Spinal Cord and Spinal Nerves

Central Nervous System (CNS)

brain and the spinal cord.

Brain

manages higher cognitive functions.

Spinal Cord

Serves as the main communication between the brain and the body (along with cranial nerves).

Ends between LI and L.2. BUT spinal nerves go below L2!!!

End resembles cone, named the conus medullaris.

Protection of Nervous Tissue

Bony structures: cranial bones and vertebral column

Membranes: Meninges (dura mater, arachnoid mater, and pia mater.)

Spaces found next to membranes: epidural space, subdural space, subarachnoid space.

Epidural space

space between the dura mater and the vertebra. It has fat. (outermost)

“Above dura mater”

Dura Mater

outermost and toughest layer.

Subdural Space

underneath the dura mater and contains interstitial fluid.

Arachnoid Space

middle layer and it resembles a spider web. (under subdural space)

Subarachnoid space

underneath the arachnoid mater, contains cerebrospinal fluid (CSF).

Pia matter

innermost and most flexible layer
*pliable - hugs the spinal cord, contains blood vessels for oxygen and nutrients to spinal cord. *blood vessels are anchored in place by the web-like columns of the arachnoid mater above them.

Menenges and spaces oder

vertebral column - epidural space - dura mater - subdural space - arachnoid mater - subarachnoid space - Pia mater

Anchoring Points of the Spinal Cord

Denticulate ligaments (laterally, Filum terminale (inferiorly)

Denticulate ligaments

Pia mater projects laterally and fuses with the arachnoid mater and dura mater, providing lateral anchoring for the spinal cord.

*Right and left denticulate ligaments extend midway between the dorsal and ventral roots along the lateral surfaces of the spinal cord.

*Most superior right and left denticulate ligaments attach to cranial dura immediately superior to the foramen magnum

Most inferior right and left denticulate ligaments found between the T12 and LI spinal roots, extending from the conus medullaris.

Filum terminale:

Anchors the spinal cord inferiorly. Remnant of embryonic tail of spinal cord, covered with pia mater.

*Extends down from end of spinal cord then fuses with arachnoid and dura mater, continuing downward through the sacral canal and sacral hiatus, and finally attaches to the dorsum of the coccyx.

Central Canal

canal in the center of the spinal cord where cerebrospinal fluid (CSF) flows.

Posterior median sulcus and anterior median fissure.

Divides spinal cord into right and left sides. Anterior is wider

*Skinny Sulcus, Fat Fissure!

The spinal cord contains gray mater and white mater.

The white mater surrounds the gray mater.

Gray mater

subdivided into horns:

The posterior gray horn - neuronal cell bodies of interneurons getting sensory info from somatic and sensory neurons.

The lateral gray horn contains the neuronal cell bodies of autonomic motor neurons.

The anterior gray horn contains the neuronal cell bodies of somatic motor neurons.

*gray because neuronal cell bodies

White mater

Subdivided into columns: Anterior, lateral, and posterior white column.

Each column is formed by bundles of nerve fibers transmitting sensory (ascending) and motor (descending) information.
Sensory information from periphery into the spinal cord and to the brain! Motor commands go from the brain into the spinal cord!
*white because nerve fibers, often surrounded by a fatty layer (myelin sheath), that transmit signals between the spinal cord and the brain.

spinal cord contains commissures

bundles of nerve fibers that connect the right and left sides of the spinal cord

gray commissure

Located within gray mater.

Made of nerve fibers crossing spinal cord, connects the gray mater on both sides of the cord.

The central canal runs through the center of the gray commissure.

anterior white commissure

located in the white mater between anterior median fissure and gray commissure.

Made of nerve fibers crossing spinal cord, communicating white mater columns on both sides of the spinal cord

Decussation

"crossing of nerve fibers" from one side to the other.

Hence, we say that the gray commissure and the anterior white commissure are the places at which nerve fibers decussate

Spinal Nerve

dorsal root and ventral root emerge from the spinal cord

peripheral processes (dendrites) of the dorsal root sensory neurons join axons of ventral root motor neurons => spinal nerve!

spinal nerves have sensory and motor nerve fibers in them => MIXED nerves (transmit both sensory information and motor information)

*Spinal nerves pass through intervertebral foramina and branches into posterior and anterior rami.

Dorsal root (posterior root)

lots of central processes (axons) of sensory neurons
Dorsal root ganglion contains neuronal cell bodies of pseudounipolar neurons

The dorsal aspect of spinal cord is related to sensory information.

"S” for Sensory! DorSal

Ventral root (anterior root)

lots of axons of motor neurons (always multipolar in structure)

The ventral aspect of spinal cord related to motor information.

Rami

The spinal nerve forms when the dorsal and ventral ROOTS come together. Spinal nerve then splits into branches called RAMI

Dorsal ramus (posterior ramus)

lots of peripheral processes (dendrites) of sensory neurons and lots of axons of motor neurons

*continues extending posteriorly and innervates the skin and muscles of the back.

Ventral ramus (anterior ramus)

lots of peripheral processes (dendrites) of sensory neurons and lots of axons of motor neurons.

It supplies everything except the skin and muscles of the back, hence, it innervates the lateral and anterior trunk as well as the limbs

Sensory and Motor Information Routes

• dorsal root ganglion (pseudounipolar neurons) receive sensory info from periphery

• Sensory receptors receive stimulus and convert it into an action potential (electrical signal).

• action potential travels through peripheral process (dendrite) of pseudounipolar sensory neurons toward cell body inside the dorsal root ganglion, through central process (axon) to terminal buttons in gray mater of the spinal cord.

• The terminal buttons of sensory neuron synapses with neurons twith a cell body inside the gray mater.

Posterior Gray Horn

cell body of sensory interneurons receiving info from the dorsal roots of spinal nerves.

Interneurons

"between" neurons, transfer sensory info from pseudounipolar sensory neurons to motor neuron with cell body in the lateral grey horn or in the ventral gray horn.

*send information upwards to our brain so we become aware of the sensation.

Lateral Gray Horn

cell bodies of autonomic motor neurons.

Autonomic motor neurons control involuntary functions in cardiac muscle, smooth muscle, and glands.

Autonomic motor neurons

responsible for controlling our internal organs and regulating visceral functions, control involuntary functions

Very important - safeguarded in lateral gray horn, with the posterior and anterior gray horns serving as buffers to protect them

*"deep" within the spinal cord, primarily concentrated in thoracic region

*We are not consciously aware - these happen automatically

Anterior Gray Horn

neuronal cell bodies of somatic motor neurons that reach skeletal muscles.

Related to voluntary control of skeletal muscles.

Structure of a Spinal Nerve

Formed by peripheral processes (dendrites) of sensory neurons and axons of motor neurons.

Nerve fibers wrapped in connective tissue layers.

Layers: Endoneurium, perineurium, and epineurium.

Endoneurium

Layer around individual nerve fiber

Endoneurium-wrapped nerve fibers are bundled together into fascicle

Perineurium

Layer around bundles of endoneurium-wrapped nerve fibers (fascicles) and continues outward, blending with the connective tissue around nerve endings and muscle junctions.

Epineurium

Outermost layer of the spinal nerve, provides tensile strength(movements and stretching), gives strength and resistance to damage!

The dura mater surrounding spinal cord extends laterally and encases the spinal nerves as they exit the spinal cord through the intervertebral foramen.

Wraps around groups of perineurium and forms invaginations between the perineurium, containing blood vessels (nourishing the spinal nerve)

Spinal nerve anatomy

31 pairs,

8 pairs of cervical nerves (C1-C8) *8 bc C1 spinal nerve between the occipital bone atlas

12 pairs thoracic (T1-T12)

5 pairs lumbar (L1-L5)

5 pairs sacral (S1-S5)

1 pair coccygeal (Col)

Spinal nerve exit

C1 exits the vertebral canal passing between the occipital bone and the Cl vertebra.

All spinal nerves above C7 exit through the intervertebral foramen superior to corresponding vertebrae.

C8 exits through the intervertebral foramen between the C7 and T1 vertebrae.

T1-L5 exit inferior to the corresponding vertebrae.

S1-S4 branch into anterior and posterior rami within sacrum, and pass through the respective anterior and posterior sacral foramina.

S5 and CoI exit through the sacral hiatus.

spinal cord enlargements (2)

cervical enlargement related to the innervation of upper limbs,

lumbosacral enlargement related to the innervation of lower limbs.

cauda equina

spinal nerves after the end of the spinal cord are

lumbar (L1-L5), sacral (S1-S5), and coccygeal (Co1)

Plexus

Complex networks from merge of adjacent Anterior ramus nerve fibers. Enable distribution of motor and sensory innervation to specific regions.

*Nerves originating from plexuses cannot be linked to a single spinal nerve.

*cervical plexus, brachial plexus, lumbar plexus, sacral plexus, and coccygeal plexus.

Dermatomes

Specific areas of skin innervated by sensory fibers from a single spinal nerve. D for dermis

Damage to a spinal nerve can affect corresponding dermatome or myotome (damage to a specific spinal nerve can be identified based on the affected dermatome and/or myotome)

*Somatic sensory neurons from a specific spinal nerve pick up stimuli from a specific dermatome.

Myotome

group of skeletal muscles innervated by a single spinal nerve. Myo for Muscles

*somatic motor neurons control the skeletal muscles underneath the specific dermatome.

Cutaneous Fields

Overlap multiple dermatomes due to contributions from multiple spinal
nerves.

*Damage to cutaneous field indicates peripheral nerve damage, not spinal nerve damage.

Cervical plexus

Interweave of nerve fibers from anterior ramus of C1, C2, C3, C4 and half of C5 spinal nerves.
Innervates the anterior and lateral aspects of the neck, as well as superior shoulders and chest. Divided into sensory and motor branches.

*phrenic nerve most important

Phrenic nerve

Mixed nerves, anterior ramus of C3, C4, and C5 spinal nerves.

Motor nerve fibers provide ability to innervate the diaphragm (most important skeletal muscle).

Its sensory fibers provide sensory information from the central tendon of the diaphragm, fibrous pericardium, mediastinal and diaphragmatic pleura, and the diaphragmatic peritoneum.

Diaphragm,

Our lungs sit on top of the diaphragm. When the phrenic nerve causes the contraction of the diaphragm, it goes down (descends), volume of the thoracic cavity increases, pressure decreases (sucking the air into our lungs).

As the diaphragm relaxes, it returns to its upside-down U shape, leading to a decrease in the volume of the thoracic cage, pressure increases, causing air to move out of our lungs.

Sensory branches of cervical plexus

provide exclusively sensory information!

Somatic sensory nerve fibers monitor the skin of the scalp, neck, axilla, and chest, as well as proprioception of those areas.

*lesser occipital nerve, great auricular nerve, transverse cervical nerve, supraclavicular nerve.

lesser occipital

Cervical plexus nerve. Interweaving of sensory nerve fibers from the anterior ramus of C2 and C3 spinal nerves, provides cutaneous sensation from the posterosuperior scalp

great auricular nerve

Cervical plexus nerve. Interweaving of sensory nerve fibers from the anterior ramus of C2 and C3 spinal nerves, cutaneous sensation from the external ear and the skin over the parotid gland.

transverse cervical nerve

Cervical plexus nerve. Interweaving of sensory nerve fibers from the anterior ramus of C2 and C3 spinal nerves, provides cutaneous sensation from the anterior neck

supraclavicular nerve

Cervical plexus nerve. Interweaving of sensory nerve fibers from the anterior ramus of C3 and C4 spinal nerves, provides cutaneous sensation from the supraclavicular area and upper thoracic region

(it shares the cutaneous innervation of the skin over the pectoralis major and deltoid muscles with the cutaneous branches of the upper intercostal nerves).

Motor branches

exclusively motor information!

Somatic motor nerve fibers provide motor innervation to neck and infrahyoid muscles and diaphragm

ansa cervicalis.

Interweaving of nerve fibers from the anterior ramus of C1, C2, and C3 spinal nerves, innervates the sternocleidomastoid and infrahyoid muscles (swallowing and speech.)
* U shaped nerve loop

Brachial plexus

anterior ramus of second half of C5 plus C6, C7, C8, and T1 spinal nerves. innervate upper limbs!!!

organized in: Root (specific nerve, only anterior ramus), Trunks (anterior ramus arrangements), Divisions, Cords, and Branches (Final nerves that will innervate the skin)

*posterior ramus of Cl to C8 spinal nerves innervate the muscles and skin in our back

Brachial plexus trunks

arrangement of anterior ramus from specific spinal nerves joining together, each Trunk splits into Anterior and Posterior Divisions.

Middle Trunk (C7), Superior Trunk (C5 and C6), Inferior Trunk (C8 and Tl)

Anterior Divisions of superior and middle trunks form the Lateral Cord.

Anterior Division of the inferior trunk forms the Medial Cords

Posterior Division from superior, middle and inferior trunks merge and form the Posterior Cord.

Axillary nerve

C5 and C6 nerve fibers in the Posterior Cord. Brachial plexus nerve

Innervates axilla and shoulder area - deltoid and teres minor muscles, and the skin of the deltoid region.

Radial Nerve.

C5 to T1 nerve fibers in the Posterior Cord. Brachial plexus nerve.

Innervates the posterior aspect of the entire upper limb - all muscles and skin in the posterior aspect of the arm and forearm, as well as the skin in the posterior aspect of the hand lateral to digit 4 (in other words, all the posterior skin of the hand except for the pinky).

Musculocutaneous Nerve.

C5, C6, C7 nerve fibers in the Lateral Cord. Brachial plexus nerve

Innervates muscles on the anterior aspect of the arm, and the skin of the lateral forearm - coracobrachialis, biceps brachii, and brachialis muscles.

Median Nerve.

C5, C6, C7 nerve fibers in Lateral Cord merge with C8 and T1 nerve fibers of Medial Cord. Brachial plexus nerve.

Innervates the palmar skin as well as few muscles in the hand, and the majority of the muscles in the anterior aspect of the forearm.

*The median nerve does not innervate the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus (ulnar nerve)

Ulnar Nerve

C8 and T1 nerve fibers in the Medial Cord. Brachial plexus nerve

Innervates the dorsal skin of the hand is medial to digit 4, the majority of the muscles in the hand, and the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus

Root, Trunks, Divisions, Cords, and Branches - mnemonic

Always Remember To Drink Cold Beer. (A for anterior ramus)

Thoracic Spinal nerves

Tl to T12 spinal nerves.

Exit through the intervertebral foramen inferior to their corresponding vertebrae then branch into anterior ramus and a posterior ramus.

The posterior ramus of T1 to T12 spinal nerves supply the muscles and skin in our back!

The anterior ramus of T1 is part of the brachial plexus.

The anterior ramus of T2 to T12 not part of nay plexus

Anterior ramus of T2 to T12

Not part of any plexus and are known as the intercostal nerves because they are found running run between (inter) ribs
Innervate the skin and skeletal muscles of the anterior and lateral aspects of thorax and abdomen, as well as visceral via sympathetic innervation.

Thoracic region of the spinal cord.

Most of the sympathetic trunks arise from the thoracic region of the spinal cord.

Thoracic nerves are responsible for sensory and motor information in the thorax and abdomen:

T2 to T5 innervate the skin and muscles in the thorax.

T6 to T8 innervate the skin and muscles in both the thorax and abdomen.

T9 to T12 innervate the skin and muscles in the abdomen.

Lumbar Spinal Nerves

(L1-L5) exit through the intervertebral foramen inferior to their corresponding vertebrae then branch into anterior and posterior ramus.

*The anterior ramus of LI, L.2, L3, and half of I4 spinal nerves are part of the lumbar
plexus.

*The anterior ramus of the second half of L4 and the L5 spinal nerves merge, form the lumbosacral trunk, and are part of the sacral plexus.

*The posterior ramus of L1, L2, L3, L4 and L5 spinal nerves are not part of any plexus and innervate the skin and muscles in our back (lumbar region).

Lumbar Plexus

Formed by the interweave of nerve fibers from the anterior ramus L1, L2, L3, and half of L4 spinal nerves

The lumbar plexus innervates the anterior and lateral aspects of the abdominal wall (lumbar region), external genitalia, and anteromedial aspects of our lower limbs!

"Only Fans" plexus: it innervates the external genitalia, and it has the famous Obturator nerves and Femoral nerves innervating the thigh area

Femoral nerve:

Innervates skin and muscles in anterior aspect of the femur!

*a branch of the femoral nerve extends down to the leg and innervates the skin over the medial side of the leg and foot.
(anterior ramus of L2, L3 and L4 spinal nerves)

Lumbar plexus nerve

Obturator nerve:

passes through the obturator foramen of the os coxa bone and reaches medial side of the thigh. innervates the adductor muscles and the skin in the medial aspect of the thigh.

(L2, L3 and L4 spinal nerves)

Lumbar plexus nerve

Genitofemoral nerve:

innervates the genitals and femoral area nearby

cremaster muscle and skin in area

(L1 and L2)

Sacral Spinal Nerves

S1 to S5 spinal nerves.

The anterior ramus of S1, S2, S3, and half of S4 spinal nerves are part of the sacral plexus.

The anterior ramus of the second half of S4 and the S5 spinal nerves are part of the

coccygeal plexus.

The posterior ramus of S1, S2, S3, S4 and S5 spinal nerves are not part of any plexus and innervate the skin and muscles in our back (sacral region).

Sacral Plexus

It is formed by the interweave of nerve fibers from the anterior ramus of the second half of L4 spinal nerve, L5, S1, S2, S3 spinal nerves, and half of S4 spinal nerves.

The sacral plexus innervates the skin and muscle of the pelvis and lower limb (specifically the posterior thigh and the entire leg).

Sciatic nerve

largest nerve of our entire body.

(anterior ramus of L4, L5, S1, S2, and S3 spinal nerves)

extends down posterior aspect of our thighs splits into the tibial nerve and common fibular nerve behind the knee. innervates the skin and muscles in the posterior aspect of our thigh.

Sacral plexus nerve

tibial nerve

extends down posterior aspect of our leg and innervates the skin and muscles there

extends down even further and gives rise to nerves that innervate the skin and muscles in the foot.

Sacral plexus nerve

Common fibular nerve

Extends more laterally "fibula bone side”, remains "common" until it splits into the superficial fibular nerve and the deep fibular nerve.

Sacral plexus nerve

Superficial fibular nerve

Stays "superficially" and innervates the muscles (fibularis longus and fibularis brevis) in the lateral aspect of the leg as well as the skin in the anterolateral leg and dorsum of the foot.

Sacral plexus nerve

Deep fibular nerve

Deep and ends on top of the tibia bone, innervates the muscles in the anterior aspect of the leg and contributes to the innervation of the foot muscles.

Sacral plexus nerve

Pudental nerve:

Innervates the perineal muscles (muscles at the floor of the pelvic cavity), the skin over the external genitalia in females and males, as well as the skin around the anus, anal canal, and vaginal canal. The pudendal nerve provides sensation in the skin of the penis, clitoris, and vagina!

(anterior ramus of S2, S3 and S4 spinal nerves)

Sacral plexus nerve

Coccygeal Spinal Nerve

Co1 spinal nerve.

The anterior ramus of Co1 spinal nerve is part of the coccygeal plexus.
The posterior ramus of Co1 spinal nerve is not part of the coccygeal plexus and innervates the skin in the posterior aspect of the coccyx.

Coccygeal Plexus

Interweave of nerve fibers from the anterior ramus of the second of S4 spinal nerve, S5, and Col spinal nerves.

anococcygeal nerve

arises from the coccygeal plexus, innervates the skin over the coccyx

reflex

Unconscious, involuntary, and fast reaction to a stimulus, react faster than we think

*tests are used to check for damage to the brain stem or spinal cord.

Two types: Cranial and spinal.

Reflex Arc Pathway

Information goes from periphery to central nervous system and back to the periphery.

Sensory information goes up to the brain, but the reaction occurs faster than brain processing

Stimulus: pain(nocireceptors), temperature (thermoreceptors), light (photoreceptors), sound, stretching (mechanoreceptors).

Cranial reflex

Integrating center at the brainstem.

Example: Tracking movement of eyes when reading.

Spinal reflex

Integrating center at the spinal cord.

Example: Patellar reflex.

Patellar Reflex

Reflex hammer causes the stretching of muscle spindle sensory receptor in the quadriceps femoris muscle.

Stretching stimulus transduced into electrical signal = nerve impulse = action potential.

The nerve impulse flows through the sensory neuron and arrives at dorsal gray horn of spinal cord.

Sensory neuron synapses with: Inhibitory interneuron connecting dorsal and ventral gray horns.

Inhibitory interneuron releases inhibitory neurotransmitters that will ensure the inhibition of motor neurons that innervate muscles that must be relaxed for us to be able to move away from danger.

Motor neuron at ventral gray horn.
Sensory neuron releases excitatory neurotransmitters that excite motor neuron at ventral gray horn leading to the contraction of muscles that allow us to move away from danger.

*Patellar reflex assesses the integrity of the lumbar plexus and specific spinal nerves (L2, L3, L4), which are the spinal nerves that form the femoral nerve.