Chapter 13 - Spinal Cord & Reflexes Flashcards
4 Functions of the Spinal Cord
- Conduction
- Neural Integration
- Locomotion
- Reflexes
Conduction
nerve fibers send sensory and motor information ↑ and ↓ the spinal cord.
Neural integration
spinal neurons receive input from multiple sources, integrate (compute!) it, and execute appropriate output (e.g., bladder control)
Locomotion
spinal cord contains central pattern generators. Ex. groups of neurons that coordinate repetitive sequences of contractions for walking.
Reflexes
involuntary responses to stimuli that are vital to posture, coordination, and protection.
Spinal Cord
cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull.
– Occupies the upper 2/3 of vertebral canal
– Inferior margin of SC ends at the L1 vertebral bone or slightly beyond!!!!
The Spinal Cord
- Cervical 8
- Thoracic 12
- Lumbar 5
- Sacrum 5
1 Coccygeal nerve
Why are there 8 cervical nerves, but only 7 cervical vertebrae (bones) ?
Cl nerve comes out between the skul and first vertebrae. Then there numbered after that
Why is there little risk of spinal cord damage with these procedures?
The spinal cord does not extend lower than L1/L2
Surface Anatomy
31 pairs of spinal nerves enter/exit between the vertebrae on both sides.
Cervical Enlargement
- nerves to upper limb
- takes care of the arm
Lumbar enlargement
- nerves to pelvic region and lower limbs
- for your legs, sensory and motor regions make this place larger
Medullary cone
- cord tapers to a point inferior to lumbar enlargement
- tip of spinal cord
Cauda equina
- bundle of nerve roots that occupy the vertebral canal from L2 to S5
- horse tail. bunch of nerves coming down from cone
Where are the Enlargements?
In the cervical & Lumbar regions
- skinner in thoracic because you have neuron that controls posture, core muscles.
- in very large motor unit groups
P.A.D.
-Pia mater
- Arachnoid mater
- Dura mater
- These same 3 fibrous membranes surround the spinal cord
Dural sheath
surrounds spinal cord and is separated from vertebrae by epidural space.
Arachnoid
membrane adheres to dura and is separated from pia by fibers spanning the subarachnoid space that is filled with cerebrospinal fluid (CSF)
Lumbar puncture
spinal tap) takes sample of CSF
Pia
delicate membrane that follows contours of spinal cord.
Safe place to do lumbar puncture
L3/L4 or L4/L5
Spina Bifida
congenital defect in which one or more vertebrae fail to form a complete vertebral arch for enclosure of the spinal cord.
Prevention of Spina Bifida
Folic acid (a B vitamin now added to flour), also in prenatal vitamins
– Note:
Defect occurs during the first 4 wks of devel., so folic acid supplements for mothers must begin 3 mo. before conception.
Gray Matter
neuron cell bodies with little myelin
– Site of information processing, synaptic integration
White matter
abundantly myelinated axons
– Carry signals from one part of the CNS to another
Gray Matter
Pair of posterior (dorsal) horns
– Posterior (dorsal) root of spinal nerve carries only sensory fibers (afferent)
Pair of thicker anterior (ventral) horns
– Anterior (ventral) root of spinal nerve carries only motor fibers (efferent)
* Gray commissure connects R & L sides
* Central canal lined with ependymal cells and filled with CSF
White Matter
Consists of myelinated axons that course up (ascending) and down (descending) the cord providing communication between different levels of the CNS.
* Columns (funiculi) —three pairs of these white matter bundles – Posterior (dorsal), lateral, and anterior (ventral) columns
– Tracts or fasciculi = subdivisions of each column
Spinal Tracts
Fibers in a given Tract have similar origin, destination and function.
– Ascending tracts—carry sensory information up
– Descending tracts—carry motor information down
Decussation
crossing of the midline that occurs in many tracts so that brain senses and controls contralateral side of body
Contralateral
when the origin and destination of a tract are on opposite sides of the body
Ipsilateral
when the origin and destination of a tract are on the same side of the body; does not decussate
Where are ascending and descending tracts found?
ventral horn
Sensory nerves
Carry signals from sensory receptors to the CNS
Motor nerves
Carry signals from CNS to muscles and glands
Mixed nerves
Consists of both afferent and efferent fibers
Both sensory and motor fibers can also be described as:
- Somatic or visceral
– General or special
Epineurium
dense irregular CT that wraps entire nerve
Perinerium
layers of overlapping squamous cells that wrap fascicles: bundles of nerve fibers
Endoneurium
loose CT external to neurilemma (mb of one axon)
Nerve
a cord-like organ composed of numerous nerve fibers (axons) bound together by CT.
The nerve fibers of PNS are surrounded by what?
Schwann cells forming neurilemma and myelin sheath around the axon.
Blood vessels penetrate CT coverings
Provide plentiful blood supply
Ganglion
- cluster of neurosomas (cell bodies) in the PNS
- Enveloped in an endoneurium CT continuous with that of the nerve
- Bundles of nerve fibers leading into, thru, & out of the ganglion.
- Posterior (dorsal) root ganglion (DRG) associated with spinal nerves. Sensory!
Which connective tissue wraps each nerve fiber
endoneurium
3 Diseases of Motor Neurons
- Destruction of motor neurons
- Leads to skeletal muscle atrophy from lack of innervation.
Poliomyelitis & Iron Lungs
- spreads by fecal contamination of water
- Destroys motor neurons in brainstem & ventral horn of SC
- Symptoms: muscle pain, weakness, & loss of some reflexes
- Then paralysis, muscular atrophy, & possibly respiratory arrest (diaphragm is skel muscle!)
ALS
- Destruction of motor neurons and muscular atrophy
– Also sclerosis (scarring) of lateral regions of the spinal cord
– Astrocytes fail to reabsorb the neurotransmitter glutamate from the tissue fluid - Accumulates to toxic levels
- Early signs: muscular weakness; difficulty
speaking, swallowing, and using hands
– Sensory and intellectual functions remain unaffected
Shingles
- Disease of sensory nerves
- caused by the virus traveling down the sensory nerves by fast axonal transport (anterograde) when immune system is compromised
- Prevention: Vaccination
Mixed spinal nerves
become roots which become rootlets before joining the spinal cord
What are the two enlargements of the spinal cord
cervical and lumbar
Anterior ramus
- In thoracic region, it gives rise to intercostal nerve.
- In other regions, anterior rami form plexuses.
Posterior ramus
innervates the muscles and joints in that region
of the spine and the skin of the back
Meningeal branch
reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments.
Cervical plexus
- in the neck, C1 to C5
- Supplies neck and phrenic nerve to the diaphragm
Brachial plexus
- near the shoulder, C5 to T1
- Supplies upper limb and some of shoulder and neck
- Median nerve—carpal tunnel syndrome
Lumbar plexus
- in the lower back, L1 to L4
- Supplies abdominal wall, anterior thigh, and genitalia
Sacral plexus
- in the pelvis, L4, L5, and S1 to S4
- Supplies remainder of lower trunk and lower limb
Coccygeal plexus
- S4, S5, and Co1
Phrenic nerve
keeps you alive
- controls the diaphragm, major muscle of respiration
Median nerve
innervate the arm
Femoral nerve
innervates the leg
sciatic nerve
supplies remainder of lower trunk and lower limb
Reflexes
lead to stimulation of glands or muscle
Reflexes require stimulation
Not spontaneous actions, but responses to sensory input
Reflexes are quick
Involve few, if any, interneurons and minimum synaptic delay
Reflexes are involuntary
Occur without intent and are difficult to suppress
Reflexes are stereotyped/automated
Occur essentially the same way every time
Somatic receptors
- In skin, muscles, or tendons
- Skin - touch or nociception
- Proprioceptors— in muscles or tendons, sense monitor body position and movement of body parts
Ex. muscle spindle, GTO
Afferent nerve fibers
Carry information from receptors to dorsal (posterior) horn of spinal cord or to the brainstem
Intergrating center
One or more synapses in the CNS
Efferent nerve fibers
Carry motor impulses to muscles
Effectors
The muscles that carry out the response
Somatic Reflexes
for skeletal muscle.
Visceral reflexes
for skeletal muscles and cardiac muscles + glands
Muscle spindle
- stretch receptors embedded in skeletal muscles.
- Enables brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture.
Speed of change
sense muscle length and body movement
A spindle contains:
intrafusal muscle fibers within it
– Rest of the muscle’s fibers (those generating force for movement) are extrafusal fibers
gamma motor neuron
innervates the ends of an
intrafusal fiber and keeps it taut
The midportion of the intrafusal fiber contains
- Primary afferent fibers– monitor fiber length and speed of length
changes - Secondary afferent fibers – monitor length only
Ex. of spindle function
help you to keep upright when standing on a boat
Myotatic reflex
when a muscle is stretched, it “fights back” and contracts.
Myotactic helps:
- helps maintain equilibrium and posture
- Head starts to tip forward as you fall asleep
- Muscles contract to raise the head
patellar reflex
monosynaptic reflex
- one synapse between the afferent and efferent neurons
Reciprocal inhibition
reflex phenomenon that prevents
muscles from working against each other by inhibiting antagonist when agonist is excited (see next slide, what color is the inhibiting neuron?)
Withdrawal reflex
triggers contraction of the flexors and relaxation of the extensors in that limb.
Flexor reflex
the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus (nociceptor used)
Polysynaptic reflex arc
pathway in which signals travel over many synapses on their way to the muscle.
Crossed extension reflex
contraction of extensor muscles in limb opposite of the one that is withdrawn
- Maintains balance by extending other leg
Intersegmental reflex
one in which the input and output occur at different levels (segments) of the spinal cord
– Pain in foot causes contraction of abdominal muscles
Golgi Tendon Organs
proprioceptors in a tendon near its junction with a muscle
– Golgi tendon organ: 1 mm long, nerve fibers entwined in collagen fibers of the tendon
Tendon reflex
in response to excessive tension on the tendon.
– Inhibits muscle from contracting strongly (recall the video about the climber).
– Moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone.
Nerve fibers
Golgi Tendon Organ
Tendon bundles
Muscle fibers
Complete transection
complete severance of cord
Spinal shock
loss of sensory & motor function below injury
Spinal cord trauma Above C4
poses threat of respiratory failure
Hemiplegia
paralysis on one side of the body
Paraplegia
Paralysis of both lower limbs
Quadriplegia
paralysis of all four limbs
Paresis
partial paralysis or weakness of the limbs
