Nervous System II Flashcards
Learning objectives for Spinal Cord & Spinal Nerve Lecture
- Know the basic anatomy of the spinal cord
- Understand the development the nervous system
- Know the basic anatomy of the vertebral column
- Understand how the spinal cord is protected and supported within the vertebral column: cells, meninges (membranes) and cerebrospinal fluid
- Know some basic functions of the spinal cord
- Know the basic organisation of the peripheral nerves that originate from the spinal cord
This lecture is not comprehensive - basics only
Spinal cord - an overview
Part of the CNS
Protected by vertebral column & 3 specialised membranes; suspended in CSF
Average brain weight: 1500g (text book). (1300-1400g)
Average weight of spinal cord = 35g.
Average length = 45cm
(Axons can be up to 1m long)
Starts at the foramen magnum of occipital bone
Occupies upper 2/3rds of vertebral column
Ends at L1-2 - vertebral column grows faster than spinal cord
Gives rise to 31 pairs of spinal nerves
Outside composed of white matter (myelinated axons)
Inside composed of gray matter (nerve cell bodies, dendrites & glia cells)
Spinal cord - function
Basic function:
* Conduct information about the body up and down its length
* Connects brain and other regions of the spinal cord
* Neural integration:
- input from multiple sources
- integrates
- appropriate output
* Reflexes (protective role) NS Lecture 4
spinal cord
White and Gray Matter of spinal cord photo 2
Nervous system development - neurulation = revision
Neural Tube - brain & spinal cord
Neural canal - fluid filled spaces in CNS
Neural crest cells = most of the PNS
(sensory nerves, autonomic nerves,
ganglia = collection of nerve cell bodies in PNS),
Schwann cells,
neurulation
Cells of the Spinal Cord (CNS) and Spinal Nerves (PNS) = revision
Spinal cord neurons = multipolar neurons (mostly)
Central nervous system
Spinal cord supporting cells (glia) = same as brain
(CNS supporting cells)
Spinal nerve neurons = unipolar neurons (sensory)
+ multipolar neurons (motor)
Schwann cells (PNS glia)
PNS & CNS
White and Gray Matter of spinal cord photo 1
White and Gray Matter
Nerve cell bodies of sensory neurons (unipolar) are found in dorsal root ganglia (group of nerve cell bodies in PNS)
White matter forms tracts = bundles of axons that travel up & down the spinal cord
(NS lecture 4)
Polio
Poliomyelitis (polio = grey) (myelon = marrow)
= inflammation of the grey matter
A crippling and potentially fatal infectious illness unique to humans.
Spreads through food and water from one person to another via the fecal-oral route.
Polio virus spends most of its time in the gut (hence fecal-oral route of infection).
But
Occasionally gets into CNS - can cause spinal polio
= inflammation of the motor neurons in the ventral horn of spinal cord
(also brain stem or motor cortex)
Death of motor neurons causes paralysis for life.
No cure.
Prevention = vaccination.
dorsal and ventral horns of spinal cord
Meninges
Compared to the brain:
What is the same?
What differs?
* Dura (single layer)
* & fat above
* Pia mater (anchors)
(i) Filum terminale (ii) Denticulate ligaments
Meninges photo
Vertebral Column
Label:
Vertebral body
Vertebral arch
Vertebral foramen
Spinous process
Transverse process
All of the vertebral foramina = vertebral canal
Spinal cord (CNS) sits in the vertebral canal.
Vertebral Column photo
intervertbral discs photo
Spinal nerves (PNS) leave the vertebral column via spaces between vertebrae
Cranial & spinal nerves exit through foramina (foramen = opening, hole, passage)
= Boundary between CNS and PNS
Spinal nerves (PNS)
31 pairs - doesn’t correspond to the number of vertebra (only 1 coccygeal nerve)
Spinal cord ends at Lumbar vertebra 1-2 (starts to taper at medullary cone)
Cauda equina = nerve roots in vertebral canal L2-S5
Spinal nerve names reflect the level of the vertebral column from which they emerge (intervertebral foramina).
C=cervical, T=thoracic, L=lumbar, S=sacral,
Co=coccygeal
Cervical spinal nerves named differently because there are 8
・・
C1-7 exit above vertebral bodies 1-7
C8 exits below vertebral body C7
Remainder exit below associated vertebral body
The part of the body supplied by each pair of spinal nerves segment
Enlargements give rise to nerves of the limbs:
Cervical - upper limb
Lumbar - lower limb
More nerve cells
cervical photo
Spinal nerve exit points differ according to region
Spinal nerves (PNS) photo
thoracic photo
Spinal nerve exit points differ according to region
Lumbar & Sacral photo
Spinal nerve exit points differ according to region
Ratio of white to gray matter depends on spinal level
Sacral region conveys tracts (white matter) from sacral region only
Cervical conveys tracts from sacral, lumbar, thoracic & cervical regions
Also differences in amount of gray matter.
Cervical and lumbar enlargements due to extra gray matter associated with limb innervation.
cervical and thoracic
lumbar and sacral
Rootlets, Roots & Rami photo
Rootlets, Roots & Rami
Rootlets combine to form roots
Dorsal (sensory) & ventral (motor) roots combine to form spinal nerves (mixed)
Spinal nerves branch soon after the spinal nerve exits the intervertebral foramen
Main branches
Dorsal primary ramus (= branch) innervates deep back muscles + associated dermis
: Ventral primary ramus innervates everything else
Rootlets, Roots & Rami continued full lecture sldie photo
learning challenge lecture slide photo
Plexuses
Spinal Nerve Plexuses - ventral rami only
Ventral rami branch and merge repeatedly to form webs of nerves
All regions except thoracic.
* Cervical - parts of face, neck, diaphragm (muscle for critical for respiration)
* Brachial - innervation of upper limb
* Lumbosacral - innervation of pelvic region and lower limb
Afferent (sensory) and efferent (motor) nerves
Result is that named nerves arising from a plexus receive contributions from several spinal nerves
e.g. Phrenic nerve (innervates the diaphragm) receives contributions from spinal nerves C3, 4 & 5.
Functions (?)
protection - e.g. if C5 is damaged you can still breath
FYI - Plexuses also occur in the autonomic nervous system
Learning objectives for Somatic Reflexes &
Spinal Tracts Lecture
Understand the role of simple reflexes
* Understand the role of interneurons in reflexes
* Understand the compensatory actions of other body parts during a reflex
* Know the difference between ascending and descending
spinal tracts
* Describe one ascending and one descending tract
* Understand the impact of spinal injury on reflexes and tracts
This lecture is not comprehensive - basics only
plexuses photo
Dermatomes - sensory
All spinal nerves except C1 receive sensory input from a specific band/ region of skin - dermatome
Roughly speaking = map cutaneous (skin) regions innervated by each spinal nerve.
Roughly because overlap at the edges - up to 50%
Clinical benefit:
Use this knowledge to asses level of spinal
cord injury
ninal nerve (CN V
Diagnose Shingles - distribution of lesions
follow dermatomes
Chicken pox virus remains in the dorsal root
ganglion for life
Virus is kept in check by the immune system
If immune system is compromised, the virus can travel via fast axonal transport along
sensory nerve fibres to the skin.
Anterograde of retrograde?
Causes blisters & sometimes intense pain
which can last for years
No cure
Prevention - vaccination
reflexes photo
Reflexes - simple!
Reflexes are quick, involuntary, stereotyped reaction to stimuli - impact muscles & glands. They are protective.
Features:
1. Require stimulation, i.e. need input
2. Quick - very few neurons involved (minimal or no interneurons = minimal synaptic delay)
3. Involuntary - automatic - no need for stimulus to register at the brain - awareness comes later
4. Stereotyped - response is same every time - predictable
Somatic Reflex Arc photo
Patellar tendon stretch reflex is used for diagnosis.
In life, movements and therefore responses are often more complicated.
Scenario:
Wading at the beach,
step on glass (sounds awful but better than a stone fish)
Response:
Withdrawal reflex = lift injured foot by flexing the knee
(E ipsilateral reflex response - sensory input & motor output are on same side of the body)
aun tese belanense on the other side of the body so you
Crossed extension reflex - stabilise opposite limb so you
don’t lose balance.
(= contralateral reflex arc - sensory input & motor output
are on opposite sides of the body)
Response = Polysynaptic & multilevel - takes more time
than a monosynaptic reflex
photo showing reflexes with pain
Somatic sensory (ascending) pathways photo
Fine two-point discrimination
Fine two-point discrimination = ability to identify two points applied to the skin as two and not one.
* Different areas of the body discriminate different distances between two points.
* This is because sensory neurons detect stimuli within an area called a receptive field.
* For each receptive field, the same neuron is stimulated.
* Depending on the region of the body, receptive fields can be quite large or very small.
White Matter Tracts photo
White Matter Tracts
White matter exhibits 3 anatomically distinct regions = 3 x funiculus (funis = cord; funiculus = little cord): dorsal (posterior), lateral, ventral (anterior)
Axons in funiculi can be organised into small structural units = fasiculus (fascis = bundle; fasiculus = little bundle). The easier name = tract.
All nerve fibres (axons) in a named tract have a similar origin, destination and function.
Sensory and motor tracts travel up and down the spinal cord.
Ascending tracts carry sensory information
Descending tracts carry motor information
Note: diagram shows ascending tracts on one side and descending on the other but both types of tracts occur in both side of the spinal cord.
Tract Anatomy
Some tracts exhibit decussation = fibres cross to the opposite side of the body (contralateral origin & destination)
Some do not decussate = fibres stay on same side of body (ipsilateral origin & destination)
There are many too tracts to consider in this lecture - we will consider one sensory pathway and one motor pathway
Both with decussation.
Tract Anatomy photo
Somatic sensory (ascending) pathways
Carry signals up the spinal cord (e.g. proprioception, fine two-point discrimination, pressure,
light touch, pain, temperature, itch & tickle).
Typically, 3 neurons are involved in a sensory pathway.
First order neuron - detects stimulus (unipolar afferent neuron)
Second order neuron - synapses at the thalamus
Third order neuron - carries signal to brain (e.g. postcentral gyrus)
Primary somatosensory cortex - postcentral gyrus photo
Motor (descending) pathways
Carry motor signals down the spinal cord.
Signals originate in the brain.
Typically, 2 neurons are involved in a motor pathway.
Lower motor neuron -cate in he ventralm of spanateod gray mater)
Upper motor neurons can excite or inhibit lower motor neurons.
2 pathways in image = corticospinal tract = largest descending tract in humans
Light green = vental (anteron coricopinat tad (i emaring 15% 1mb))
Both decussate. Contralateral motor control.
Lateral corticospinal tract innervates skeletal muscle that controls skilled limb movements, e.g. typing, playing a guitar, dribbling a soccer ball, dancing.
Motor (descending) pathways photo
Primary motor cortex - precentral gyrus photo
Primary motor cortex - precentral gyrus photo
Spinal injury - what is lost and what remains?
Spinal cord trauma - paralysis:
Most common in 16-30yo males (high-risk behaviour)
Most commonly caused by car & motorcycle accidents, followed by
sports injuries.
Complete severance (transection) of the spinal cord
= interruption in pathways to the brain described in this lecture.
Initially, experience spinal shock:
Flaccid paralysis below the site of injury & absence of reflexes.
Patient retains urine and faeces.
As spinal shock passes, somatic reflexes reappear
(e.g. muscle & tendon reflexes) - first in toes, then feet, then legs.
Autonomic reflexes also reappear.
Patient now becomes incontinent because rectum & bladder empty
reflexively in response to stretch.
(Depends on level of spinal cord damage)
Initial flaccid paralysis changes to spastic paralysis because spinal
reflexes lack inhibitory control from the brain.
Spastic paralysis involves uncoordinated muscle spasms which can
contort the limbs.
Learning challenge photo
Somatic Reflex Arc:
- Somatic receptors (skin, muscles, tendons, joints)
- Afferent nerve fibres (sensory)
- Integrating centre = point of synaptic contact
- Efferent nerve fibres (motor)
- Effector (muscles)
Somatic Reflex Arc:
photo
Soz! It’s never that simple. photo
photo
Within the CNS an aggregation of functionally
related nerve cell bodies is called:
a nucleus
Within the PNS an aggregation of like nerve
cell bodies is called:
a ganglion
A tract is
a bundle of axons in the brain and spinal cord (CNS).
A nerve is
a bundle of axons in the peripheral nervous system (PNS).
The skeletal location of the “boundary” between the central and peripheral nervous
system.
The intervertebral foramina - where the peripheral nerves leave the .
vertebral canal.
What purpose do the foramina and fissures of the skull and vertebral column serve?
Passage of nerves and blood vessels. .
Which bone does the pons lie on?
Occipital Bone
Which bone does the olfactory bulb rest on? What are the holes in this bone for?
Ethmoid (cribriform plate). Holes=passage for olfactory nerves into the nose.
How does the length of the spinal cord compare to the length of the vertebral
column?
Why?
It is shorter. .
Cell division ceases and nervous tissue stops growing at around 2-3 years of .
age before the individual reaches their adult height. .
s this nerve cell a sensory or
motor neuron? Give a reason for your answer.
Motor. Neuron is multipolar and the axon terminates at the neuromuscular junction. .
