Spinal Cord and Spinal Nerves

A&P I - Test 3

Skull and vertebral column

Bony encasement

Meninges

Three distinct protective, connective tissue coverings that encircle the spinal cord and brain

Cerebrospinal fluid

between 2 of the layers of meninges and suspends the nervous tissue of the CNS

Vertebral canal

The vertebral foramen of each vertebrae stacked

Dura mater

Most superficial layer
Thick, strong, dense irregular connective tissue
Continuous with epineurium

Arachnoid mater

Middle layer
Avascular
Comprised of cells and thin loosely arranged collagen and elastic fibers

Subdural space

Between the dura mater and the arachnoid mater (contains interstitial fluid)

Pia mater

Deepest layer
Thin transparent connective tissue layer
Adheres to the surface of the spinal cord and brain
Many blood vessels that supply oxygen and nutrients to the spinal cord
Contains denticulate ligaments

Denticulate ligaments

Thickenings of pia mater that fuse with the other two layers to prevent displacement of the spinal cord

Subarachnoid space

Between the arachnoid mater and pia mater (contains cerebrospinal fluid)

Medulla oblongata

Most inferior part of the brain where the spinal cord extends from

Cervical enlargement

Nerves to and from the upper extremities arise here

Lumbrosacral enlargement

Nerves to and from the lower extremities arise here

Conus medullaris

Terminal end of the spinal cord at the superior border of the 2nd lumbar vertebra

Filum terminale

An extension of pia mater that arises from the conus medullaris (connective tissue)

Spinal nerves

Paths of communication between the spinal cord and specific regions of the body

Roots

2 bundles of axons that connect each spinal nerve to a "segment" of the spinal cord by rootlets

Rootlets

Even smaller bundles of axons that the roots divide into adjacent to the spinal cord

Posterior root and rootlets

Contain only sensory axons from receptors in the skin, muscles, and internal organs
Conduct impulses from receptors to the CNS

Anterior root and rootlets

Contain axons of motor neurons
Conduct impulses from CNS to effectors (muscles and glands)

Cauda equinea

Collection of spinal nerves within the inferior vertebral column inferior to the conus medullaris

Gray mater

Dendrites, cell bodies, unmyelinated axons, neuroglia

White mater

Bundles of myelinated axons

Myelinated

Areas of axons surrounded by multilayered lipid and protein coverings (myelin sheath) that significantly increases the speed of nerve impulse conduction

Grey commissure

Central portion of the grey mater connecting the two sides

Central canal

Small space spanning the entire length of the spinal cord, filled with cerebrospinal fluid

Anterior white commissure

Connects white mater of the two sides

Nuclei

Clusters of neuron cell bodied grey mater

Sensory nuclei

Receive input via sensory neurons

Motor nuclei

Provide output to effector tissue via motor neurons

Posterior grey horns

Contain axons of incoming sensory neurons and cell bodies and axons of interneurons

Anterior grey horns

Contain somatic motor nuclei that provide nerve impulses to skeletal muscles

Lateral grey horns

Contain automatic motor nuclei that regulate the activity of cardiac muscle, smooth muscle, and glands

Funiculi

Regions or anatomic division of the white mater

Tracts (fasciculi)

Bundles of axons traveling up and down the spinal cord

Sensory tracts (ascending)

Bundles of axons conducting impulses toward the brain

Motor tracts (descending)

Bundles of axons conducting impulses away from the brain

Endoneurium

Wraps individual axons (both myelinated and unmyelinated)
Consists of a mesh of collegen fibers, fibroblasts, and macrophages

Nerve fascicles

Bundles of axons and their endoneurium

Perineurium

Thicker layer of connective tissue surrounding nerve fascicles (axon bundles)

Epineurium

Outermost covering of the entire nerve
Consists of fibroblasts and thick collagen fibers

Rami

Branches of the spinal nerve after passing through the intervertebral foramen

Posterior ramus

Serves the deep muscles and skin of the posterior surface of the trunk

Anterior ramus

Serves muscles and structures of the upper and lower limbs and the lateral and anterior surfaces of the trunk

Meningeal branch

Reenters the vertebral cavity through the intervertebral foramen
Serves vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges

Communicating rami

Components of the autonomic nervous system

Cervical plexus

Formed by the anterior rami of C1-C5, serves the head, neck, and diaphragm

Phrenic nerve

Supplies motor fibers to the diaphragm

Brachial plexus

Formed by anterior rami of C5-C8 and T1 bilaterally
Provides the nerve supply skin and muscles of the shoulder and upper extremities

Axillary nerve

Deltoid and teres minor muscles

Musculocutaneous nerve

Anterior muscles of the arm

Radial nerve

Muscles of posterior arm and forearm

Median nerve

Muscles of anterior forearm and some of the hand

Ulnar nerve

Muscles of anteromedial forearm and most of the hand

Lumbar plexus

Formed by anterior rami of L1-L4 bilaterally
Supplies skin and muscles of anterolateral abdominal wall, external genitalia, parts of the lower limbs

Femoral nerve

Flexors of hip, extensors of knee

Obturator nerve

Adductors of hip joint

Sacral plexus

Formed by anterior rami of L4-L5 and S1-S4
Located mostly anterior to the sacrum
Supplies muscles and skin of buttocks, perineum, lower limbs

Sciatic nerve

Knee extensors, hip adductors, and actually two nerves bonded together (tibial and common fibular nerve)

Tibial nerve

Primarily plantar flexors of foot and toes

Common fibular nerve

Controls muscles on the anterior and lateral leg (primarily dorsiflexors) and foot

Pundendal nerve

Muscles of perineum

Coccygeal plexus

Formed from S4-S5 and Co1

Anococcygeal nerves

Supply a small area of skin to the coccygeal region

Intercostal nerves

T2-T12
Anterior rami directly connect to structures they supply

Anterior ramus of T2

Supplies intercostal muscles of 2nd intercostal space, skin of chest, axilla and medial aspect of upper arm

Anterior rami of T2-T6

Supply intercostal muscles and lateral and anterior skin of the chest wall

Anterior rami of T7-T12

Supply the intercostal muscles, abdominal muscles, and overlying skin

Posterior rami of intercostal nerves

Supply deep back muscles and skin of posterior thorax

Dermatomes

Sensory neurons that serve a specific, predictable segment of the body
These neurons carry nerve impulses to the brain and spinal cord

Two main functions of spinal cord and homeostasis

1) Nerve impulse propagation (occurs in the tracts of the white matter)
2) Integration of incoming and out going information (occurs in the grey matter)

Reflex

A fast, involuntary sequence of actions that occurs in response to a particular stimulus

Spinal reflex

Integration takes places in the grey matter of the spinal cord

Cranial reflex

Integration occurs in the brainstem

Somatic reflex

Response involves contraction of skeletal muscles

Autonomic reflex

Response involves smooth muscle, cardiac muscle, and glands

Reflex arc

A relatively direct connection between a sensory neuron and a motor neuron that allows an extremely rapid response to a stimulus, often without conscious brain involvement.

1. Sensory receptor

- Dendrites of sensory neuron or sensory receptor receive a stimulus
- Produces a generator potential
- Causes one or more nerve impulses of the sensory neuron

2. Sensory neuron

- Sensory nerve impulse propagates from the sensory receptor along the axon to the axon terminals (located in the grey matter of the spinal cord)
- Relay neurons send impulse to the brain allowing for conscious awareness that the reflex has occurred (body jerking before you realize someone's touched you)

3. Integrating center

One or more regions of the grey matter that relay impulses from sensory to motor neurons

Monosynaptic reflex arc

A single synapse between a sensory and motor neuron

Polysynaptic reflex arc

More than one synapse and more than two types of neurons (involves interneurons)

4. Motor neuron

Impulse triggered by the integrating center propagate out of the CNS along a motor neuron to an effector

5. Effector

Muscle or gland responds to a motor nerve impulse

Stretch reflex

Controls muscle length

Muscle is stretched

- Contraction of the stretched muscle
- Relaxation of the antagonistic muscles
- Nerve impulses to the brain to allow consciousness of the nerve impulses

Tendon reflex

Controls muscle tension

Muscle tension increases

- Causes relaxation of the muscle before tension becomes so great that a tendon is torn
- Causes antagonistic muscles to contract
- Causes an impulse to the brain for conscious awareness of the states of muscle tension

Flexor and cross extensor reflexes

Protective effect

Intersegmental reflex arc

Impulse from one sensory nerve, activates interneurons that ascend and descend the spinal cord and activate motor neurons of multiple spinal segments

Monoplegia

Paralysis of one limb

Diplegia

Paralysis of both upper limbs or both lower limbs

Paraplegia

Paralysis of both lower limbs

Hemiplegia

Paralysis of one side of the body

Quadriplegia

Paralysis of all four limbs

Complete transection

Severed spinal cord causing loss of sensation and all voluntary movement below the level of transection

Transection of C3 or above

No function maintained below the neck, diaphragm is paralyzed

Lower cervical transection

Some arm and chest muscle movement, diaphragm may or may not be affected

High thoracic transection

Arm function remains intact, lower extremity function lost

Mid thoracic transection

Maintain control of trunk above the umbilicus