Spinal Cord and Spinal Nerves

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Spinal Cord and Spinal Spinal Cord and Spinal NervesNerves

Spinal CordSpinal Cord

• Enclosed in the vertebral canal, extends from the foramen magnum of the skull to the first or second lumbar vertebra where it terminates in the cone shaped conus medullaris

• Spinal meninges; Dura Mater – outerArachnoid Mater – middlePia Mater – inner

• Dura Mater – composed of dense, irregular connective tissue

• Arachnoid Mater – It is an avascular covering with a spider’s web arrangement of delicate collagen fibers and some elastic fibers

• Pia Mater – A vascular and thin transparent connective tissue layer composed of interlacing bundles of collagen fibers and some fine elastic fibers

• Subarachnoid space – between the arachnoid mater and pia mater which contains cerebrospinal fluid

The dura mater and arachnoid meningeal coverings extend beyond the conus medullaris, approximately to the level of S2

• Filium terminale – a fibrous extension of the pia mater, extends farther and attaches to the posterior coccyx

• 31 pairs of spinal nerves, which exit via the intervertebral foramina

• Since the spinal cord doesn’t extend to the end of the vertebral column, the spinal nerves emerging from the inferior end must travel through the vertebral canal until reaching the appropriate intervertebral foramina

• This collection of spinal nerves are called the cauda equina

Spinal Cord / Gray MatterSpinal Cord / Gray Matter

• Looks like an H

• Posterior or dorsal horns – posterior projections

• Anterior or ventral horns – anterior projections that contain cell bodies of motor neurons

• Lateral Horns – In the thoracic and lumbar regions there is a lateral outpocketing of gray matter

• Gray commissure – central area of gray matter

• Dorsal root – Sensory fibers enter the cord here

• Dorsal Root Ganglia – Cell bodies of sensory neurons located here

• Ventral Roots – Motor neurons leave the cord here

• Spinal nerves – are formed from the fusion of the dorsal and ventral roots

Spinal Nerves;

8 pairs of cervical nerves

12 pairs of thoracic nerves

5 pairs of lumbar nerves

5 pairs of sacral nerves

1 pair of coccygeal nerves

Spinal Cord / White MatterSpinal Cord / White Matter

• The anterior median fissure and the posterior median sulcus divide the spinal cord into R. and L. sides

• White matter is divided into columns;

Posterior Funiculus, Anterior Funiculus, and Lateral Funiculus

• Each column contains distinct bundles of nerve axons called tracts

Two Types of Tracts

1. Sensory (ascending) tracts – conduct nerve impulses toward the brain

2. Motor (descending) tracts – conduct impulses down the cord

Connective Tissue Coverings of Connective Tissue Coverings of Spinal NervesSpinal Nerves

• A fiber is a single axon within an endoneurium

• A fascicle is a bundle of fibers within a perineurium

• A nerve is a bundle of fascicles within an epineurium

Spinal Nerves and Nerve PlexusSpinal Nerves and Nerve Plexus

• Each nerve divides into dorsal and ventral rami

• Rami contains both motor and sensory rami

• Dorsal rami – serve the skin and musculture of the posterior body trunk at their approximate level of emergence

• Ventral rami of spinal nerves T2 –T12 – pass anteriorly to supply the muscles of intercostal spaces, and the skin and muscles of the anterior and lateral trunk

• Ventral rami of all other nerves – form complex networks of nerves called plexuses

• The plexuses diverge again to form peripheral nerves

Four Plexuses

1. Cervical

2. Brachial

3. Lumbar

4. Sacral

1. Cervical Plexus

Supplies the skin and muscles of the head, neck, and upper part of the shoulders and diaphragm

• Damage to the spinal cord above the origin of the phrenic nerves causes respiratory arrest.

• Breathing stops because the phrenic nerves stops sending impulses to the diaphragm

2. Brachial Plexus

Supplies the upper extremities and a number of neck and shoulder muscles

• In carpal tunnel syndrome there is damage to the median nerve

3. Lumbar Plexus

Supplies the anterolateral abdominal wall, external genitals, and part of the lower extremities

• Femoral Nerve - Largest nerve arising from the lumbar plexus

4. Sacral Plexus

Supplies the buttocks and part of the lower extremities

• Sciatic Nerve – Largest nerve arising form the sacral plexus

• Sciatica – Pain that extends from the buttock down the back of the leg due to injury to the sciatic nerve

Spinal Cord PhysiologySpinal Cord Physiology

• The white matter tracts are highways for nerve impulse conduction to and from the brain

Spinal Cord PhysiologySpinal Cord Physiology

• The gray matter receives and integrates incoming and outgoing information

Spinal Cord Physiology / Sensory Spinal Cord Physiology / Sensory TractsTracts

Sensory information from receptors travels up the spinal cord via the;

1. Spinothalamic tracts

2. Posterior column tract

Spinothalamic tracts carry pain, touch, and temperature impulses

Posterior columns carry proprioception and vibration

Spinal Cord Physiology / Motor Spinal Cord Physiology / Motor TractsTracts

Motor info. travels from the brain down the spinal cord to muscles and glands via the;

1. Pyramidal tracts

2. Extrapyramidal tracts

Pyramidal tracts control voluntary, purposeful motor movement of eyes, mouth, face, arms, and legs

Extrapyramidal tracts control more complex things like the accessory adjustments we make to muscle tone to adjust our posture

Spinal Cord Physiology / ReflexesSpinal Cord Physiology / Reflexes

• The gray matter of the spinal cord serves as an integrating center for spinal reflexes

Spinal Cord Physiology / ReflexesSpinal Cord Physiology / Reflexes

• A reflex is a fast, predictable, automatic response to changes in the environment that helps to maintain homeostasis

Spinal Cord Physiology / Reflex ArcSpinal Cord Physiology / Reflex Arc

Five functional components;

1. Receptor

2. Sensory Neuron

3. Motor Neuron

4. Integrating Center Neuron

5. Effector

Stretch ReflexStretch Reflex

1. Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles

2. In response to being stretched the muscle spindle generates a nerve impulse along a somatic sensory neuron and enters the spinal cord via the posterior root

3. In the integrating center of the spinal cord the sensory neuron makes an excitatory synapse with a motor neuron in the anterior gray horn

4. If the excitation is strong enough a nerve impulse is propagated along the motor neuron which extends from the spinal cord to the muscle

5. ACh is released at the NMJ which triggers a muscle action potential in the stretched muscle (effector) and the muscle contracts

Spinal Cord and Spinal Nerves

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