CHAPTER 11: The spinal cord

gray ramus comunicans

unmyelinated postganglionic fibers that innervate glands and smooth muscles in the body wall or limbs

Cauda equina

'horse-tail' - complex of the filum terminale and the long dorsal and ventral roots - a bundle of spinal nerves

Describe the anatomical features of the spinal cord.

- A spinal cord is approximately 45 cm long and 1.5 cm wide - in the sectional view, it has an outer layer of white matter and an inner layer of gray matter surrounding a small central canal

Identify the three layers of connective tissue of the spinal nerve and identify the major peripheral branches of a spinal nerve.

- the layers of connective tissue: outer epineurium, middle perineurium and inner endoneurium -the major peripheral branches: dorsal rami, ventral rami and communicating rami

A typical spinal cord has how many pairs of spinal nerves, and where does the spinal cord end?

- there are 31 pairs of spinal nerves -it ends in the conus medullaris

Describe the gross anatomical features of a cross section of the spinal cord.

-Anterior median fissure (a deep groove along the anterior and ventral surface) -Posterior median sulcus (a shallow longitudinal groove) -White matter (myelinated + unmyelinated axons) -Gray matter (cell bodies, neuroglia, unmyelinated axons) -central canal (a passageway containing CSF) -dorsal root of each spinal nerve -ventral root of each spinal nerve - dorsal root ganglia -spinal nerve (contain the axons of sensory and motor neurons)

Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands.

-White matter is organized according tot he area of the body innervated

Structural organization of gray matter.

-posterior gray horn: contains somatic and visceral sensory nuclei - lateral gray horn: located only in thoracic and lumbar segments; contains visceral motor nuclei - anterior gray horn: somatic motor nuclei

Perineurium

Middle layer of the connective tissue in the spinal nerve; divide the nerve into a series of compartments with axon bundles, called fascicles

Reflex Classification III: Complexity of the Circuit

Monosynpatic reflexes: simplest reflex arc; involving only one synapse in the CNS - delay between the stimulus and response is minimized Polysynpatic reflexes: involve at last one interneuron in addition to a sensory and motor neuron; produces complex responses

Babinski reflex

Positive response: fanning of toes due to lack of inhibitory control over reflex response from descending motor pathways --> common in infants Negative response: (aka the Plantar reflex) toes curl due to development of normal reflex response

Name the pairs of spinal nerves

8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

Nerve plexus

A complex, interwoven network of nerves.

Define a nerve plexus trunk and cord.

A nerve plexus trunk is a large bundle of axons contributed by several spinal nerves; a nerve plexus cord is a smaller branch of nerves that originates from the trunk.

List the components of a reflex arc.

A reflex arcs include: - a receptor - a sensory neuron - a motor neuron - a peripheral effector - interneurons may or may not be present as well

Describe a dermatone.

A specific bilateral sensory region monitored by a single pair of spinal nerves.

Identify the basic characteristics of the polysynaptic reflex.

All polysynpatic reflexes involve pools of interneurons, are intersegmental in distribution, involve reciprocal inhibition, and have reverberating circuits (positive feedback).

Describe reflexes

All reflexes are rapid, unconscious patterned responses to a physical stimulus, which restore or maintain homeostasis.

Describe the brachial plexus and the major nerves associated.

Brachial plexus: a network of nerves formed by branches of spinal nerves segments C4-T1, en route to innervating the upper limbs. The major nerves: dorsal scapular, long thoracic, suprascapular, medial and lateral pectoral, subscapular, thoracodorsal, axillary, medial antebracial cutaneous, radial, musculocutaneous, median and ulnar nerves.

Describe the cause of shingles.

Caused by a reactivation of the varicella-zoster virus (VZV), the same herpes virus that causes chickenpox. Once reactivated, the virus stimulates painful inflammation of the nerve ganglia and cause skin eruptions in a pattern that corresponds to the affected dermatone.

Jendrassik maneuver

Clasping of hands that overemphasizes the patellar reflex; produces a big kick, not a twitch

What are common characteristics and various classifications of reflexes?

Classified according to their development (innate vs. acquired), the nature of resulting motor response (somatic vs. visceral), the complexity of the neural circuit involved (polysynaptic vs. monosynaptic), and the site of information processing (spinal vs. cranial)

Epidural space

Contains blood vessels, areolar tissue and a protective padding of adipose tissue --> meant to cushion the spinal cord

Rami

a branch from the spinal nerve; some carry visceral motor fibers of the ANS

Filum terminale

a slender strand of fibrous tissue that extends from the inferior tip of the conus medullaris to the 2nd sacral vertebra ; anchors the spinal cord in the coccyx

Lumbar puncture/spinal tap

Used to detect meningitis; a needle is inserted into the subarachnoid space (where CSF is) inferior to the tip of the conus medullaris

Identify and describe the three spinal meninges.

Dura mater: outermost component of the cranial and spinal meninges; contains dense bundles of collagen fibers arranged along a longitudinal axis Arachnoid mater: the middle layer that encloses cerebrospinal fluid; a simple squamous epithelium (arachnoid membrane) and subrarachnoid space Pia mater: the innermost layer of the meninges firmly bound to the underlying neural tissue; meshwork of elastin and collagen fibers

Define gray and white ramus.

Gray ramus: bundle of postganglionic sympathetic nerve fibers that are distributed to effectors in the body wall, skin, and limbs by way of a spinal nerve White ramus: a nerve bundle containing the myelinated preganglionic axons of sympathetic motor neurons en route to sympathetic ganglia.

reverbration

collateral branches extend back toward the source of the impulse and further stimulate presynaptic neurons (positive feedback)

White matter

contains a large number of myelinated and unmyelinated axons (mostly myelinated, hence the white colour)

Spinal nerve

contains axons of both sensory and motor neurons, moving in different directions

Ventral root

contains axons of motor neurons that extend into the periphery to control somatic and visceral effectors

Dorsal root

contains axons of the neurons whose cell bodies are in the dorsal root ganglion; carries sensory information

sympathetic nerve

contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity

Dorsal ramus

contains somatic motor and visceral motor fibers that innervates the skin and skeletal muscles of the back

Reflex Classification I: Development

Innate reflexes: comes about during development and are genetically or developmentally programed; generally appear in a predictable sequence from simplest responses to complex motor patterns Acquired reflexes: learned rather than pre-established; rapid and automatic and enhanced by repetition

Endoneurium

Innermost layer of connective tissue in the spinal nerve. Consists of delicate layer of collage, fibroblasts and ECF matrix

Ipsilateral vs. Contralateral reflexes

Ipsilateral: same side of the body involved in sensory and motor responses Contralateral: opposite side

dorsal root ganglion

contains the cell bodies of sensory neurons whose axons carry information to the spinal cord; each segment has a pair of dorsal root ganglion, one on each side

convergence

several neurons synapse on a single postsynaptic neuron; same motor neurons can be subject to conscious and subconscious control

Denticulate ligaments

extend from the pia mater, through the arachnoid mater to the dura mater --> prevent lateral movement of the spinal cord and anchors it

divergence

spread of information from one neuron to several neurons (or one pool to several pools); allows for a broad distribution from a specific input

Epineurium

outermost covering of the spinal nerve dense network of collagen fibers for protection

neural circuits

pattern of interaction among neurons that hint at their functional classification

Gray commissures

posterior and anterior to the central canal; contain axons that cross from one side of the spinal cord tot he other

Communicating rami

present in the thoracic and cervical enlargements; contain axons of the sympathetic neurons

Conus medullaris

the tapered, conical portion of the spinal cord inferior to the lumbar enlargement

Describe reinforcement as it pertains to spinal reflexes.

Reinforcement is an enhancement of a spinal reflex through the facilitation of motor neurons involved in reflexes.

Differentiate between sensory and motor nuclei.

Sensory: receive and relay sensory information from peripheral receptors Motor: issue motor commands to peripheral effectors

Reflex Classification II: Nature of Response

Somatic reflexes: involuntary control of skeletal muscles (e.g. withdrawal reflex); essential because they are rapid and immediate Visceral reflexes: autonomic, control or adjust the activities of smooth muscle, cardiac muscle, glands and adipose tissue

Reflex Classification IV: Processing Site

Spinal reflexes: important events occur in the nuclei of the spinal cord (intersegmetnal: involve mutliple segments of the spinal cord and can produce very complex and coordinated responses) Cranial reflexes: important events occur in the nuclei of the brain

Define stretch reflex. In the patellar reflex, identify the response observed and the effectors involved.

Stretch reflex: a monosynaptic reflex that provides automatic regulation of skeletal muscle length. Patellar reflex: the response observed is a leg extension, and the effectors involved are the quadriceps femoris muscles.

Describe the flexor reflex.

The flexor reflex is an example of a withdrawal reflex that contracts the flexor muscles of a limb in response to painful stimuli; it has a protective function.

Describe the composition of gray matter of the spinal cord.

The gray matter of the spinal cord is composed o cell bodies of neurons, neuroglia and unmyelinated axons.

List the major nerve plexuses.

The major plexuses are brachial, cervical, lumbar, and sacral plexues.

Spinal cord

The spinal cord receives sensory input and sends motor commands on spinal nerves

white ramus communicans

carries visceral motor fibers to sympathetic ganglion. they are myelinated

Meningitis

inflammation of the meninges caused by bacteria, viruses or microorganisms --> leads to the build up of fluid, or an edema

Ventral ramus

innervates structures in the lateral and anterior trunk and limbs