Chapter 13: The Spinal Cord and Spinal Nerves

Question 1

What are the 3 layers of protections for the nervous tissue?

Answer 1

Vertebral column
Meninges
Space between two of the mineral membranes that contains CSF that suspends and cushions the tissue

Question 2

Define meninges and describe the 3 layers

Answer 2

Meninges are protective, connective tissue coverings that encircle the spinal cord and brain - layers superficial to deep:

Dura mater - thick strong layer of dense irregular tissue

Arachnoid matter - thin avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibre (web-like arrangement)

Pia mater - thin transparent layer that adheres to the surface of the spinal cord and brain; thin squamous to cuboidal cells within interacting collagen and elastic fibers; supplies oxygen and nutrients to the spinal cord

Question 3

Define epidural space

Answer 3

A space between the dura mater and wall of the verbal canal that protects the spinal cord with a cushion of fat and connective tissue

Question 4

Define subdural space

Answer 4

A space containing interstitial fluid between the dura matter and arachnoid mater

Question 5

Define subarachnoid space

Answer 5

A space containing shock-absorbing CSF between the arachnoid mater and pie mater

Question 6

Define the cervical and lumbar enlargements

Answer 6

Cervical - superior enlargement that extends from C4 to T1; nerves to and from the upper limbs arise from this englargement

Lumbar - inferior enlargement that extends from T9 to T12; nerves to and from the lower limbs arise from this enlargement

Question 7

Define conus medullaris is and filum terminale

Answer 7

Inferior to the lumbar enlargement the spinal cord terminates as a tapering conical structure called the conus medullaris ending at disk between L1-L2

The filum terminale arises from the conus medullaris, an extension of the pit mater that extends inferiorly, uses with the aracnoid and dura mater, anchoring the spinal cord to the coccyx

Question 8

Describe the structure/naming of spinal nerves

Answer 8

Spinal nerves are the paths of communciation between the spinal cord and specific regions of the body

The naming of the nerves is based on the segment in which they are located: 
Cervical nerves (C1-C8)
Thoracic nerves (T1-T12) 
Lumbar nerves (L1-L5) 
Sacral nerves (S1-S5)
Coccygeal nerves (Co1)

Question 9

Define the cauda equina

Answer 9

The roots of the lower spinal nerves angle inferiorly alongside the filum terminale in the vertebral canal like wisps of hair, collectively named cauda equina

*This is due to the spinal cord being shorter than the column and thus the lower nerves do not leave the column at the same level they exit the cord

Question 10

Describe the following inner anatomy of the spinal cord:

Anterior median tissue 
Posterior median sulcus 
Gray commissure 
Central Canal 
Anterior/ventral commissure

Answer 10

Anterior median fissure - wide groove

Posterior median sulcus - narrow furrow

Gray commissure - gray matter forming the crossbar of the H in the spinal cord

Central Canal - small space in centre of gray commissure filled with CSF that extends the entire length of spinal cord

Anterior/ventral commissure - anterior to gray commissure that connects the white matter of the right and left sides of spinal cord

Question 11

Differentiate between the posterior and anterior gray horns

Answer 11

Horns are gray matter on each side of the spinal cord

Posterior horns contain cell bodies and axons of interneurons as well as axons of incoming sensory neurons

Anterior horns contain somatic motor nuclei which provide nerve impulses for contraction of skeletal muscles

Question 12

Describe lateral gray horns

Answer 12

They are between the posterior and anterior gray horns but are only present in the thoracic and upper lumbar segments of the spinal cord

They contain autonomic motor nuclei that regulate activity of cardiac muscle, smooth muscle, and glands

Question 13

Describe the columns in the spinal cord

Answer 13

Columns are the white matter on each side of the of the spinal cord, subdivided into 3 broad areas:

• Anterior white column
• Posterior white column
• Lateral white column

They are comprised of bundles of axons that enter long distances up or down the spinal cord, known as tracts

Question 14

Differentiate between sensory and motor tracts

Answer 14

Sensory (ascending) tracts conduct nerve impulses towards the brain

Motor (descending) tracts carry nerve impulses from the brain

Question 15

When sensory neurons convey input to the posterior root what are the 3 paths the sensory info may take when leaving the root?

Answer 15

1. Extend into white matter and ascent to the brain
2. Enter posterior gray horn and synapse with interneuron that extend into the white matter and ascend to brain
3. Enter posterior gray horn and synapse with interneurons that synapse with somatic motor neurons involved in spinal reflex pathways

Question 16

Describe how spinal nerves are connected to the spinal cord and differentiate between poster and anterior roots

Answer 16

Roots (2 bundles of axons) connect each spinal nerve to a segment of the corn by even smaller bundles of axons called rootlets

Posterior roots & rootlets contain only sensory axons and each root has a swelling, the posterior root ganglion which contains the cell bodies of sensory neurones

Anterior root & rootlets contain axons of motor neurons, conducting impulses from CNS to effectors

Question 17

Explain why a spinal nerve is considered a mixed nerve

Answer 17

Spinal nerves are parallel bundles of axons and their associated neuroglia class are wrapped in several layers of connective tissue

A typical nerve has two connections to the cord: a posterior root (sensory axons) and an anterior root (motor axons) and is thus a mixed nerve

Question 18

Describe the following connective tissue coverings of a spinal nerve:

Endoneurium
Fascicle
Perinerium
Epineurium

Answer 18

Endoneurium - the innermost layer that wraps individual axons within a nerve that consists of collagen fibers, fibroblasts, and macrophages

Fascicle -bundles of groups of axons with their endoneurium

Perinerium - middle layer covering fascicles that is thicker and consists of up to 15 layers of fibroblasts within a networking of collagen fibers

Epineurium - outermost covering around the entire nerve that consists of fibroblasts and thick collagen fibers

Question 19

Define ramus (rami pl.)

Answer 19

After passing through its invertrral foramen, a spinal nerve divides into several branches, known as rami

The posterior ramus serves the deep muscles of the skin on the posterior side of trunk; the anterior ramus serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk

Question 20

Define meningeal branch

Answer 20

Nerves give off a meningeal branch that renters the vertebral cavity though the invertebrael foramen and supplies the vertebrae, vertebrae ligaments, blood vessels of spinal cord, and meninges

Question 21

Define the term plexus, and describe the anatomic importance of a plexus

Answer 21

Axons from the anterior rami of spinal nerves, except for T2-T12, do not go directly to the body structures they supply but instead form networks on both the left and right sides of the body by joining with anterior rami of adjacent nerves, known as a plexus

Question 22

Describe the following principal plexuses:

Cervical plexus 
Brachial plexus 
Lumbar plexus 
Sacral plexus 
Coccygeal plexus

Answer 22

Cervical plexus - formed by C1-C4, with contributions from C5 that supplies the skin and muscles of the head, neck and superior part of shoulders

Brachial plexus - formed by C5-C8 and T1 that supplies almost the entire nerve supply of the shoulders and upper limbs

Lumbar plexus - formed from L1-L4 that has minimal intermingling of fibres as in brachial plexus that supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs

Sacral plexus - formed from L4-L5 and S1-S4 that is situated anterior to the sacrum and supplies the buttocks, perineum, and lower limbs

Coccygeal plexus - formed from S4-S5 and the coccygeal nerves which supply a small area of skin in the coccygeal region

Question 23

Define intercostal nerves

Answer 23

the anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses and are known as intercostal nerves or thoracic nerves

They directly connect to the structures they supply in the intercostal spaces

Question 24

Describe the clinical significance of dermatomes

Answer 24

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

Knowing which spinal cord segments supply each dermatome makes it possible to locate damaged regions of the spinal cord

There is overlap but little loss of sensation can indicate damage and anaesthesia may be required at multiple adjacent nerves may be required during surgeries

Question 25

What are the functions (3) of the spinal cord and spinal nerves?

Answer 25

White matter contains sensory and motor tracts, for conduction of sensory nerve impulses toward the brain and motor nerve impulses toward effector tissues

Gray matter is a site for integration (summing) of SPSPs and IPSPs

Connect the CNS to the sensory receptors, muscles, and glands in the body

Question 26

Describe the spinothalamic tract and posterior column

Answer 26

Nerve impulses from sensory receptors propagate up the spinal cord to the brain along two main routes on each side: the spinothalamic tract and posterior column

Spinothalamic tract - conveys nerve impulses for sensing pain, warmth, coolness, itching, ticking, deep pressure, and crude touch

Posterior column - consists of the gracile fascicles and cuneate fasiculus that converts impulses from discriminative touch, light pressure, vibration, and conscious proprioception

Question 27

Differentiate between the direct and indirect motor pathways

Answer 27

Direct pathway includes lateral corticspinal, anterior corticospinal and corticobulbar tracts; they convey impulses that originate in the cortex and are destined to cause voluntary movements

Indirect pathways include rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, and medial reticulospinal tracts; covers impulses from brain stem to cause automatic movements, help coordinate body movements with visual stimuli, and maintain skeletal muscle tone

Question 28

Define a reflex

Answer 28

A fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus

Some are inborn, while others are learned or acquired

Question 29

Describe the following reflexes:

Spinal reflex
Cranial reflex
Somatic reflex
Autonomic (visceral) reflex

Answer 29

Spinal - integration occurs in the spinal cord gray matter (ex. patellar reflex - knee jerk)

Cranial - integration occurs in the brain stem (ex. eye tracking movements)

Somatic - contraction of skeletal muscles

Autonomic - involve responses of smooth muscle, cardiac muscle, and glands

Question 30

Define a reflex arc and its 5 functional components

Answer 30

The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit)

1. Sensory receptor - responds to stimulus by producing a generator/receptor potential that triggers an impulse if it potential reached depolarization threshold
2. Sensory neuron - axons conducts impulses from receptor to axon terminals in gray matter of spinal or brain stem and relay neurons sent impulses to the area of the brain that allows conscious awareness of reflex
3. Integrating center - one or more regions within CNS that relay impulses from sensory to motor neurons
4. Motor neuron - axon conducts impulses from integrating centre to effector
5. Effector - muscle or gland that responds to motor impulses - its action is called a reflex

Question 31

Differentiate between monosynaptic reflex arc and polysynaptic reflex arc

Answer 31

Monosynaptic - a reflex pathway having only one synapse in the CNS

Polysynaptic - a reflex pathway that involves more than two types of neurons and more than one CNS synapse

Question 32

Describe how a stretch reflex operates (5 steps)

Answer 32

A stretch refex causes contraction of a skeletal muscle in response to stretching of a muscle - it occurs via a monosynaptic reflex arc; it controls muscle length

1. Stretching stimulates sensory receptor in muscle called muscle spindles
2. Muscle spindle generate impsules along a somatic sensory neuron through posterior root of spinal nerve into the spinal cord
3. In spinal cord (integrating center) sensory neurons makes an excitatory synapse with a motor meuron in the anterior gray horn
4. Motor neuron sends impsule to the axon terminalis that forms neuromuscular junctions (NMJs) with skeletal muscle fibers of stretched muscle
5. ACh is released at NMJ that riggers AP in effector muscle, causing contraction

Question 33

Contract ipsilateral and contralateral reflex

Answer 33

Ipsilateral - sensory nerve impulse enter the spinal cord on the same side from which the motor nerve impulses leave it
- All monosynaptic reflexes are ipsilateral

Contralateral - sensory impulse enter one side of the spinal cord and motor impulses exist on the opposite side

Question 34

What is the relationship between the brain and muscle tone?

Answer 34

The brain regulates muscle spindle sensitivity through pathways to smaller motor neurons that innovate specialized muscle fibers within the muscle spindles themselves

By adjusting how vigorously a muscle spindle responds to stretching, the brains etc an overall level of muscle tone, which is a small degree of contraction present while the muscle is at rest

Question 35

Define reciprocal innervation

Answer 35

The components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists

This prevents conflict between opposing muscles and is vital in coordinating body movements

Question 36

Define tendon reflex

Answer 36

Operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn

It is also ipsilateral

Question 37

Describe the operation of a tendon reflex

Answer 37

1. Increased tension stimulates sensory recepepts in tendons called tendon organs
2. Sensory neurones are excited
3. Within the spinal cord (integrating center) sneosyr neuron activated inhibitory neuron
4. Motor neon is inhibited (hyperpolarization) and thus generated fewer nerve impulses
5. Muscle attached to same neon relaxes and receives excess tension

Question 38

Describe the flexor or withdrawal reflex

Answer 38

An ipsilateral polysynaptic reflex arc that results in response to a painful stimulus

1. Stepping on a tack stimulates sensory receptors
2. Excited sensory neuron
3. Within integrating center (spinal cord) sensory neuron activated interneurons in several spinal cords segments as several muscle groups must contract
4. Motor neurons are excited
5. Flexor muscles contract and withdraw led

• Also an intersegmental reflex arc

Question 39

Describe the intersegmental reflex arc

Answer 39

When a nerve impsule from one sensory neurons ascends and decants int he spinal cord and activates interneurons in several segments of the spinal cord

Thus, it is able to stimulate more than one effector

Question 40

Describe a crossed extensor reflex

Answer 40

This reflex helps you maintain your balance

1. Stepping on tack stimulates sensory receptor in right foot
2. sensory neuron excited
3. Within integrating centre, sensory neurone activates several interneurons
4. Motor neurons excited
5. Extensor muscles contract and extend left leg to support the body

• This is a contralateral reflex arc

Question 41

Describe two clinically important reflexes:

Patellar reflex (knee jerk) 
Babinski reflex

Answer 41

Patellar reflex - extension of the leg atet knee by contract of the quad fermoris muscle by tapping patella ligament; blocked by damage to the sensory/motor nerves supplying the muscle and exaggerated in disease involved motor tracts descending from the higher centres of the brain to spinal cord

Babinski reflex - great toe extends with/without a lateral fanning of the other toes after gently stroking of the lateral outer margins of the sole; due to incomplete myelination in the corticospinal tract; after age 1.5 the negative babinski is the normal response

Question 42

Describe spinal cord compression

Answer 42

spinal cord compression may result from fractured vertebrae, herniated discs, tumors, osteoporosis, or infection

compression can destroy neural tissue causing pain, weakness/paralysis, or loss of sensation

Question 43

Describe shingles

Answer 43

An acute infection of the PNS caused by herpes zoster

After chickenpox the virus retreats to a posterior root ganglion and if reactivated it may overcome the immune system, travelling down the sensory neurons of the skin causing blisters along the particular nerve belonging to the infected posterior root ganglion

Question 44

Describe poliomyelitis (polio)

Answer 44

Polio is caused by a virus marked by fever, headache, stiff neck and back, deep muscle pain, and loss of certain somatic reflexes

Can cause death if virus invades neurons in viral centres that control breathing and heart function