The Spinal Cord, Spinal Nerves, and Spinal Reflexes Flashcards
Regions of the spinal cord
- Cervical
- Thoracic
- Lumbral
- Sacral
Gray matter
Neuron cell bodies
White matter
Myelinated axons
Spinal nerves
C1-C8 T1-12 L1-L5 S1-S5 CO1
Where does the spinal cord extend to?
From the brain to L1-L2
Posterior median sulcus
Shallow longitudinal groove in the spinal cord
Anterior median fissure
Deep groove along the anterior surface of the spinal cord
Central canal
Internal passageway
Functions of cerebrospinal fluid (CSF)
Acts as a shock absorber and a diffusion medium for dissolved gases, nutrients, chemical messengers, and wastes
Where is there the most gray matter?
In areas dedicated to sensory and motor control of the limbs
Cervical enlargement
Supplies nerves to the shoulder and upper limbs
Lumbosacral enlargement
Supplies nerves nerves to pelvis and lower limbs
Conus medullaris
Conical, tapered region of the spinal cord
Filum terminale
Slender strand of fibrous tissue that extends from the inferior tip of the conus medullaris to the sacral vetebra
Part of coccygeal ligament
Spinal ganglia
Contain cell bodies of sensory neurons
Posterior roots
Made up of axon roots of sensory neurons
Anterior roots
Made up of axons of motor neurons that extend into periphery to control somatic and visceral effectors
Rootlets
Branch out from roots of spinal nerves
Spinal nerve
Sensory and motor roots bound together to form a spinal nerve
White ramus communicans
Containing myelinated axons
Gray ramus communicans
Containing unmyelinated axons fibres that innervate flands and smooth muscles in the body walls or limbs
Posterior ramus
Providing sensory and motor innervation to the skin and muscles of the back
Anterior ramus
Supplying ventrolateral body surface, structures in the body wall and lumbs
Mixed nerves
Containing both afferent and efferent fibres
Spinal nerves are mixed nerves
How many spinal nerves are there?
31, each associated with adjacent vertebrae
How are cervical nerves named?
The cervical nerve takes the name of the vertebra immediately below it
How are the thoracic, lumbar and sacral nerves named?
The nerve takes the name of the vertenbra immediately above it
Spinal meninges
Series of specialised membranes surrounding the spinal cord that provide stability and shock absorption
3 layers of spinal meninges
- Dura mater
- Arachnoid mater
- Pia mater
Dura mater
- Outermost
- Dense collagen fibres
Epidural space
Between the dura matter and the walls of the vertebral canal
Contains areolar tissue, blood vessels and adipose tissue
Arachnoid mater
- Middle layer
Subarachnoid space
Filled with CSF
Lumbar puncture, spinal tap
Needle draws CSF from subarachnoid space
Pia mater
- Innermost
- Meshwork of elastic collagen fibres firmly bound to neural tissue
Denticulate ligaments
Extend from pia mater through the arachnoid mater to dura mater
Provide lateral movement
What prevents longitudinal movement?
Dural connections at the foramen magnum and the coccygeal ligament
Meningitis
Inflammation of the meningeal membranes
Spinal anaesthesia
Anaesthetics injected into subarachnoid space of the spinal cord
Horns
Areas of gray matter on each side of the spinal cord
Posterior, lateral and anterior
Gray commissures
Posterior and anterior
Nuclei
Masses of gray matter within the CNS
Sensory nuclei
Receive and relay sensory information from peripheral receptors
Motor nuclei
Issue motor commands to peripheral effectors
Posterior horns
Contains somatic and visceral sensory nuclei
Anterior horns
Function in somatic motor control
Lateral horns
Contain visceral motor neurons
Posterior white columns
Lie between the posterior horns and the posterior median sulcus
Anterior white columns
Lie between the anterior horns and the anterior fissure
Anterior white commissure
Connect anterior white columns
Region where axons cross from one side of the spinal cord to the other
Lateral white column
White matter between anterior and posterior columns on each side
Tract
Bundle of axons in the CNS
Short tracts
Carry sensory and motor signals between segments of the spinal cord
Long tracts
Connect spinal cord with the brain
Ascending tracts
Carry sensory information toward the brain
Descending tracts
Convey motor commands to the spinal cord
Layers of spinal nerves
- Epineurium
- Perineurium
- Endoneurium
Epineurium
- Outermost layer
- Dense network of collagen fibres
Perineurium
- Middle layer
- Cover one fascicle
Endoneurium
- Innermost layer
- Surround individual axons
Peripheral nerves
Branch from spinal nerves
Innervate body tissues and organs
Dermatome
Specific bilateral region of the skin surface monitored by a single pair of spinal nerves
What happens when a spinal nerve of spinal ganglion is damaged or infected?
Corresponding region of skin loses sensation
Peripheral neuropathies
Regional losses of sensory and motor function most often resulting from nerve trauma or compression
Shingles
Attacks neurons within the posterior roots of spinal nerves and sensory ganglia of cranial nerves
Nerve plexus
Complex, interwoven network of nerves
Four major plexuses
- Cervical plexus
- Brachial plexus
- Lumbar plexus
- Sacral plexus
Cervical plexus
Consists of the anterior rami of spinal nerves C1-C5
Phrenic nerve
Major nerve of the cervical plexus
Supply the diaphragm
Brachial plexus
Innervates pectoral girdle and upper limb
Consists of anterior rami of spinal nerves C5-T1
Trunks
Large bundles of spinal nerve axons
Cords
Smaller branches of trunks
Musculocutaneous nerve
Formed from lateral cord
Median nerve
Made of medial cord and median nerve
Lumbar and sacral plexuses
Innervate pelvic girdle and lower limbs
Lumbar plexus
Contains axons from the anterior rami of spinal nerves T12-L4
Major nerves of lumbar plexus
Genitofemoral nerve
Lateral femoral cutaneous nerve
Femoral nerve
Sacral plexus
Contains axons from the anterior rami of spinal nerves L4-S4
Major nerves of sacral plexus
Sciatic nerve
Pudendal nerve
What does the sciatic nerve branch into?
Fibular and tibial nerve
Reflexes
Automatic responses
Motor responses to specific stimuli
Sensory information
- Sympathetic nerve carries sensory information from the visceral organs
- Anterior ramus carries sensory information from ventrolateral surface, structures in the body wall, and the limbs
- Posterior ramus carries sensory information from the skin and skeletal muscles of the back
- Posterior root of each spinal nerves carries sensory information to the spinal cord
Motor commands
- Anterior root of each spinal nerve contains the axons of somatic motor and visceral motor neurons
- Spinal nerve forms just lateral to the invertebral foramen, where the posterior and anterior roots unite
- Posterior ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles of the back
- The axons in the relatively large anterior ramus supply the ventrolateral body surface, structures in the body wall and the limbs
- White ramus communicans is the first branch from the spinal nerve and carries visceral motor fibres to a nearby sympathetic ganglion
- The gray ramus communicans contains postganglionic fibres that innervate glands and smooth muscles in the body wall or limbs
Which ganglionic branches are myelinated?
Preganglionic axons - white ramus
Postganglionic axons - gray ramus
Rami communicantes
White and gray rami together
Where are white and gray rami found?
White - between T1 and L2
Graya - associated with each spinal nerve
Reflex arc
- Arrival of a stimulus and activation of a receptor
- Activation of a sensory neuron
- Information processing in the CNS
- Activation of a motor neuron
- Response by a peripheral effector
Receptor
Specialised cell or the dendrites of a sensory neuron
Sensitive to chemical of physical changes
Respond to stimuli that cause or accompany tissue damage
Innate reflexes
Result from connections that form between neurons during development e.g. withdrawal reflex
Acquired reflexes
Learned motor patterns e.g. experienced driver steps on the brakes when trouble appears
Repetition enhances acquired reflexes
Somatic reflexes
Provide a mechanism for the involuntary control of the muscular system
Immediate
Visceral reflexes
Control the activities of other systems
Monosynaptic reflexes
Involve the simplest reflex arc
Sensory neuron innervate a motor neuron directly
Motor neuron performs the information processing
Polysynaptic reflexes
Have at least one interneuron between the sensory neuron and the motor neuron
Longer delay between stimulus and response
Spinal reflexes
The important interconnections and processing events occur in the spinal cord
Cranial reflexes
Reflexes processed in the brain
Intersegmental reflex arcs
Most complicated polysynaptic reflex arcs
Many segments interact to produce a coordinated highly variable motor response
Stretch reflex
Monosynaptic
Automatically regulates skeletal muscle length
e.g. patellar reflex (knee-jerk)
Stimulus (increase in muscle length > sensory neuron activated > immediate motor neuron response (muscle contracts) > stimulus counteracted
Muscle spindles
Sensory receptors involved in the stretch reflex
Intrafusal muscle fibres
A bundle of small, specialised skeletal muscle fibres
Extrafusal muscle fibres
Large skeletal muscle fibres that surround muscle spindles
Gamma motor neurons
Motor neurons innervating intrafusal fibres
Gamma efferents
Axons of motor neurons innervating intrafusal fibres
Function of muscle spindles
Stretching muscle spindles produce a sudden burst of activity in the sensory neurons > stimulation of motor neurons > rapid muscle shortening
Postural reflexes
Maintains a person’s upright posture
Tendon reflex
Monitors the tension produced during muscular contractions and prevents damage to tendons
Withdrawal reflex
Move affected portions of the body away from a source of stimulation
Flexor reflex
A withdrawal reflex affecting muscles of a limb
Crossed extensor reflex
Complements withdrawal reflexes
What do all polysynaptic reflexes involve?
- Involve pools of interneurons
- Are intersegmental in distribution
- Involve reciprocal inhibition
- Have reverberating circuits, which prolong the reflexive motor response
- Can cooperate to produce a coordinated response
Reinforcement
Enhancement of spinal reflexes produced by facilitation
Plantar reflex
Replaces Babinski reflex in infants