Lecture chapter 13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes

Meningitis

is an inflammation of the meninges

Dura mater

outermost layer

Epidural space

The space between the dura mater and the bony vertebra

Subdural space

The space between the dura mater and the arachnoid mater

Subdural hematoma

blood hemorrhage into the subdural space which can be fatal due to high arterial blood pressure

Arachnoid mater

middle layer

Subarachnoid space

The space between the arachnoid mater and the pia mater

lumbar puncture or spinal tap

is a procedure that withdraws CSF from this space so it can be analyzed. It is performed below L3.

Subarachnoid hemorrhage

is possible and is usually less fatal than a subdermal hematoma due to the lower venous blood pressure

Pia mater

innermost layer

Denticulate ligaments

Anchors the pia mater to prevent lateral (side- to-side) movement of the spinal cord

Filum terminale

Thread-like extension of the pia mater that extends from the conus medullaris and joins fibers of the dura mater known as the coccygeal ligament

Cervical enlargement

supplies nerves to the shoulder and upper limbs

Lumbar enlargement

supplies nerves to the pelvis and lower limbs

Conus medullaris

the tapered, conical end of the spinal cord

Cauda equina

nerve roots extending from the conus medullaris inferiorly into the vertebral canal; looks like a “horse’s tail”

Anterior median fissure

deep depression on the anterior/ventral side of the spinal cord

Posterior median sulcus

shallow depression on the posterior/dorsal side of the spinal cord

Anterior/ventral gray horns contain

Somatic motor nuclei

• Delivers motor commands to somatic effectors (skeletal muscles)

Lateral gray horns contain

Visceral motor nuclei

• Delivers motor commands to visceral effectors (cardiac muscle, smooth muscle, glands, adipose tissue)

Posterior/dorsal gray horns contain:

Somatic sensory nuclei

• Receives somatic sensory information from peripheral receptor

  • touch, pressure, pain, temperature, vibration from skin/joints/muscles

Visceral sensory nuclei

• Receives visceral sensory information from peripheral receptors

  • Pain, pressure or vibration from internal organs

Gray commissure

this is an area of crossover between the gray matter on the left side of the spinal cord and the gray matter on the right side of the spinal cord

central canal

is a space within the gray commissure that is lined with ependymal cells and filled with cerebrospinal fluid

Ascending (sensory/afferent) tracts

are conducting action potentials toward the brain

Descending (motor/efferent) tracts

are conducting action potentials away from the brain toward effectors

dorsal root

brings sensory information into the spinal cord

dorsal root ganglion

contains the cell bodies of unipolar sensory neurons surrounded by satellite cells

Ventral root

brings motor commands from the spinal cord and toward effectors

Dermatomes

Specific bilateral regions of skin that deliver sensory information to a single pair of spinal nerves

Peripheral Neuropathy

is the loss of sensory or motor function to a dermatome often due to

• Nerve compression or trauma, Illness etc.

Epineurium (outermost layer)

Dense network of collagen fibers that covers the outside of the nerve

Perineurium (middle layers)

Surrounds bundles of axons called fascicles

Endoneurium (innermost layer)

Surrounds each individual axon

• This is not the myelin, it surrounds the myelin

Dorsal ramus

carries sensory information from the skin and skeletal muscles of the back

Ventral ramus

carries sensory information from the ventrolateral body surface, structures in the body, wall, and the limbs

White rami communicans (white ramus) (T2 – T12)

communicans is the first branch from the spinal nerve and carries visceral motor fibers to a nearby sympathetic ganglion

Gray rami communicans (gray ramus)

Communicans contains preganglionic fibers that innervate glands and smooth muscles in the back, body wall or limbs.

The nerve plexuses give rise to various ?

peripheral nerves

Ventral rami T1 – L2 give rise to the ?

thoracic nerves

All other ventral rami give rise to ?

nerve plexuses

Formed from the ventral rami of spinal nerves C1-C5

Cervical plexus

Innervates neck, thoracic cavity, diaphragmatic muscles / Gives rise to the: Phrenic nerve

Phrenic nerve

innervates the diaphragm for breathing

Formed from the ventral rami of spinal nerves C5–T1

Brachial plexus

• Innervates the pectoral girdle and upper limbs / Gives rise to the: Axillary nerve, Musculocutaneous nerve, Radial nerve, Median nerve, Ulnar nerve

Axillary nerve

motor control of the shoulder

Musculocutaneous nerve

• Sensory from the skin of the lateral surface of the forearm

• Motor control of the flexor muscles of the arm

Radial nerve

• Sensory from the skin of the posterolateral surface of the arm and forearm

• Motor control of the extensor muscles of the arm and forearm; thumb abduction

Median nerve

• Sensory from skin of the anterolateral surface of the hand

• Motor control of the flexor muscles of the lateral forearm;

Ulnar nerve

• Sensory from skin of the medial surface of the hand

• Motor control of the flexor muscles of the medial forearm and digits; thumb adduction

Carpal tunnel syndrome

Repetitive movements can cause swelling in this area, compressing the median nerve

Formed from the ventral rami of spinal nerves T12–L4

Lumbar plexus

• Gives rise to the Femoral nerve, Obturator nerve

femoral nerve

• Sensory from the anteromedial thigh, medial leg and foot

• Motor control of the flexor muscles of the thigh (hip flexors)

Obturator nerve

• Sensory from the skin of the medial surface of the thigh

• Motor control of the adductor muscles of the thigh

Formed from the ventral rami of spinal nerves L4–S4

Sacral plexus

• Gives rise to the Pudendal nerve and Sciatic nerve

Pudendal nerve

• Sensory from the skin of the external genitalia

• Motor control of the muscles of the perineum

Sciatic nerve

Runs along the posterior thigh

• Motor control of the hip extensors and hip adductor muscles

neuronal pools

is Interneurons organized into functional groups of interconnected neurons (circuits)

Divergence

A circuit for spreading stimulation to multiple neurons or neuronal pools in the CNS

• Example: Visual information from the eye is delivered to many different areas of the brain

Convergence

A circuit for providing input to a single neuron from multiple sources

• Example: subconscious & conscious control of the diaphragm in breathing—two neuronal pools synapse on the same motor neurons that control the diaphragm

Serial processing

A mechanism in which neurons or pools work sequentially

• Example: pain signals pass sequentially through two neuronal pools to reach the brain for conscious awareness of pain

Parallel processing

A mechanism in which neurons or pools process the same information simultaneously

• Example: step on a bee. Signals spread through several neuronal pools so you can shift your weight, lift your foot, and yell in pain at about the same time

Reverberation

Collateral branches of neurons extend back and continue stimulating presynaptic neurons

• Examples: maintenance of consciousness, breathing, muscle coordination

reflex arc

is the route (circuit) that action potentials follow to produce a reflex

Five Steps in a Simple Neural Reflex Arc

1. Arrival of stimulus & activation of a sensory receptor

2. Activation of sensory neuron

• Action potential in a sensory neuron

3. Information processing by one or more interneurons

• Action potential in an interneuron

• The interneuron is usually located in the brain or the spinal cord

4. Activation of motor neuron

• Action potential in a motor neuron

5. Response of peripheral effector

• Generally the response opposes original stimulus (negative feedback)

Innate reflexes

Basic neural reflexes formed before birth

• Examples: withdrawal, chewing, visual tracking

Acquired reflexes

Rapid, automatic learned motor patterns

• Example: braking a car in emergency

Somatic reflexes

Trigger involuntary control of skeletal muscles

Stretch reflex

Example: patellar (knee-jerk) reflex

Superficial reflexes

triggered by stimulation of skin, mucous membranes

• Example: Plantar reflex

Visceral reflexes (autonomic reflexes)

Control other types of effectors such as smooth muscle, cardiac muscle, or glands

Cranial reflexes

Sensory information & motor commands travel though cranial nerves & processing occurs in the brain

Spinal reflexes

Sensory information & motor commands travel though spinal nerves & processing occurs in the spinal cord without any input from the brain

Tendon Reflex

Prevents skeletal muscles from developing too much tension which can tear a tendon

• Sensory receptors in tendons are called Golgi tendon organs

Withdrawal Reflexes

Moves a body part away from stimulus (pain or pressure)

ipsilateral Reflex

Meaning the reflex occur on the same side of the body as the stimulus

contralateral reflex

Meaning the response also occurs on the opposite side of the stimulus

Facilitation of Spinal Reflexes by the Brain

Results in reinforcement (enhancement) of a spinal reflex

Inhibition of Spinal Reflexes by the Brain

Suppresses a spinal reflex from happening

EX: The Plantar Reflex - toes curl under

• normal response in adults

The Babinski Reflex - toes fan out

• Normal in infants because their spinal cord is not yet fully developed