Sem 2 Module 2 - The Spinal Cord Flashcards
What is in the Spinal Cord?
- Is composed of interneurons
- Receives and directs incoming sensory information to procession centres in the brain
- Receives and relays motor output from the brain
- Initiates spinal reflexes
What is the gross anatomy of the Spinal Cord?
- Approximately 42cm long and 14mm wide
- Extends from foramen magnum to L2 in adults and L4 in infants
- Terminates in a cone shaped structure —> conus medullaris
- filum terminale —> a strand of fibrous tissue that extends from the conus medullaris and anchors the spinal cord to the coccyx
What are the spinal nerves?
Spinal nerves are:
- Short nerves
- Contain both sensory and motor axons (mixed nerves)
- Connect to the spinal cord at one end and branch to form peripheral nerves at the other
How do the spinal nerves attach
Spinal nerves attach to the spinal cord by paired nerve roots —-> dorsal and ventral roots
What is the Ventral root?
Contains the axons of motor neurons (issue motor commands to effector gland and muscles)
What is the dorsal root?
Contains the axons of sensory neurons (relays sensory input from receptors to the spinal cord)
How is the spinal cord protected?
The spinal cord is protected by the vertebral column, cerebrospinal fluid and the spinal meninges
What are the spinal meninges?
- Are continuous with the brain meninges
- Include the dura, arachnoid and pia mater
- Contain CSF within the subarachnoid space
- Provide stability and protection for the spinal cord
- Contain blood vessels that deliver oxygen and nutrients to cord tissue
What is a lumbar puncture (spinal tap)?
- Involves the insertion of a needle into the subarachnoid space beyond the L3
This procedure:
- Does not damage spinal cord
- Used to withdraw CSF for diagnostic testing
- Used to administer medications
What are the spinal meninges separated by?
Epidural space
What is the epidural space?
- Filled with adipose tissue and blood vessels
- Site of anaesthetic injection
- Aare secured to the spinal cord by denticulate ligaments
The spinal cord consists of grey and white matter and is divided into left and right halves by two grooves - what are they?
Anterior (ventral) median fissure
Posterior (dorsal) median sulcus
What is grey matter?
- Composed of the cell bodies and dendrite of neurons, unmyelinated axons and neuroglia
- Varies in size and shape down the spinal cord
- Surrounds the central canal
The grey matter is organised into grey horns - what are they?
- Posterior (dorsal) grey horns
- Lateral grey horns (thoracic & lumbar regions only)
- Anterior (ventral) grey horns
What are the cell bodies of neurons within the grey matter are organised into functional groups called?
Nuclei
- Sensory nuclei and motor nuclei
What are sensory nuclei?
Contain the cell bodies of spinal cord interneurons
- Receives and integrate sensory information entering the spinal cord
What are motor nuclei?
Contain the cell bodies of motor neurons
- Receive and relay motor output generated by the brain
- Generate motor output that mediates spinal reflexes
What is white matter?
- Superficial to grey matter (opposite to the brain)
- Composed of myelinated (mainly) and unmyelinated axons
What are the white columns that white matter is organised into?
- Posterior (dorsal) white columns
- Lateral white columns
- Anterior (ventral) white columns
Anterior white commissure —-> site of axon cross over
The axons within the white mater are functionally organised into spinal tracts - what do these tracts relay?
Relay information between spinal cord segments or, between the spinal cord and brain
- Ascending tracts —> relay sensory information
- Descending tracts —> relay motor information
Often reveal their origin, destination and spinal cord location in their name:
e.g. lateral spinothalamic tract
- Origin = spine
- Destination = thalamus
- Tract location = lateral white column
What are neural pathways?
Spinal cord tracts belong to neural pathways that connect the body periphery to the brain
These pathways:
- Consist of 2-3 neurons
- Usually decussate within the spinal cord or brainstem
- Are paired symmetrically on each side of the spinal cord and brain
What are Somatosensory pathways?
Sensory pathways of the somatic nervous system convey sensory information, from receptors to the brain, through a chain of three successive neurons:
What are these neurons?
- First-order neuron
- Second-order neuron
- Third-order neuron
What are first-order neurons?
First-order neurons (sensory neurons):
Conduct sensory input from receptors to the spinal cord.
- The cell body resides in a dorsal root ganglion
- The axon enters the spinal cord via a dorsal root and synapses with a second-order neuron within the sensory nuclei
What are second-order neurons?
Second-order neurons (interneurons):
Conduct sensory input to the thalamus or cerebellum
- The cell body resides in the sensory nuclei of the posterior grey horns
- The axon travels in a specific spinal cord tract
What are third-order neurons?
Third-order neurons (interneurons):
Conduct sensory input from thalamus to a specific brain destination
e.g. primary somatosensory cortex
What are the major somatosensory pathways?
- Posterior column pathway (dorsal column-medial lemniscal pathways)
- Anterolateral pathway
- Spinocerebellar pathway
What is the posterior column pathway?
- Begins at a peripheral receptor
- Involves the fasciculus gracilis and fasciculus cuneatus spinal cord tracts (within the posterior white columns)
- Ends within the primary somatosensory cortex
What does the posterior column pathway relay?
Sensations of:
- Fine touch
- Pressure
- Vibration
- Proprioception
What is the Anterolateral pathway?
- Begins at a peripheral receptor
Involves
- Anterior spinothalamic tracts –> relay crude touch, pressure tickle and itch sensations (within anterior white column
- Lateral spinothalamic tracts –> relay pain and temperature sensations (within lateral white column)
Ends with the primary somatosensory cortex
What is the Spinocerebellar pathway?
- Begins at proprioceptors in skeletal muscles, tendons and joints
Involves the:
- Anterior Spinocerebellar tracts
- Posterior Spinocerebellar tracts
- Within the lateral white columns
Ends within the cerebellum
Relays sensation of proprioception
What do motor pathways do?
Motor pathways convey motor output from the brain to skeletal muscles via a chain of two successive neurons:
- Upper motor neurons (pyramid neurons)
- Lower motor neurons
What are Upper motor neurons (interneurons)?
Conduct motor output from the primary motor cortex to the spinal cord:
- Cell bodies located in primary motor cortex
- Axons travel in a specific spinal tract and synapse with lower motor neurons in the motor nuclei of the anterior grey horns
What are Lower motor neurons?
Conduct motor output from the spinal cord to skeletal muscles:
- Cell bodies located in the anterior grey horns (motor nuclei) - Axons exit the spinal cord via ventral root and travel to skeletal muscles
What are the two main somatic motor pathways?
- Lateral corticospinal pathway
2. Anterior corticospinal pathway
What is the lateral corticospinal pathway?
- Begins in the primary motor cortex
- Upper motor neurons travel in the lateral corticospinal tract
- Ends at the skeletal muscle sot he limbs
- Relays motor output that controls voluntary skeletal muscles contractions
What is the Anterior corticospinal pathway?
- Begins in the primary motor cortex
- Upper motor neurons travel in the anterior corticospinal tract
- Ends at the skeletal muscles of the axial skeleton (neck and trunk), pectoral and pelvic girdles
- Relays motor output that controls voluntary skeletal muscles contractions
What are the spinal reflexes?
The spinal cord mediates somatic reflexes
- Sensory information is processed, and a reflex response generated, within the spinal cord
- Lower motor neurons relay the motor output to effector skeletal muscle
What are somatic reflexes?
- Rapid, involuntary motor responses generated by the spinal cord in response to a stimulus
- Can be modified by the primary motor cortex
What can somatic reflexes be used to assess the condition of?
- Lower motor neurons
- Spinal cord
- Primary motor cortex
- Corticospinal tracts
What is the general structure of a neuron?
The axons (fibres), blood and lymphatic vessels that comprise nerves are enclosed by three connective tissue layers.
What are the three connective tissue layers?
- Epineurium
- Perineurium
- Endoneurium
What is the epineurium tissue layer?
Tough fibrous outer layer that encloses entire nerve
What is the Perineurium tissue layer?
Middle layer that surrounds bundles of axons called fascicles
What is the Endoneurium tissue layer?
Innermost layer that surrounds each individual axon
Nerves can be classified as?
- Sensory nerves
- Motor nerves
- Mixed nerves
- Cranial nerves (12 pairs)
- Spinal nerves
What do sensory nerves contain?
Contain the axons of sensory neurons
What do motor neurons contain?
Contain the axons of motor neurons
What do mixed nerves contain?
Contain both sensory and motor axons
Where do cranial nerves arise from?
Arise from the brain to innervate the head and neck, visceral organs
Where do spinal nerves arise from?
Arise from the spinal cord and branch into peripheral nerves that innervate all body areas except some head and neck regions
What are the spinal nerves interconnected called?
After emerging from the vertebral column, most spinal nerves (not thoracic) branch and interconnect to form nerve networks called nerve plexuses
What are the four main plexuses?
- Cervical plexus
- Brachial plexus
- Lumbar plexus
- Sacral plexus
What is the cervical plexus?
- Arises from spinal nerves C1 to C5
- Provides sensory innervation from the:
- Skin of the head (posterior), neck and shoulders
- Phrenic nerve - Provides motor innervation to:
- Neck and shoulder muscles
- Phrenic nerve - Main nerve —-> Phrenic nerve
What is the Brachial plexus?
- Arises from spinal nerves C4 to T1
- Provides sensory innervation from the skin of the:
- Shoulders
- Arms
- Hands - Provides motor innervation to:
- Shoulder muscles
- Arm muscles
- Hand muscles - Main nerve —-> Axillary, Musculocutaneous, Median, Radial, Ulnar
What is the Lumbar plexus?
- Arises from spinal nerves L1 to L4
- Provides sensory innervation from the skin of the:
- Lower abdomen
- Hips
- Anterior & medial thigh, leg & foot - Provides motor innervation to:
- Abdominal wall muscles
- Hip muscles
- Medial & anterior thigh muscles - Main nerve—> femoral nerve
What is the sacral plexus?
- Arises from spinal nerves L4 to S4
- Provides sensory innervation from the skin of the:
- Buttocks
- Posterior thigh
- Leg & foot - Provides motor innervation to:
- Gluteal muscles
- Posterior thigh muscles
- Leg & foot muscles - Main nerve—> Sciatic nerve
What is a Dermatome?
An area of skin innervated by the sensory fibres of a single spinal nerve
What are dermatomes useful for?
Clinically, dermatomes are useful in identifying spinal nerve damage and locating spinal cord injuries
- Spinal cord and spinal nerve injuries can result in a loss of sensation from the dermatome they innervate - Painful rash and blisters may also appear in affected dermatome e. g. shingles ----> herpes zoster virus - Attacks the sensory neurons of a spinal nerve ---> painful rash and blisters appear in corresponding dermatome
What are cranial nerves?
- 12 pairs of nerves that originate directly from the brain (forebrain and brainstem)
- Innervate structures of the head and neck, and visceral organs
- Can be classified as sensory nerves, motor nerves or mixed nerves
- Named according to the structures they innervate or their functions
- Numbered using Roman numerals
I
1
Olfactory
II
2
Optic
III
3
Oculomotor
IV
4
Trochlear
V
5
Trigeminal
VI
6
Abducens
VII
7
Facial
VIII
8
Vestibuloccoclear
IX
9
Glossopharyngeal
X
10
Vagus
XI
11
Accessory
XII
12
Hypoglossal
What is different about the Vagus nerve (CN X)?
- Only cranial nerve to extend beyond head/neck region
Motor innervation - Parasympathetic motor to visceral organs —-> heart, lungs, abdominal organs
- Somatic motor to pharynx and larynx
Sensory innervation
- Thoracic & abdominal viscera
- Baroreceptors
- Chemoreceptors
- Taste buds
- Proprioceptors (larynx and pharynx)
What are cranial nerve reflexes?
- Are rapid, involuntary motor responses generated by the brain in response to a stimulus
e. g. - Pupillary light response
- Auditory reflex
What happens in nerve repair?
If damaged, the sensory and motor axons within a peripheral nerve can regenerate providing:
- The cell body remains intact
- Schwann cells that produce myelin remain active
- Severed axon endings are closed together
In general axon regrowth never matches what existed pre-trauma
What are cranial nerves?
- Arise from the brain (mainly brainstem)
- Most extend into the periphery to supply sensory and motor innervation to the skin, muscles and gland of the head and neck
- Vagus nerve innervates thorax and abdomen
- Mediate cranial reflex responses
