The Spinal Cord (CNS) and Peripheral Nervous System Spinal Cord (CNS) Meninges and Injections/Punctures Tracts and Information Pathways Spinal nerves

The Spinal Cord (CNS) and Peripheral Nervous System

Spinal Cord (CNS)

• Meninges and Injections/Punctures

• Tracts and Information Pathways

• Spinal nerves

• Medical conditions: Spinal cord injury, shingles

Peripheral Nervous System

• Structure of a Nerve and Its Wrappings

• The Twelve Cranial Nerves

• Reflexes: Monosynaptic and Polysynaptic

• Branches of the Peripheral System

o Comparison between autonomic and somatic

Sympathetic division

Parasymphathetic division

• Developmental Aspects of the Nervous System

Spinal Cord Extends from the medulla

oblongata to the region of T12

Below L2 is the cauda equina (a collection of spinal nerves)

Enlargements occur in the cervical and lumbar regions

31 pairs of spinal nerves branch off it

Meninges follow cord all the way to the spinal hiatus of the sacrum

Figure 7.18

at L2

C7

Lumbar Puncture vs. Epidural Space Injection

Lumbar puncture for CSF below L2 in larger subarachnoid space

3

2

1

Epidural injection (steroids, anaesthetic) into epidural space

Spinal Cord Anatomy Exterior white mater – conduction tracts (axons) – is myelinated

Internal gray matter - mostly cell bodies, is unmyelinated

• Dorsal (posterior) horns (mostly association/interneurons)

• Anterior (ventral) horns (motor nerves of somatic system)

Central canal filled with cerebrospinal fluid


Spinal Cord (CNS)



• Spinal nerves











Figure 12.32

Somaticsensoryneuron

Dorsal root (sensory)

Dorsal root ganglion

Visceralsensory neuron

Somaticmotor neuron

Spinal nerve

Ventral root(motor)

Ventral horn(motor neurons)

Dorsal horn (interneurons)

Visceralmotorneuron

Interneurons receiving input from somatic sensory neurons

Interneurons receiving input from visceral sensory neurons

Visceral motor (autonomic) neurons

Somatic motor neurons

What’s in the Gray Matter of the Spinal Cord?

White Matter: Spinal Cord Information Pathways

Dorsal columns carry only ascending/afferent sensory tracts

Lateral and anterior columns carry both

• Ascending/afferent sensory tracts

• Descending/efferent motor tracts

Pathways are composed of 2-3 neurons in a chain or relay

Pathways cross from one side of the CNS to the other (decussate)

Ascending fibers add on laterally in cord as they join - they “map” in the tract based on where they enter or exit (somatotopy)

Ascending/Afferent Sensory Tracts only in

posterior columns Ascending/ Afferent Sensory) AND

Descending/Efferent Motor) Tracts in

lateral and anterior columns

Dorsal/posterior

Ventral/anterior

Three Ascending Pathways to Somatosensory Cortex

Dorsal column-medial lemniscal pathway

Precise, “straight-through” transmission of inputs from single, localized body surface receptors. In dorsal columns.

Conveys information about muscle or tendon stretch. Do not contribute to conscious sensation. In lateral columns.

Transmission of pain, temperature, and coarse touch. In anterior-ventral and lateral columns.

Anterolateral (spinothalmic) pathways

Spinocerebellar tracts

Descending Pathways and Tracts

Direct pyramidal pathways stimulate skeletal muscles

Indirect (Extrapyramidal system), e.g. the rubrospinal tract


Spinal Cord (CNS)



• Spinal nerves











Spinal Nerves

There is a pair of spinal nerves at the level of each vertebrae (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) = a total of 31 pairs

Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord

Spinal nerves are named for the region from which they arise

Nerve C8 emergesBelow vertebra C7

Anatomy of Spinal Nerves Spinal nerves divide soon after

leaving the spinal cord

• Dorsal ramus – serves the skin and muscles of the posterior trunk

• Ventral ramus – forms a complex of networks (plexus) for the anterior part of the body spinal

nerve

posterior

anterior


Spinal Cord (CNS)



• Spinal nerves

• Medical conditions: spinal cord injury, shingles










Areas of the Skin Served By Single Spinal Nerves

Dermatomes

• Used to diagnose spinal cord injury

• Pain in a particular skin area reflects trouble in a specific spinal nerve and spinal location

• Can help to locate the site of damage in the spinal cord

• Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness

Shingles (Herpes zoster) is a viral infection of sensory neurons to the skin

Scaly, painful blisters confined to a narrow strip of skin on one side of the body trunk

Infects sensory skin neurons

Caused by latent infection (resurgence) of chicken pox virus when immune system is weakened as an adult

Mostly in people over 50.


Spinal Cord (CNS)



• Spinal nerves











Peripheral Nervous System (The PNS)

Nerves and ganglia outside the central nervous system

Nerve = bundle of neuron fibers

Neuron fibers are bundled by connective tissue; one fiber = 1 axon or cell

Endoneurium surrounds each fiber, just outside of Schwann cells

Groups of fibers are bound into fascicles by perineurium

Fascicles are bound together by epineurium

one neuron (nerve fiber)

Classification of Nerves Mixed nerves – both sensory and motor fibers

present in the nerve

Afferent (sensory) nerves – carry impulses toward the CNS

Efferent (motor) nerves – carry impulses away from the CNS

The Twelve Cranial Nerves (I-V) I Olfactory nerve – purely sensory for smell; ask

patient to identify oil of cloves and vanilla

II Optic nerve – purely sensory for vision; observe eye, test patient with eye chart

III Oculomotor nerve – mostly motor fibers to eye muscles, some proprioreceptive afferents; examine pupil size and reflex, ability to follow objects with the eye

IV Trochlear – mostly motor fibers to extrinsic eye muscles; test patient’s ability to follow objects with eye

V Trigeminal nerve – 3 divisions:

• Opthalmic (tested by corneal reflex) carrying sensory for skin of anterior scalp, eyelid, nose

• Maxillary (tested with pain, touch temperature using safety pin) carrying sensory from nasal cavity, palate, upper lip, cheek

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The Twelve Cranial Nerves (Vi-XIII) VI Abducens nerve – motor fibers to eye muscles

(lateral rectus); test by having patient follow object side-to-side

VII Facial nerve – sensory for taste; motor fibers to the face (test with ability to taste sweet salt, sour, bitter and close eyes, smile, whistle, make tears); five major branches: temporal, zygomatic, buccal, mandibular, cervical

VIII Vestibulocochlear nerve – sensory for balance (semicircular canals) and hearing; test with air and bone conduction with tuning fork

IX Glossopharyngeal nerve – sensory for taste and touch, pressure, pain from posterior tongue; motor fibers to the swallowing muscles in pharynx; test for gag and swallowing reflex, cough, taste, uvula position

X Vagus nerves – sensory (including aortic arch baroreceptors, respiration) and motor fibers for pharynx, larynx, and viscera (heart rate, breathing, digestive activity); test by gag, swallowing reflexes

XI Accessory nerve – mostly motor fibers to neck and upper back (trapezius, sternocleidomastoid); test for head rotation strength and shrugging against resistance

XII Hypoglossal nerve – mostly motor fibers to tongue allowing food manipulation; test by tongue protraction and retraction)

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Spinal Cord (CNS)



• Spinal nerves











Reflexes Classification by Acquisition

• Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus

o E.g. withdrawing your arm when fingers burned

• Learned (acquired) reflexes result from practice or repetition

o Example: driving skills, knitting

Classification by Function

• Somatic (drives skeletal muscle)

• Autonomic/visceral if activates visceral effectors in glands or smooth and cardiac muscle

Classification by Number of Synapses

• Monosynaptic - e.g. any stretch reflex like knee-jerk reflex

• Polysynaptic - e.g.

o Golgi tendon reflex causing muscle relaxation and lengthening

o Flexor/withdrawal reflexes causing flexion of muscles to pull limb away

o Crossed-extensor reflexes casuing withdrawal and opposing extensor activity like sudden pain in the foot and shifting of weight

Figure 13.14

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5

Basic Reflex Arc

Figure 13.17 (1 of 2), step1

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle and inhibition of its antagonist.

The events by which muscle stretch is damped

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

Stretch Reflexes are Monosynaptic Pathways Triggered By Muscle Stretching

Figure 13.17 (1 of 2), step1

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle and inhibition of its antagonist.

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.




Sensoryneuron


Spinal cord

1

Stretch Reflexes are Monosynaptic Pathways Triggered By Muscle Stretching

Figure 13.17 (1 of 2), step 2

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.


The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.




Sensoryneuron


Spinal cord

12

Figure 13.17 (1 of 2), step 3a





Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.



Sensoryneuron


Spinal cord

12

3a

Figure 13.17 (1 of 2), step 3b





Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.

Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).



Sensoryneuron


Spinal cord

12

3a 3b

Figure 13.17 (2 of 2)

The patellar (knee-jerk) reflex—a specific example of a stretch reflex

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

1

2

3a

3b

1

2

3a3b 3b

Figure 13.18

+ Excitatory synapse– Inhibitory synapse

Quadriceps strongly contracts. Golgi tendon organs are activated.

Afferent fibers synapse with interneurons in the spinal cord.

Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferent impulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord

Quadriceps(extensors)

Golgitendon

organHamstrings

(flexors)

1 2

3a 3b

Golgi Tendon Reflex: A Polysynaptic Reflex

Figure 13.19

Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

Interneurons

Efferentfibers

FlexorinhibitedExtensorstimulated

+ Excitatory synapse– Inhibitory synapse

Crossed Extensor Reflex: A Polysynaptic Reflex


Spinal Cord (CNS)



• Spinal nerves











Autonomic Nervous System The involuntary branch of

the nervous system

Consists of only motor nerves

Innervate smooth and cardiac muscle and glands

Make adjustments to ensure optimal support for body activities

Operate via subconscious control

Divided into two divisions

• Sympathetic division

• Parasympathetic division

Skin, muscle, and

joint sensors

Visceral organ

sensors

somatic sensory fibers

visceral sensory fibers

(visceral afferents)

and reflexes

inhibitory stimulatory

Efferent Pathways Somatic nervous system

• A, thick, heavily myelinated somatic motor fiber makes up each pathway from the CNS to the muscle

ANS pathway is a two-neuron chain

1. Preganglionic neuron (in CNS) has a thin, lightly myelinated preganglionic axon

2. Ganglionic neuron in autonomic ganglion has an unmyelinated postganglionic axon that extends to the effector organ

Differences Between Somatic and Autonomic Nervous Systems

Somatic Autonomic

Nerves one motor neuron, always stimulatory

preganglionic and postganglionic nerves

(two neurons connected end to end); stimulatory and inhibitory types

Effector organs skeletal muscle smooth muscle, cardiac muscle, and glands

Nerurotransmitters always uses acetylcholine uses ACh in pre-ganglionic neurons; either ACh or

norepinephrine in post-ganglionic fibers

Comparison of Somatic and Autonomic Nervous Systems

Closer to spinal cord than target

Closer to target than spinal cord

Anatomy of the Sympathetic Division Originates from T1 through L2

Short pre-ganglionic neuron and long postganglionic neuron transmit impulses from CNS to the effector

Ganglia are closer to the spinal cord than the target

Norepinephrine and epinephrine are neurotransmitters to the effector organs

Anatomy of the Parasympathetic Division Originates from the brain

stem (cranial nerves III,VII, IX, X) and S1 through S4

Long pre-ganglionic neuron and short postganglionic neuron transmit impulses from CNS to the effector

Terminal ganglia are at the effector organs (closer to target than spinal cord)

Always uses acetylcholine as a neurotransmitter

Autonomic Functioning Sympathetic – “fight-or-flight”

• Response to unusual stimulus

• Takes over to increase activities

• Remember as the “E” division = exercise, excitement, emergency, and embarrassment

Parasympathetic – housekeeping activites

• Conserves energy

• Maintains daily necessary body functions

• Remember as the “D” division - digestion, defecation, and diuresis

Sympathetic and Parasympathetic Compared

Sympathetic Division 14.2 Parasympathetic Division 14.6

Neurotransmitters and Receptor Types Cholinergic fibers release the neurotransmitter acetylcholine (Ach)

• All ANS preganglionic axons

• All parasympathetic postganglionic axons

• Post synaptic membranes have cholinergic receptors

Adrenergic fibers release the neurotransmitter norepinephrine (NE)

• Most sympathetic postganglionic axons

• Post synaptic membranes have adrenergic receptors

o Two general class types

Alpha (), e.g. 1: constricts blood vessels & sphincter, dilates pupils

Beta (), e.g. 1: increases heart rate, stimulates renin release (BP)

2: inhibitory; dilates blood vessels and bronchioles, relaxes smooth muscle walls of GI tract, bladder, uterus


Spinal Cord (CNS)



• Spinal nerves











Interactions of the Autonomic Divisions Most visceral organs have dual innervation

Dynamic antagonism allows for precise control of visceral activity

• Sympathetic division increases heart and respiratory rates, and inhibits digestion and elimination

• Parasympathetic division decreases heart and respiratory rates, and allows for digestion and the discarding of wastes

Four Steps of Regeneration of Peripheral Nerves

Axon becomes fragmented at injury site

Macrophages clean off the dead axon distal to the injury

Axon sprouts or filaments grow through a regeneration tube formed by Schwann cells

Axon regenerated and a new myelin sheath forms

Developmental Aspects of Fetal Nervous System

Measles (rubella)

Lack of oxygen

Radiation and drugs

Lack of folic acid

Premature birth problems

Aging of the Brain and Nervous System No more neurons are formed

after birth, but growth and maturation continues for several years; brain atrophies in old age

Loss of sympathetic nervous activity and slowing of vasoconstriction (e.g. headrushes, dizziness)

Circulatory problems

Dry eyes, constipation

Need for regular checkups when elderly

Enlargement of the subarachnoid space (left) and loss of white matter (right)

Shrinkage of hippocampus (left) and enlargement of ventricles (right)


Spinal Cord (CNS)



• Spinal nerves











Documents

The Spinal Cord (CNS) and Peripheral Nervous System Spinal Cord (CNS) Meninges and Injections/Punctures Tracts and Information Pathways Spinal nerves