The Spinal Cord and Spinal Nerves

The brachial plexus provides almost the entire nerve supply of the shoulders and upper limbs Five large terminal branches arise from the brachial plexus:

(1) The axillary nerve supplies the deltoid and teres minor muscles. (2) The musculocutaneous nerve supplies the anterior muscles of the arm. (3) The radial nerve supplies the muscles on the posterior aspect of the arm and forearm. (4) The median nerve supplies most of the muscles of the anterior forearm and some of the muscles of the hand. (5) The ulnar nerve supplies the anteromedial muscles of the forearm and most of the muscles of the hand.

The Tendon Reflex ● Polysynaptic reflex. ● Control muscle tension by causing muscle relaxation when muscle tension is great. ● Sensory receptors

- Golgi tendon organs. ● ↑ Tension applied to the tendon → tendon organ stimulation → nerve impulse → spinal cord →motor neuron causes muscle relaxation and relieves tension.

The roots (anterior rami) of spinal nerves C5-C8 and T1 form the _____________________(BRĀ-kē-al), which extends inferiorly and laterally on either side of the last four cervical and first thoracic vertebrae (Figure 13.9a). It passes above the first rib posterior to the clavicle and then enters the axilla.

brachial plexus

What is the difference between a horn and a column in the spinal cord? As spinal nerves branch from the spinal cord, they pass laterally to exit the vertebral canal through the intervertebral foramina between adjacent vertebrae. However, because the spinal cord is shorter than the vertebral column, nerves that arise from the lumbar, sacral, and coccygeal regions of the spinal cord do not leave the vertebral column at the same level they exit the cord. The roots of these lower spinal nerves angle inferiorly alongside the filum terminale in the vertebral canal like wisps of hair. Accordingly, the roots of these nerves are collectively named the_____________________

cauda equina

dermatome

Dermatome is the area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal nerve (Cranial Nerve V).

intercostal nerves

nerves between the ribs

Distribution of Spinal Nerves

Branches Plexuses Intercostal Nerves

brachial plexus

C5-C8, T1

The white matter of the spinal cord, like the gray matter, is organized into regions. The anterior and posterior gray horns divide the white matter on each side into three broad areas called columns: _________________ Each column in turn contains distinct bundles of axons having a common origin or destination and carrying similar information. These bundles, which may extend long distances up or down the spinal cord, are called tracts. Recall that tracts are bundles of axons in the CNS, whereas nerves are bundles of axons in the PNS. _________________consist of axons that conduct nerve impulses toward the brain. Tracts consisting of axons that carry nerve impulses from the brain are called ________________________. Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain.

(1) anterior (ventral) white columns (2) posterior (dorsal) white columns (3) lateral white columns . Sensory (ascending) tracts motor (descending) tracts

Meninges The meninges are three protective, connective tissue coverings that encircle the spinal cord and brain. From superficial to deep they are the__________________

(1) dura mater (2) arachnoid mater (3) pia mater

dura mater

). The most superficial of the three spinal meninges is a thick strong layer composed of dense irregular connective tissue. The dura mater forms a sac from the level of the foramen magnum in the occipital bone, where it is continuous with the meningeal dura mater of the brain, to the second sacral vertebra. The dura mater is also continuous with the epineurium, the outer covering of spinal and cranial nerves.

Functions of the Spinal Cord and Spinal Nerves

1.The white matter of the spinal cord contains sensory and motor tracts, the "highways" for conduction of sensory nerve impulses toward the brain and motor nerve impulses from the brain toward effector tissues. 2.The spinal cord gray matter is a site for integration (summing) of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). 3.Spinal nerves and the nerves that branch from them connect the CNS to the sensory receptors, muscles, and glands in all parts of the body. QBased

spinal nerves

31 pairs; mixed nerves. Cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5) and coccygeal.

Branches

A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches (Figure 13.6). These branches are known as rami (RĀ-mī = branches). The posterior (dorsal) ramus (RĀ-mus; singular form) serves the deep muscles and skin of the posterior surface of the trunk. The anterior (ventral) ramus serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk. In addition to posterior and anterior rami, spinal nerves also give off a meningeal branch (me-NIN-jē′-al). This branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges. Other branches of a spinal nerve are the rami communicantes (kō-mū-ni-KAN-tēz), components of the autonomic nervous system that will be discussed in

Connective tissue coverings of spinal nerves:

Epineurium, perineurium and endoneurium: Fascicles

Important nerves that arise from the brachial plexuses are

Erb-Duchenne palsy (waiter's tip)- loss of sensation along the lateral side of the arm. Wrist drop- inability to extend the wrist and fingers.

Cervical Plexus

Formed by the anterior Formed by the anterior rami of C1-C5. rami of C1-C5. Phrenic nerves- Phrenic nerves- important nerves from important nerves from the cervical plexuses. the cervical plexuses.

brachial plexus

Formed by the anterior rami of C5-C8 & T1. Supplies the shoulders and upper limbs. Roots → trunks → divisions → cords → nerves.

coccygeal plexus

Formed by the anterior rami of S4-S5 and the coccygeal nerves. Supplies a small area of skin in the coccygeal region.

● Median nerve palsy- numbness, tingling and pain in the palm and fingers. ● Ulnar nerve palsy- inability to abduct or adduct fingers ● Winged scapula- the arm cannot be abducted beyond the horizontal position.

Injuries to the Brachial Plexus

Distribution of Spinal Nerves

Spinal nerves branch and their braches are called rami: Posterior (dorsal) ramus Anterior (ventral) ramus Plexuses: a network of axonsAnterior rami except T2-T12 form plexuses.

The Stretch Reflex ● Causes contraction of a skeletal muscle in response to stretching of the muscle. ● Monosynaptic reflex. ● Patellar or knee-jerk reflex:

Stretching of a muscle →activation of muscle spindles →sensory neuron →spinal cord→motor neuron → muscle contraction. ● Ipsilateral.

pia mater

This innermost meninx is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. It consists of thin squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers. Within the pia mater are many blood vessels that supply oxygen and nutrients to the spinal cord. Triangular-shaped membranous extensions of the pia mater suspend the spinal cord in the middle of its dural sheath. These extensions, called denticulate ligaments are thickenings of the pia mater. They project laterally and fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side (Figure 13.1a, b). Extending along the entire length of the spinal cord, the denticulate ligaments protect the spinal cord against sudden displacement that could result in shock. Between the arachnoid mater and pia mater is a space, the subarachnoid space, which contains shock-absorbing cerebrospinal fluid.

arachnoid mater

This layer, the middle of the meningeal membranes, is a thin, avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibers. It is called the arachnoid mater because of its spider's web arrangement of delicate collagen fibers and some elastic fibers. It is deep to the dura mater and is continuous through the foramen magnum with the arachnoid mater of the brain. Between the dura mater and the arachnoid mater is a thin subdural space, which contains interstitial fluid.

As noted earlier, a typical spinal nerve has two connections to the cord:

a posterior root and an anterior root

The pathway followed by nerve impulses that produce a reflex is a reflex arc. A reflex arc includes:

a. sensory receptor b. sensory neuron c. integrating center d. motor neuron e. effector

Sensory and Motor Tracts As noted previously, one of the ways the spinal cord promotes homeostasis is by conducting nerve impulses along tracts. Often, the name of a tract indicates its position in the white matter and where it begins and ends. For example, the ___________________ is located in the anterior white column; it begins in the _____________________(superficial gray matter of the cerebrum of the brain) and ends in the_________________. Notice that the location of the axon terminals comes last in the name. This regularity in naming allows you to determine the direction of information flow along any tract named according to this convention.

anterior corticospinal tract cerebral cortex spinal cord

A transverse section of the spinal cord reveals regions of white matter that surround an inner core of gray matter . The white matter of the spinal cord consists primarily of bundles of myelinated axons of neurons. Two grooves penetrate the white matter of the spinal cord and divide it into right and left sides. The ____________________ is a wide groove on the anterior (ventral) side. The posterior median __________________ is a narrow furrow on the posterior (dorsal) side. The gray matter of the spinal cord is shaped like the letter H or a butterfly; it consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia. The ________________ (KOM-mi-shur) forms the crossbar of the H. In the center of the gray commissure is a small space called___________________; it extends the entire length of the spinal cord and is filled with cerebrospinal fluid. At its superior end, the central canal is continuous with the fourth ventricle (a space that contains cerebrospinal fluid) in the medulla oblongata of the brain. Anterior to the gray commissure is the ________________white commissure, which connects the white matter of the right and left sides of the spinal cord.

anterior median fissure sulcus gray commissure the central canal anterior (ventral)

Pia mater This innermost meninx is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. It consists of thin squamous to cuboidal cells within interlacing bundles of collagen fibers and some fine elastic fibers. Within the pia mater are many blood vessels that supply oxygen and nutrients to the spinal cord. Triangular-shaped membranous extensions of the pia mater suspend the spinal cord in the middle of its dural sheath. These extensions, called________________ are thickenings of the pia mater. They project laterally and fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side (Figure 13.1a, b). Extending along the entire length of the spinal cord, the denticulate ligaments protect the spinal cord against sudden displacement that could result in shock. Between the arachnoid mater and pia mater is a space, the subarachnoid space, which contains shock-absorbing cerebrospinal fluid.

denticulate ligaments

The skin over the entire body is supplied by somatic sensory neurons that carry nerve impulses from the skin into the spinal cord and brain. Each spinal nerve contains sensory neurons that serve a specific, predictable segment of the body. One of the cranial nerves, the trigeminal (V) nerve, serves most of the skin of the face and scalp. The area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal (V) nerve is called a ___________________________(Figure 13.7). The nerve supply in adjacent dermatomes overlaps somewhat. Knowing which spinal cord segments supply each dermatome makes it possible to locate damaged regions of the spinal cord. If the skin in a particular region is stimulated but the sensation is not perceived, the nerves supplying that dermatome are probably damaged. In regions where the overlap is considerable, little loss of sensation may result if only one of the nerves supplying the dermatome is damaged. Information about the innervation patterns of spinal nerves can also be used therapeutically. Cutting posterior roots or infusing local anesthetics can block pain either permanently or transiently. Because dermatomes overlap, deliberate production of a region of complete anesthesia may require that at least three adjacent spinal nerves be cut or blocked by an anesthetic drug.

dermatome

spinal meninges

dura mater, arachnoid mater, pia mater

Each spinal nerve and cranial nerve consists of many individual axons and contains layers of protective connective tissue coverings (Figure 13.5). Individual axons within a nerve, whether myelinated or unmyelinated, are wrapped in _________________, the innermost layer. The endoneurium consists of a mesh of collagen fibers, fibroblasts, and macrophages. Groups of axons with their endoneurium are held together in bundles called____________________, each of which is wrapped in ______________________ , the middle layer. The perineurium is a thicker layer of connective tissue. It consists of up to 15 layers of fibroblasts within a network of collagen fibers. The outermost covering over the entire nerve is the _________________ . It consists of fibroblasts and thick collagen fibers. Extensions of the epineurium also fill the spaces between fascicles. The dura mater of the spinal meninges fuses with the epineurium as the nerve passes through the intervertebral foramen. Note the presence of blood vessels, which nourish the spinal meninges . You may recall from Chapter 10 that the connective tissue coverings of skeletal muscles—endomysium, perimysium, and epimysium—are similar in organization to those of nerves.

endoneurium perineurium epineurium

In the _________________ of the spinal cord and brain, clusters of neuronal cell bodies form functional groups called____________. Sensory nuclei receive input from receptors via sensory neurons, and motor nuclei provide output to effector tissues via motor neurons. The gray matter on each side of the spinal cord is subdivided into regions called ___________________. The________________ gray horns contain axons of incoming sensory neurons as well as cell bodies and axons of interneurons. Recall that cell bodies of sensory neurons are located in the posterior (dorsal) root ganglion of a spinal nerve. The ___________________________ contain somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles. Between the posterior and anterior gray horns are the lateral gray horns, which are present only in thoracic and upper lumbar segments of the spinal cord. The lateral gray horns contain autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands.

gray matter nuclei horns posterior (dorsal) anterior (ventral) gray horns

dermatome

instrument used to cut skin (in thin slices for skin grafts)

Intercostal Nerves The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are known as______________. These nerves directly connect to the structures they supply in the intercostal spaces. After leaving its intervertebral foramen, the anterior ramus of nerve T2 innervates the intercostal muscles of the second intercostal space and supplies the skin of the axilla and posteromedial aspect of the arm. Nerves T3-T6 extend along the costal grooves of the ribs and then to the intercostal muscles and skin of the anterior and lateral chest wall. Nerves T7-T12 supply the intercostal muscles and abdominal muscles, along with the overlying skin. The posterior rami of the intercostal nerves supply the deep back muscles and skin of the posterior aspect of the thorax.

intercostal nerves or thoracic nerves

cervical plexus

is formed by the roots (anterior rami) of the first four cervical nerves (C1-C4), with contributions from C5 (Figure 13.8). There is one on each side of the neck alongside the first four cervical vertebrae.

spinal cord

is roughly oval in shape, being flattened slightly anteriorly and posteriorly. In adults, it extends from the medulla oblongata, the inferior part of the brain, to the superior border of the second lumbar vertebra.In newborn infants, it extends to the third or fourth lumbar vertebra. During early childhood, both the spinal cord and the vertebral column grow longer as part of overall body growth. Elongation of the spinal cord stops around age 4 or 5, but growth of the vertebral column continues. Thus, the spinal cord does not extend the entire length of the adult vertebral column. The length of the adult spinal cord ranges from 42 to 45 cm (16-18 in.). Its maximum diameter is approximately 1.5 cm (0.6 in.) in the lower cervical region and is smaller in the thoracic region and at its inferior tip.

Spinal nerves: are associated with the spinal cord and, like all nerves of the peripheral nervous system (PNS), are parallel bundles of axons and their associated _________________ wrapped in several layers of connective tissue. Spinal nerves connect the CNS to sensory receptors, muscles, and glands in all parts of the body. The 31 pairs of spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge . Not all spinal cord segments are aligned with their corresponding vertebrae. Recall that the spinal cord ends near the level of the superior border of the second lumbar vertebra (L2), and that the roots of the lumbar, sacral, and coccygeal nerves descend at an angle to reach their respective foramina before emerging from the vertebral column. This arrangement constitutes the cauda equina.

neuroglial cells

A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches (Figure 13.6). These branches are known as _____________________ The ___________________________ serves the deep muscles and skin of the posterior surface of the trunk. The ____________________ serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk. In addition to posterior and anterior rami, spinal nerves also give off a _________________________. This branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges. Other branches of a spinal nerve are the ________________________, components of the autonomic nervous system that will be discussed in

rami posterior (dorsal) ramus anterior (ventral) ramus meningeal branch rami communicantes

The _________________surround the spinal cord and are continuous with the _____________________ which encircle the brain. All three spinal meninges cover the spinal nerves up to the point where they exit the spinal column through the intervertebral foramina. The spinal cord is also protected by a cushion of fat and connective tissue located in the _______________________, a space between the dura mater and the wall of the vertebral canal . Following is a description of each meningeal layer.

spinal meninges cranial meninges epidural space

Since the brachial plexus is so complex, an explanation of its various parts is helpful. As with the cervical and other plexuses, the roots are the anterior rami of the spinal nerves. The roots of several spinal nerves unite to form__________________in the inferior part of the neck. These are the superior, middle, and inferior trunks. Posterior to the clavicles, the trunks diverge into divisions, called the anterior and posterior divisions. In the axillae, the divisions unite to form____________called the lateral, medial, and posterior cords. The cords are named for their relationship to the axillary artery, a large artery that supplies blood to the upper limb. The branches of the brachial plexus form the principal nerves of the brachial plexus.

trunks cords

reflex

● A reflex is an automatic, sudden, involuntary response to a stimulus. ● When the integration takes place in the spinal cord, the reflex is a spinal reflex.

Important nerves that arise from the brachial plexuses are

● Axillary nerve ● Musculocutaneous nerve ● Radial nerve ● Median nerve ● Ulnar nerve

lumbar plexus

● Formed by the anterior rami of L1-L4. ● Supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs. ● Femoral nerves, obturator nerves.

sacral plexus

● Formed by the anterior rami of L4-L5 and S1- S4. ● Supplies the buttocks, perineum, and lower limbs. ● Gives rise to the largest nerve in the body- the sciatic nerve.

sensory and motor tracts

● The name of the tract often indicates its location in the white matter and where it begins and ends. ● The white matter contains both sensory (In Blue) and motor (In Red) tracts.