Ch 16 spinal cord

¾ of an inch in diameter, 16–18 in. long

Extends from foramen magnum to L1 vertebra

function: pathway for sensory and motor impulses

•reflexes

inferior end of spinal cord

•“Horse’s tail”—axons that extend inferiorly from spinal cord’s conus medullaris

•Thin strand of pia mater within cauda equina that attaches conus medullaris to coccyx

•the inferior cervical part of the spinal cord and innervates the upper limbs

•extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs

•Posterior median sulcus

•Anterior median fissure

•8 cervical nerves (C1–C8)

•12 thoracic nerves (T1–T12)

•5 lumbar nerves (L1–L5)

•5 sacral nerves (S1–S5)

•1 coccygeal nerve (Co1)

between dura and periosteum of vertebra

Epidural anesthesia is given here

single meningeal layer (in contrast to double-layered dura around brain)

•Stabilizes spinal cord

•Fuses with connective tissue around spinal nerves

potential space separating dura mater from deeper arachnoid mater

real space filled with cerebrospinal fluid (CSF)

innermost meninx, adheres to the spinal cord

-delicate layer of elastic and collagen fibers

-Has paired, lateral triangular extensions called denticulate ligaments, anchor spinal cord laterally to the dura mater

•Gray matter: dendrites and cell bodies of neurons, unmyelinated axons, and glial cells

•White matter: Myelinated axons

Anterior horns house somas of somatic motor neurons

Lateral horns contain somas of autonomic motor neurons

-Only in T1–L2

Posterior horns contain axons of sensory neurons and cell bodies of interneurons

Gray commissure contains unmyelinated axons for communication between right and left side

-houses a narrow central canal

called nuclei:

•Sensory nuclei in posterior horns contain interneuron cell bodies of somatic sensory nuclei and visceral sensory nuclei

•Motor nuclei in the anterior horns contain somatic motor nuclei

•Autonomic motor nuclei are in the lateral horns

White matter is external to spinal gray matter

Partitioned into three funiculus:

•Posterior funiculus

•Lateral funiculus

•Anterior funiculus (connected by white commissure)

Axons within each funiculus are organized into smaller structural units called tracts

•Endoneurium

•Perineurium

•Epineurium

Multiple anterior rootlets arise from the spinal cord

•contain motor axons only

•Motor axons arise from cell bodies in the anterior and lateral horns of the spinal cord

Multiple posterior rootlets

• sensory axons only

•Sensory axons arise from cell bodies in the posterior root ganglion, which is attached to the posterior root

Each anterior root and its corresponding posterior root unite within the intervertebral foramen

•both motor and sensory axons

- spinal nerves (C1–C7) exit the intervertebral foramen above the vertebra of the same number

•spinal nerves (C8) exit above the first thoracic vertebra

•The remaining spinal nerves exit below the vertebra of the same number

•Because the spinal cord is shorter than the vertebral canal, roots of lumbar and sacral spinal nerves travel inferiorly to reach their respective intervertebral foramen

After leaving intervertebral foramen, spinal nerve splits into rami

•posterior ramus innervates deep muscles and skin of the back

•anterior ramus innervates the anterior and lateral portions of the trunk and the upper and lower limbs

-Each anterior ramus splits into multiple branches

-Many of the anterior rami form nerve plexuses

-rami communicantes, extend between the spinal nerves and the sympathetic trunk ganglia

segment of skin supplied by a single spinal nerve

•All spinal nerves except C1 innervate a segment of skin

dermatome map: Skin divided into sensory segments

•Anesthesia in a region could indicate specific spinal nerve damage

•Shingles rash -blisters along dermatome

referred visceral pain- pain in one organ is mistakenly referred to a dermatome (potentially nowhere near the organ in pain)

network of interweaving anterior rami of spinal nerves

-The major plexuses: the cervical, brachial, lumbar, and sacral plexuses

Anterior rami of spinal nerves T1–T11 -spaces between adjacent ribs

Spinal nerve T12 is called a subcostal nerve, because it arises below the ribs

With the exception of spinal nerve T1, the intercostal nerves do not form plexuses

-Part of T1 helps form brachial plexus; and another lies in first intercostal space

-T2 innervates intercostal muscles of the second intercostal space and is sensory for axilla and medial surface of arm

-T3–T6 innervate intercostal muscles and are sensory for the anterior chest wall

-T7–T12 innervate intercostal muscles, abdominal muscles, and overlying skin

formed by anterior rami of spinal nerves C1–C4

Branches of the plexus innervate anterior neck muscles, skin of the neck, and parts of the head and shoulders

originated primarily from C4 of C3 and C5

•Travels through the thoracic cavity to innervate the diaphragm

-nerves that supply the upper limbs

Each plexus is formed by the anterior rami of spinal nerves C5–T1

-innervates the pectoral girdle and the entire upper limb of one side

Anterior rami of C5–T1

•Superior trunk: Nerves C5 and C6

•Middle trunk: Nerve C7

•Inferior trunk: Nerves C8 and T1

anterior division and a posterior division

The anterior and posterior divisions converge to form three cords:

•Posterior cord

•Medial cord

•Lateral cord

•Axillary nerve

•Median nerve

•Musculocutaneous nerve

•Radial nerve

•Ulnar nerve

C5, C6

Formed from posterior cord, posterior division of the brachial plexus

motor innervation

Deltoid (arm abductor)

Teres minor (lateral rotator of arm)

Cutaneous Innervation

Superolateral arm

C6-T1

Formed from medial and lateral cords, anterior division of the brachial plexus

motor innervation

-Most anterior forearm muscles (flexor carpi radialis, thumb, later 2 lumbrical)

Cutaneous Innervation

Palmar aspects and dorsal tips of lateral 3-1/2 digits (thumb, index finger, middle finger, and 1/2 of ring finger

C5-C7

Formed from the lateral cord, anterior division of the brachial plexus

motor innervation

Anterior arm muscles (flex humerus, flex elbow joint, supinate forearm)

Coracobrachialis, Biceps brachii, Brachialis

Cutaneous Innervation

Lateral region of forearm

C5-T1

Formed from the posterior cord, posterior division of the brachial plexus

Motor Innervation

Posterior arm muscles

(extend forearm)

Triceps brachii

Posterior forearm muscles

Supinator

Brachioradialis

Cutaneous Innervation

Posterior region of arm

Posterior region of forearm

Dorsal aspect of lateral three digits (except their distal tips)

C8-T1

Formed from the medial cord, anterior division of the brachial plexus

Motor Innervation

Anterior forearm muscles (flexors of wrist and digits)

Flexor carpi ulnaris

Intrinsic hand muscles

Hypothenar muscles

Cutaneous Innervation

Dorsal and palmar aspects of medial 1-1/2 digits (little finger, medial aspect of ring finger)

anterior rami of spinal nerves L1–L4

-The lumbar plexus is subdivided into an anterior division and a posterior division.

-The main nerve of the posterior division is the femoral nerve

-The main nerve of the anterior division is the obturator nerve

L2-L4

Motor Innervation

Anterior thigh muscles

Quadriceps femoris (knee extensor)

Rectus femoris

Vastus medialis Iliopsoas (hip flexor) Sartorius

Cutaneous Innervation

Anterior thigh

L2-L4

Motor Innervation

Medial thigh muscles (adductors of thigh)

Adductors

Gracilis

Pectineus¹

Obturator externus (lateral rotator of thigh)

Cutaneous Innervation

superomedial thigh

anterior rami of spinal nerves L4–S4

-The lumbar and sacral plexuses are sometimes considered together as the lumbosacral plexus

-The anterior rami are organized into an anterior division and a posterior division

largest and longest nerve

the tibial division and common fibular division wrapped in a common sheath

•Tibial nerve

•Common fibular nerve

-•Along the lateral knee, the common fibular nerve splits into the deep and superficial fibular nerves

-Additional branches from the tibial and common fibular nerves serve the calf, leg, and foot

•Sural, lateral sural cutaneous, and medial sural cutaneous nerves

L4-S3

Motor Innervation

Posterior thigh muscles (extend thigh and flex leg)

Long head of biceps femoris

Semimembranosus

Part of adductor magnus

Posterior leg muscles (plantar flexors of foot, flexors of knee)

Flexor hallucis longus

Gastrocnemius

Soleus

Popliteus

Tibialis posterior (inverts foot)

Plantar foot muscles (via medial and lateral plantar nerve branches)

Cutaneous Innervation

heel

L4-S2

(Divides into deep fibular and superficial fibular branches)

Motor Innervation

Short head of biceps femoris (knee flexor); see also deep fibular and superficial fibular nerves

L4-S1

Motor Innervation

Anterior leg muscles (dorsiflex foot, extend toes)

Tibialis anterior (inverts foot)

Extensor hallucis longus

Dorsum foot muscles (extend toes)

Extensor digitorum brevis

Cutaneous Innervation

dorsal interspace between 1st and 2nd toe

L5-S2

Motor Innervation

Lateral leg muscles (foot evertors and plantar flexors)

Fibularis longus

Fibularis brevis

Cutaneous Innervation

anteroinferior part of leg most of dorsum of foot

rapid, preprogrammed, involuntary reactions to a stimulus

Survival mechanism against potentially harmful stimuli

Properties of reflexes:

•A stimulus is required to initiate the reflex

•A rapid response requires that few neurons are involved and synaptic delay is minimal

•A preprogrammed response occurs the same way every time

•An involuntary response ensures no intent or preawareness is required

neural circuit of a reflex

•Stimulus activates sensory receptor

•Nerve impulse travels through sensory neuron to the CNS

•Interneurons process information about the stimulus

•Motor neurons send impulses to effector

•Effector (muscle or gland) brings about response

reflex arc Both the receptor and effector organs are on the same side

Sensory axons synapse directly on motor neurons, whose axons project to the effector

polysynaptic reflex arc:

•Initiated by a painful stimulus

•transmitted to the spinal cord

•Interneurons receive the information and stimulate motor neurons to direct flexor muscles to contract in response

•Simultaneously, antagonistic extensor muscles are inhibited so that the traumatized body part may be quickly withdrawn

monosynaptic reflex arc:

•Stretch in a muscle is monitored by stretch receptors called muscle spindles

•The fibers within a spindle are intrafusal muscle fibers (innervated by gamma motor neurons)

•Most muscle fibers in a muscle are not in spindles, and so are extrafusal muscle fibers (innervated by alpha motor neurons)

•When a stimulus stretches a muscle, the muscle reflexively contracts

•Example: knee jerk reflex; the stimulus stretches the quadriceps and it reflexively contracts

Golgi tendon reflex-

•Golgi tendon organs are nerve endings in tendons near a muscle–tendon junction

•As a muscle contracts, force is exerted on its tendon, resulting in tension in the tendon and activation of the Golgi tendon organ

•Nerve impulses signal interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons

•Muscle relaxes, protecting the muscle and tendon from excessive tension

Hypoactive reflex: Response is diminished or absent

Hyperactive reflex: Response is abnormally strong

CNS develops from neural tube,

cranial and spinal nerves develop from neural crest cells

Neural canal becomes central canal of spinal cord

Basal plates form anterior horns, lateral horns and anterior part of gray commissure

Alar plates form posterior horns and posterior part of gray commissure

reflex arc Impulses initiating from a receptor cross the spinal cord to activate effector organs on the opposite limb

More complex pathways exhibit a number of synapses involving interneurons within the reflex arc