Lecture 4: Nervous system III Flashcards
How are spinal nerves named?
According to their association with vertebral column
8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral
See figure
How is each nerve attached to the spinal cord?
By a ventral root and a dorsal root
See figure
Where does brain receive sensory info from?
Somatosensory systems
What happens after brain receives sensory info?
Receptors convert stimuli into action potentials that travel down the axons that enter the CNS
What root of the spinal nerves does sensory info enter the CNS?
Dorsal roots
What are afferent signals in the CNS used for?
In reflexes
Or signals may be relayed via neurons in spinal cord or brainstem to higher centres
What roots of spinal nerves carry motor impulses?
Ventral
What kind of stimuli is sensory system capable of detecting?
Mechanical stimuli (mechanoreceptors)
Pain (nociceptors) and
Temperature (thermoreceptors)
Mechanoreceptors (many types)
Types of mechanoreceptors
Neuromuscular spindles - receptors for stretch, respond to change in length (basis for stretch reflex i.e. Knee jerk)
Golgi Tendon organs - stimulated by tension in tendons (protection against damage from excessive stretch)
Where are the cell bodies of the sensory neurons of the PNS?
Dorsal root ganglia
Where do the processes of the sensory cells project?
One process of the sensory cell projects (in the spinal nerve) to the periphery where it ends in a sensory receptor
one process projects to CNS (spinal cord).
Afferent signal transduction
Signal (converted to action potential by receptor) travels from receptor to CNS
See figure
What root contains efferent fibres?
Anterior Root = Ventral Root = Motor Root
action potentials are leaving the cord
What do motor neurons innervate?
cell bodies are in the ventral horn of the cord
innervate skeletal muscle (via motor fibres)
cause contractions of the muscle (LMNs)
What do descending fibres from the cerebral cortex and brainstem activate?
Motor neurons
produce voluntary (and reflexive) movement
Where do the ventral and dorsal roots pass through?
Intervertebral foramina
They then join together and form a short spinal nerve (becomes a mixed nerve)
Supply myotomes and dermatomes
What is a myotome?
refers to muscles innervated by the motor fibres of a spinal nerve
What is a dermatome?
area of skin innervated by the sensory fibres of a spinal nerve
What would peripheral nerve injury cause (motor changes)?
flaccid paralysis followed by muscle
degeneration
What would peripheral nerve injury cause (sensory changes)?
loss of cutaneous sensibility (pinprick test)
However: the injury of one spinal cord segment
or spinal nerve rarely causes the loss of function of a whole muscle or loss of sensitivity in a dermatome
What does spinal nerve or root compression cause?
Prolapse of the nucleus pulposus
Causes herniation
Postero-lateral disc herniation
Most common herniation
Causes lower back pain, pain in buttock, thigh, leg
Prolapse of the L4/L5 disc causes compression of the L5 root
see figure
Postero-medial disc herniation
see figure
Where does the spinal cord extend from?
From foramen magnum to L1-L2
What structure marks the end of the spinal cord?
Conus medullaris (between L1 and L2)
See figure
What tethers the cord to the end of the dural sac
The filum terminale
Extension of the pia
What is below the conus medullaris?
Cauda equine
What are the two enlargements of the spinal cord
Cervical and lumbosacral
Where is cervial enlargement? What does it innervate?
C4-T1 of sc
Arm
Where is lumbosacral enlargement? What does it innervate?
L2-S3 of sc
Innervation of leg
Root length in cervical and lumbosacral region
Cervical - short and horizontal
Lumbosacral - cauda equina (horses tail)
What is caudal equina contained within?
Subarachnoid space
Internal composition of spinal cord
White matter (ascending and descending axons)
Butterfly shapedgray matter
See figure
Horns of grey matter
Dorsal horn
ventral horn
See figure
Function of dorsal horn?
Sensory
processes and relays sensory information to the brain
Through which root does sensory info enter through?
Dorsal root
Where are the cell bodies of sensory neurons located?
Dorsal root ganglion
Function of ventral horn
Motor
Contains large motor neurons projecting into muscle (via ventral root)
Another word for motor neurons
“motor efferents”
Composition of white matter of spinal cord
nerve fibres, neuroglia, & blood vessels (not neuronal cell bodies)
Why is white matter white?
high fat content (myelin)
Where do the fibres of the white matter arise from?
Cell bodies located in…
Brain (descending from cerebrum and brainstem)
Spinal cord (ascending from grey matter of spinal cord)
Periphery (Ascending from dorsal root ganglia)
(see figure)
How are axons arranged in white matter of spinal cord?
Arranged in tracts/fasciculi/bundles that have similar origin, termination and function (sensory or motor)
Number of fibres in white matter relative to cord location
White matter decreases as you move down the cord
See figure
Composition of grey matter of spinal cord
Neuronal cell bodies
Neuroglia
Dorsal horn of grey matter of spinal cord
Receives sensory input via dorsal root.
Site of numerous interneurons and projection neurons (project axons to brain)
Ventral horn of grey matter of spinal cord
Motor fn
Sends projections to skeletal muscle via ventral root
(see figure)
What is spinal cord contained in?
Meninges: dura/ arachnoid sac from foramen magnum to 2nd sacral vert. (S2)
Arachnoid is attached to dura, and is continuous with dura of the brain.
Where does conus medullaris end?
Between L1 and L2 vertebrae
Is dura attached to bone of vertebrae?
No
Epidural space of spinal cord
Between dura and vertebrae
Contains connective tissue, fat, veins
Space between arachnoid and pia of spinal cord
Filled with CSF
subarachnoid space
What is lumbar cistern?
Space between L1-2 and S2 vertebrae
Contains cauda equina
What is lumbar cistern used for?
Access to CSF without damaging cord
Spinal anasthesia
Pia matter of spinal cord
Innermost layer attached to spinal cord
What is the denticulate ligament?
Lateral projections of pia matter of spinal cord
Suspend cord with dural sac
(see figure)
What is the somatic nervous system?
Functional entity of the peripheral nervous system
Includes sensory component and a tomato-motor component
What does sensory component of somatic nervous system detect?
Pain, temperature, touch, vibration, proprioception
Sensory fibres carry information to the CNS
Where are the cell bodies of the sensory components of the somatic nervous system?
1 neutron from receptor to CNS
Cell body is in dorsal root ganglion
What does somato-motor component of somatic nervous system do?
Responsible for contraction of voluntary (skeletal) muscle (under voluntary control of the conscious mind,
as well as reflexes of skeletal muscle).
See figure
Where are the cell bodies of the somato-motor components of the somatic nervous system? Where do the axons exit?
Cell bodies of neurons innervating muscle are in ventral horn of spinal cord
Axons exit cord via ventral root and contact muscle directly (1 neuron from cord to muscle)
See figure
Systems regulated by ANS
- Cardiovascular System
- Respiratory System
- Reproductive System
- Urinary System
- Gastrointestinal System
Target tissue of ANS
i. Cardiac muscle
ii. Smooth muscle (within the walls of blood
vessels, airways, G.I. tract etc.), and
iii. Glands (e.g., salivary, sweat, adrenal medulla)
Conscious control of ANS?
In general, little conscious control (although there can be conscious influences on ANS activity)
Neurons of the afferent component of the autonomic nervous system
Sensory neurons that project from the target
tissue to the CNS via cranial and spinal nerves
Neurons of the efferent component of the autonomic nervous system
A ‘2-neuron chain’ that connects the CNS to the
target tissue
The connection between the 2 neurons is in an autonomic ganglion
See figure
What is the pre-ganglionic neuron? Where is it located? Where does it project?
Cell body of 1st neuron
is in the CNS
projects to a ganglion via a myelinated axon
What is the ganglion in the efferent portion of the autonomic nervous system? Where does it project to?
Site of connection between the pre- and post-ganglionic neuron
Houses the cell body of the post-ganglionic neuron
projects to the target tissue via a non-myelinated axon
see figure
2 divisions of autonomic nervous system (ANS)
- Sympathetic (Thoracolumbar) Nervous System (“Fight or Flight” response)
- Parasympathetic (Craniosacral) Nervous System (“Rest & Digest”)
Target tissue can be innervated by both, but this is not always the case
Sympathetic nervous system functions
Activated during ‘fight or flight’ response:
Increases heart rate and blood pressure
Increases diameter of airways
Inhibits salivary glands and decreases G.I. activity (directs blood away from G.I. tract)
Stimulates adrenal gland 0 Activates sweat glands
Dilates pupil
Where are the pre-ganglionic cell bodies of the sympathetic (thoracolumbar) NS located?
Located in thoracic and lumbar segments of spinal cord (T1 to L2)
Axons exit spinal cord via ventral root and project to sympathetic ganglia
What are sympathetic ganglia for the sympathetic (thoracolumbar) NS?
sympathetic chain: blood vessels, sweat glands, head and thorax (heart and lungs)
pre-vertebral ganglia: Abdominal and pelvic viscera
Where are the pre-ganglionic cell bodies of the parasympathetic (craniosacral) NS located?
brainstem: cell bodies of preganglionics are in specific cranial nerve nuclei and send their axons within the cranial nerves.
sacral levels of the spinal cord: The axons of these neurons exit cord via ventral root and travel within sacral nerves.
Where are parasympathetic ganglia located?
Near the target tissue (axons of post-ganglionics are short)
What do Cranial nerves in parasympathetic carniosacral NS innervate?
eye, glands of the head, viscera of thorax (heart & lungs) and abdomen (stomach, intestines, liver)
What do sacral nerves in parasympathetic carniosacral NS innervate?
pelvic viscera (lower colon, bladder, and reproductive organs)
Parasympathetic NS functions
Dominant when at rest (‘Rest & Digest’) :
Decreases heart rate and blood pressure
Decreases diameter of airways
Stimulates salivary glands and increases G.I. activity
Constricts pupil
sympathetic vs parasympathetic
Generally the 2 branches (symp. & parasymp.) are functionally antagonistic
If both innervate a muscle, 1 branch will cause contraction, while the other relaxation
Together they determine the overall activity of a gland or contractile activity of a smooth muscle
Innervation of smooth muscle of blood vessels and sweat glands
only innervated by the Sympathetic NS
Which functions require coordination between SNS and PSNS?
Bowel and bladder function (defecation and micturition)
Sexual function (erection and ejaculation)
What is the major regulator of the ANS?
Hypothalamus (part of diencephalon)
Hypothalamus projects to brainstem and down the spinal cord and regulates activity of the preganglionics of SNS and PNS
