Chapter 13 Nervous System Flashcards
Central nervous system
Brain and spinal cord
Processing centers
Peripheral nervous system
Cranial nerves and spinal nerves
Sensory input and motor response pathways
Reflexes
Quick, automatic nerve responses triggered by specific stimuli
Spinal reflexes
Controlled by the spinal cord alone without input from the brain
Example: dropping a hot pan. Reflex causes release of the pan before the information reaches the brain and the pain is perceived
Sensory Receptors
Cranial Nerves: In from the PNS sensory to brain out to Cranial motor Nerves
Spinal Nerves: In from the PNS and out to the Spinal Nerves
Effectors are
Muscles, Glands and Adipose Tissue
Spinal cord
Housed membranes (meninges) and vertebral column
Carries sensory and motor information between brain and out
Gives rise to spinal nerves
Gross anatomy of the spinal cord
About 18 in. (45 cm) long, 1/2 in. (14 mm) wide
brain only to vertebrae L1 and L2 (stops lengthening around age 4, but the vertebral column still grows)
4 regions (cervical, thoracic, lumbar, sacral)
Has bilateral symmetry
31 segments; give rise to spinal nerves
Grooves divide spinal cord into left and right
Posterior median sulcus—posterior side
Anterior median fissure—deeper, anterior groove
Central canal contains cerebrospinal fluid (CSF)
Enlargements of the spinal cord
Areas of the spinal cord that supply the limbs have more gray matter and are visibly wider.
Cervical enlargement
Supplies shoulder and upper limb
Lumbosacral enlargement
Supplies pelvis and lower limb
Distal end of the spinal cord
Conus medullaris
Tapered, conical end of cord below lumbar enlargement
Cauda equina
Nerve roots extending below conus medullaris
Looks like a horse’s tail, hence the name
Filum terminale
Thin thread of fibrous tissue at end of conus medullaris
Attaches to coccygeal ligament
Spinal roots and ganglia
Two branches form spinal nerves
Anterior root (ventral root)—axons of motor neurons
Posterior root (dorsal root )—axons of sensory
neurons
Spinal nerve roots divide into rootlets before entering or leaving the spinal cord
Spinal ganglia (also called dorsal root ganglia)
Contain cell bodies of sensory neurons that form the posterior root
Located between pedicles of adjacent vertebrae
Spinal nerves
Formed by union of posterior and anterior roots
Pairs—one from each side at each vertebral level
Each has a white ramus communicans and a gray ramus communicans that innervate glands and smooth muscle
Mixed nerves—contain both afferent (sensory) and efferent (motor) fibers
Each spinal nerve quickly divides into rami
Posterior ramus supplies skin/muscles of back
Anterior ramus supplies most of body wall, skin, limbs
Naming the spinal nerves:
Designated by vertebral region and number
C1 runs above first cervical vertebra
C8 below seventh cervical vertebra
All others named for vertebrae above
Example: T1 is below first thoracic vertebra
spinal meninges
Three specialized membranes surrounding spinal cord
Dura mater—outermost layer
Arachnoid mater—middle layer
Pia mater—innermost layer
Functions of the spinal meninges include:
Meningitis
Viral or bacterial infection of meninges
Protect spinal cord
Carry blood supply
Continuous with cranial meninges
The dura mater—outermost meningeal layer
Tough with dense collagen fibers
Continuous with cranial dura mater and fuses with periosteum of occipital bone
Distal end tapers to dense cord of collagen fibers; joins filum terminale in coccygeal ligament
Epidural space
Between vertebrae and dura mater (superficial to dura mater)
Contains loose connective and adipose tissue
Subdural space = potential space deep to dura mater
arachnoid mater—middle meningeal layer
Two components
Arachnoid membrane—weblike layer of simple squamous epithelia
Arachnoid trabeculae—network of collagen/elastic fibers between arachnoid membrane
Subarachnoid space
Space with arachnoid trabeculae, between arachnoid mater and pia mater
Filled with cerebrospinal fluid (CSF) that carries dissolved gases, nutrients, wastes
Lumbar puncture or spinal tap withdraws CSF from subarachnoid space
pia mater—innermost meningeal layer
Is a mesh of collagen and elastic fibers
Firmly attached to underlying neural structures it surrounds
Blood vessels for spinal cord are on surface of the pia mater, within subarachnoid space
Paired denticulate ligaments
Anchor pia mater to dura mater
Prevent lateral movement of spinal cord
Gray matter—cell bodies of neurons, neuroglia, and unmyelinated axons
Functional organization of gray matter
Masses of gray matter within CNS are called nuclei and are organized into regions called horns
Posterior horns—somatic and visceral sensory nuclei (incoming information from receptors)
Anterior horns—somatic motor nuclei (outgoing information to effectors)
Lateral horns—thoracic and lumbar segments; visceral motor nuclei
Gray Matter continued
Gray commissures
Narrow bands of gray matter around central canal
Axons cross here to the other side of spinal cord
Sensory or motor nucleus location within gray matter determines which body part it controls
WHITE MATTER - FUNCTION
Three columns (regions)
Posterior white columns—between posterior horns and posterior median sulcus
Anterior white columns—between anterior horns and anterior median fissure
Anterior white commissure—where axons cross from one side of spinal cord to the other
Lateral white columns—on each side of spinal cord, between anterior and posterior columns
White Matter
Tract—bundle of axons in CNS
Relay same type of information in same direction
Ascending tracts—sensory information up toward the brain
Descending tracts—motor commands down to the spinal cord
Three connective tissue layers surround spinal nerves
Epineurium—outermost; network of collagen fibers
Perineurium—middle layer; separates nerve into
fascicles (axon bundles)
Endoneurium—innermost; surrounds individual
axons
Spinal nerves
Pair of spinal nerves emerges laterally from each spinal cord segment
Form by junction of anterior and posterior roots
All spinal nerves are mixed nerves
Peripheral nerves
Form from branching and re-sorting of spinal nerves.
All are mixed nerves (sensory and motor)
Same connective tissue layers as spinal nerves (continuous with each other)
Peripheral distribution of spinal nerves
Shingles—rash/symptoms occur along dermatomes
Sensory nerves
In addition to motor impulses:
Posterior, anterior, and white rami also carry sensory information
Dermatome—specific bilateral region of skin supplied by a single pair of spinal nerves
Peripheral neuropathies—regional losses of neural function that affect dermatomes, often from nerve trauma, compression, various illnesses
Nerve plexuses
Complex, interwoven networks of nerve fibers
Formed from blended fibers of anterior rami of adjacent spinal nerves
Allows multiple spinal nerves to supply the same structures
Four major nerve plexuses
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus
The cervical plexus
Includes anterior rami of spinal nerves C1–C5
Innervates scalp behind ear, neck, and diaphragm
Major cervical plexus nerves
Phrenic nerve—from C3–C5; controls diaphragm
Lesser occipital nerve
Great auricular nerve
Transverse cervical nerve
Supraclavicular nerves
brachial plexus
Innervates pectoral girdle, upper back, upper limb
Anterior rami of C5–T1
These rami first form three large trunks (superior, middle, and inferior trunks)
Trunks re-sort their axons to form three cords (lateral, posterior, and medial cords)
Cords are named by their positions relative to the axillary artery
Most nerves of brachial plexus come off the cords; a few originate at the trunks
Major brachial plexus nerves
Musculocutaneous nerve (lateral cord)
Median nerve (lateral and medial cords)
Ulnar nerve (medial cord)
Axillary nerve (posterior cord)
Radial nerve (posterior cord)
The hand is innervated by the ulnar, median, and radial nerves
Median nerve passes through the small carpal tunnel on the anterior wrist, along with several muscle tendons
Carpal tunnel syndrome—compression of median nerve, usually when adjacent tendons are swollen (repetitive stress)
Causes change or loss of sensation in the areas supplied by the median nerve
lumbar plexus
Includes anterior rami of spinal nerves T12–L4
Major nerves
Iliohypogastric nerve
Ilio-inguinal nerve
Femoral nerve
Obturator nerve
Genitofemoral nerve
Lateral femoral cutaneous nerve
sacral plexus
Includes anterior rami of spinal nerves L4–S4
Major nerves
Sciatic nerve
Superior and inferior gluteal nerves
Pudendal nerve
Two branches of the sciatic nerve
Fibular nerve (common fibular nerve)
Tibial nerve
Sensory innervation of foot
Mapping touch/pain perception and checking muscle function can determine damage to specific peripheral nerves
Branches from these peripheral nerves supply the foot/ankle
Saphenous nerve
Sural nerve
Fibular nerve
Tibial nerve
Functional organization of neurons
Sensory neurons
About 10 million;
Motor neurons
About 1/2 million;
Interneurons
About 20 billion;
Interpret, plan, and coordinate signals coming in and out
Neuronal pools
Interneurons organized into functional groups of interconnected neurons
Each has limited input sources and output destinations
May stimulate or depress parts of brain or spinal cord
Five patterns of neural circuits in neuronal pools
Divergence
Convergence
Serial processing
Parallel processing
Reverberation
Patterns of neural circuits in neuronal pools
Divergence—spreads information from one neuron or neuronal pool to many
Especially common in sensory pathways
Convergence—several neurons synapse on a single neuron
Example: subconscious and conscious control of the diaphragm in breathing—two neuronal pools synapse with the same motor neurons
Patterns of neural circuits in neuronal pools (continued)
Serial processing—information moves along a single path, sequentially from one neuron or neuronal pool to the next
Example: pain signals pass sequentially through two neuronal pools to reach conscious brain
Parallel processing—several neurons/neuronal pools process the same information at the same time
Example: step on a bee. Signals spread through several neuronal pools so you can shift your weight, lift your foot, yell in pain at about the same time
Patterns of neural circuits in neuronal pools (continued)
Reverberation—collateral branches of neurons extend back and continue stimulating presynaptic neurons
Forms positive feedback loop; continues until synaptic fatigue or inhibition occurs
Examples: may maintain consciousness, breathing, muscle coordination
Neural reflexes
Rapid, automatic responses to specific stimuli
Basic building blocks of neural function
A specific reflex produces the same motor response each time
reflex arc
Sensory receptor
Sensory neuron
Information processing in CNS
Motor neuron
Effector
Events in a spinal reflex arc
Step 1: Stimulus activates a receptor (review receptor types, Chapter 12)
Step 2: With enough stimulation, action potential is generated in sensory neuron. Axon enters spinal cord via posterior root
Step 3: Information processing in spinal cord usually occurs at one or more interneurons
Step 4: Interneurons stimulate action potentials in motor neuron; its axon leaves via anterior root
Step 5: Motor neuron stimulates effector (muscle/gland)
Four types of classification of reflexes
Development
Motor response
Complexity of neural circuit
Site of information processing
Development of reflexes—innate or acquired
Innate reflexes
Basic neural reflexes formed before birth
Genetically programmed (inborn)
Examples: withdrawal, chewing, visual tracking
Acquired reflexes
Rapid, automatic learned motor patterns
Repetition enhances them
Examples: braking a car in emergency
Motor response
Somatic reflexes
Control skeletal muscle contractions
Superficial reflexes—stimuli in skin/mucous membranes
Stretch or deep tendon reflexes (such as patellar, or “knee-jerk,” reflex)
Immediate—important in emergencies (slipping, cutting finger)
Visceral reflexes (autonomic reflexes)
Control other effectors
Smooth muscle, cardiac muscle, or glands
Monosynaptic reflex
Single synapse—simplest reflex arc
Sensory neuron synapses directly with motor neuron
Fast response
Polysynaptic reflex
At least one interneuron between sensory neuron and motor neuron; most common
Slower response; delay increase with number of synapses involved (longer path = longer delay)
Intersegmental reflex arcs
—many spinal cord segments interact; produce variable response
Muscle spindle structure (continued)
Dendrites of sensory neurons wind around central region of intrafusal fibers.
Sensory neuron axon enters CNS in posterior root
Sensory neurons synapse in spinal cord directly with motor neurons
Gamma efferents complete reflex arc by synapsing back at the intrafusal fibers
Muscle contracts back to its resting length
Postural reflexes
ed
Include both stretch reflexes (monosynaptic) and also complex polysynaptic reflexes
Maintain normal upright posture
Often involve multiple muscle groups (e.g., back and abdominal muscles)
Maintain firm muscle tone
Extremely sensitive receptors allow constant fine adjustments to be made as need
Polysynaptic reflexes
More complicated than monosynaptic reflexes
Interneurons can control multiple muscle groups
Produce either EPSPs or IPSPs, stimulating some muscles and inhibiting others.
Examples
Tendon reflex
Withdrawal reflexes
Crossed-extensor reflexes
tendon reflex
Prevents skeletal muscles from:
Developing too much tension
Tearing or breaking tendons
Sensory receptors are Golgi tendon organs
Stimulated when collagen fibers are overstretched
Stimulate inhibitory interneurons in spinal cord
More muscle tension leads to more muscle inhibition
Withdrawal reflexes
s
Move body part away from stimulus (pain or pressure)
Example: flexor reflex in limbs; pulls hand from hot pan
Strength and extent of response depends on intensity and location of stimulu
Reciprocal inhibition
For flexor reflex to work, stretch reflex of antagonistic (extensor) muscles must be inhibited (reciprocal inhibition) by interneurons in spinal cord.
When flexors contract, extensors relax
When extensors contract, flexors relax
Reflex arcs
Ipsilateral reflex arcs
Occur on same side of body as stimulus
Stretch, tendon, and withdrawal reflexes
Crossed extensor reflexes involve contralateral reflex arcs
Occur on side opposite stimulus
Crossed extensor reflexes
Coordinated with flexor reflex
Step on something sharp; before flexor reflex can lift injured foot, crossed extensor reflex straightens opposite limb to receive body weight, then flexor reflex can occur
Maintained by reverberating circuits
Five general characteristics of polysynaptic reflexes
Involve pools of interneurons
May cause excitation or inhibition
Involve more than one spinal segment
Can activate muscles in multiple areas
Involve reciprocal inhibition
Coordinates contractions and reduces resistance
Have reverberating circuits
Prolongs reflexive motor response
Several reflexes may cooperate
To produce coordinated, controlled response
Voluntary movements and reflex motor patterns
Spinal reflexes produce characteristic response for a given stimulus.
Brain can also activate these same motor patterns through descending pathways
Can facilitate, inhibit, or “fine-tune” the established motor response
Examples: walking, running, jumping
Reinforcement of spinal reflexes
Higher centers can adjust sensitivity of reflexes by stimulating excitatory or inhibitory interneurons in brainstem or spinal cord
When excitatory synapses are chronically stimulated, postsynaptic neurons can be in general facilitation
This reinforcement enhances spinal reflexes
Inhibition of spinal reflexes
Higher centers inhibit spinal reflexes by:
Stimulating inhibitory neurons
Creating IPSPs at reflex motor neurons
Suppressing postsynaptic neurons, thus inhibiting the reflex
Plantar reflex
s
Normal in adults
Stroke lateral sole, causes reflexive toe-curling
The Babinski reflex
Normal in infants
May indicate CNS damage in adults
