The Spinal Cord and Spinal Nerves Bio 137 Anatomy & Physiology I

Spinal Cord Protection Bones Spinal cavity (Vertebral Column) Meninges – below bony coverings Dura mater Arachnoid mater Pia mater Cerebrospinal Fluid

Meninges of the Spinal Cord

Spinal Cord Meninges: Dura Mater Dura mater continues into vertebral canal as a sheath that surrounds the spinal cord The dura mater of the spinal cord is not directly attached to the bone of the vertebra An epidural space is found between the dural sheath and bony walls of vertebrae This space is filled with loose connective tissue, vessels and fat Provides a cushion for the cord

Meninges of Brain & Cord

CSF Circulation CSF circulates in lateral ventricles to 3rd and 4th ventricles of brain and to central canal of spinal cord CSF is also found in subarachnoid spaces

External Cord Anatomy The spinal cord begins as a continuation of the medulla oblongata (the most inferior portion of the brain stem) Extends from the foramen magnum of the occipital bone Terminates as the conus medullaris between L1 - L2.

External Cord Anatomy The spinal cord has two enlargements, one in the cervical area from C4–T1, and another in the lumbar area between T9–T12. The cervical enlargement correlates with the sensory input and motor output to the upper extremities. The lumbar enlargement handles motor output and sensory input to and from the legs.

External Cord Anatomy Arising from the conus medullaris is the filum terminale, an extension of the pia mater that extends inferiorly and blends with the arachnoid and dura to anchor the spinal cord to the coccyx. The cauda equina or “horses tail” are the roots of the lower spinal nerves that angle down alongside the filum terminale.

Spinal nerves below conus medullaris descend and form cauda equina

PNS: Spinal Nerves 31 segments of the spinal cord All are Mixed nerves Each gives rise to a pair of spinal nerves, which branch to various body parts All are Mixed nerves 8 cervical (numbered C1 to C8) 12 thoracic (T1 to T12) 5 lumbar (L1 to L5) 5 sacral (S1 to S5) 1 coccygeal (Co) Please keep in mind that these numbers refer to the spinal nerves

Lumbar Puncture A needle inserted into the subarachnoid space for the purpose of withdrawing CSF (for diagnosis or to reduce pressure) or to introduce a drug or contrast agent is called a lumbar puncture. CSF is often collected to diagnose meningitis or some other disease of the CNS. Agents injected into the subarachnoid space include drugs such as antibiotics, chemotherapeutic agents, or analgesics, or contrast media for radiographic procedures. The pressure of CSF in the SAS can also be measured during a lumbar puncture.

Lumbar Puncture The site used for most lumbar punctures is between the 3rd and 4th (or 4th and 5th) lumbar vertebrae - below the termination of the actual cord in the region of the cauda equina. With the needle in the SAS, CSF can be sampled. Anesthetics can also be given in this way, but using 1/10 the dose required for epidural anesthesia.

Spinal Cord Structure Cross Section

Cross Section of Spinal Cord Anterior median fissure and Posterior median sulcus divide cord into left and right halves

Spinal Cord Cross Section White matter surrounds ‘butterfly’ of gray matter WHITE MATTER GRAY MATTER Posterior horns Lateral horns Anterior horns Gray commissure Surrounds central canal Composed of Interneuronal cell bodies and some motor neuron cell bodies Posterior column (funiculi) Lateral column Anterior column Composed of bundles of myelinated axons that comprise major nerve pathways called nerve tracts

Spinal Cord Model (cross-section)

Spinal Cord Structures Dorsal Root Ganglion Cell bodies of sensory neurons (unipolar) Dorsal Root Axons of sensory neurons Ventral Root Axons of motor neurons Fusion of dorsal root & ventral root forms a spinal nerve

Spinal Cord Model (cross-section) Photo from

Internal Cord Anatomy A tract is a bundle of neuronal axons that are all located in a specific area of the cord and all traveling to the same place (higher or lower in the brain or cord). Sensory (Ascending) Tracts Sensory impulses from body to brain Motor (Descending) Tracts Motor impulses from brain to body

Sensory Tract

Motor Tract

Spinal Cord Injury If damage is to ascending tracts, lose sensory function below injury If descending tracts are damaged, lose motor function below injury

Reflexes A reflex is a fast, involuntary response to a stimulus. In a spinal reflex the integration takes place in the spinal cord, not the brain.

Reflexes A reflex arc is a pathway that a nerve impulse follows to produce a reflex. Components of a reflex arc: Sensory receptor and a sensory neuron Integrating center inside the cord A motor neuron An effector Usually some sort of muscle or a gland which makes something move or secrete (“involuntarily”).

Reflex Arc

Withdrawal Reflex At least a 3-neuron pathway Protective, prevents or limits tissue damage

Knee-Jerk Reflex 2-neuron pathway that maintains an upright position

Spinal Nerves: Nerve Plexus Spinal nerves exit out intervertebral foramina and immediately branch Anterior rami (branches) of spinal nerves (except T2-T11) combine to form a nerve plexus Fibers (anterior branches) of various spinal nerves are sorted and recombined to feed an area of the body Particular body part receives ‘nerves’ consisting of nerve fibers from different starting points of origin in cord

Nerve Plexuses The cervical plexus, formed by the anterior rami of C1-C5, serves the head, neck, and diaphragm. The phrenic nerves arise from the cervical plexus to supply the major muscle of respiration (“C3,4,5 keep the diaphragm alive”).

Brachial Plexus Formed by anterior branches C5-C8 and T1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Formed by anterior branches C5-C8 and T1 Lies deep within shoulders There are five (5) branches Supply muscles of the arms and hands and some shoulder, chest and back muscles C5 Ventral rami: C5, C6, C7, C8, T1 Trunks: upper, middle, lower C6 C7 Anterior divisions C8 Posterior divisions C5 Dorsal scapular n. T1 C6 Suprascapular n. C7 Axillary n. Lateral pectoral n. C8 Humerus Medial pectoral n. Lower subscapular n. Thoracodorsal n. T1 Musculocutaneous n. Median n. Median n. Ulnar n. Axillary n. Musculo- cutaneous n. Radial n. (a) Ulnar n. Radial n. Ulna Radius (b)

Nerve Plexuses Some of the major nerves that arise from the brachial plexuses are the: musculocutaneous nerve axillary nerve radial nerve median nerve ulnar nerve long thoracic nerve

Nerve Plexuses The ulnar nerve is the largest unprotected (by muscle or bone) nerve in the human body. It emerges from the medial and lateral cords of the brachial plexus to supply the medial half of the hand. Striking the medial epicondyle of the humerus where the nerve is exposed is referred to as bumping one’s “funny bone”. Damage to the nerve leads to abnormal sensations in the 4–5th fingers and an inability to abduct or adduct the little and ring fingers.

Nerve Plexuses The lumbar plexus is formed by the anterior rami of L1-L4 to supply the anterolateral abdominal wall, external genitalia, and part of the lower limbs. The femoral and obturator nerves come from the lumbar plexus.

Nerve Plexuses The sacral plexus is formed by the anterior rami of L4-L5 and S1-S4. It supplies the buttocks, perineum, and part of the lower limbs. It gives rise to the largest nerve in the body, the sciatic nerve.

Nerve Plexuses The coccygeal plexus is formed by the anterior rami of S4-S5 and the coccygeal nerves. It is a small plexus from which the anococcygeal nerve exits to supply a small area of skin in the coccygeal region.

Spinal Cord versus spinal nerve injuries Which is worse? Why?