of the discs - marginal osteophytosis.
Osteoarthritic changes of facet joints which is inner aged by meningeal branch of the spinal nerve.
As disc height dec and osteoarthritis develops, intervertebral foramina decrease in size. This can lead to compression of the spinal nerves - radicular pain.
Herniation of an intervertebral disc i.e. ‘slipped disc’ notion
Pain occurs due to herniated disc material pressing on a spinal nerve.
4 stages:
1. Disc degeneration - discs dehydrate and bulge
2. Prolapse: Protrusion of the nucleus pulposus into spinal canal.
3. Extrusion: The nucleus pulposus breaks through the annulus fibrosus but is still contained within the disc space
4. Sequestration: The nucleus pulposus enters the spinal canal.
The most common sites for ‘slipped disc’ are L4/5 and L5/S1.
The nerve roots are most vulnerable at two sites:
1) Where they cross the intervertebral disc - central herniation
2) Where they exit the spinal canal in the intervertebral foramen - far lateral herniation - exiting nerve root affected
Paracentral most common - posterolateral- traversing nerve root affected.
In paracentral L4/L5 disc herniation it would be the traversing nerve root L5 that is affected.
In lateral herniation it is the exiting nerve root L4 that is affected.
In central herniation it is directly towards the spinal cord which can cause cauda equina syndrome
Radicular leg pain (“Sciatica”)
Irritation or compression of nerve roots that contribute to sciatic nerve.
The pain experienced is in the back and radiates to the dermatome supplied by the affected nerve root.
Parasthesia only in dermatomes and not back.
Typical distribution of pain:
L4 sciatica: anterior thigh, anterior knee, medial leg
L5 sciatica: lateral thigh, lateral leg, dorsum of foot
S1 sciatica: posterior thigh, posterior leg, heel, sole of foot
Cauda equina syndrome
prolapsed intervertebral
disc that compresses the lumbar and sacral
nerve roots within the spinal canal.
Other causes - tumours, infection, spinal stenosis, fractures etc
Symptoms:
Bilateral sciatica
Perianal numbness
Painless retention of urine
Urinary / faecal incontinence
Erectile dysfunction
Treatment - surgical decompression within 48 hours of the onset of sphincter symptoms
Consequences- intermittent self-catheterisation to pass urine, faecal incontinence, lower limb weakness requiring a wheelchair.
