ILS 5: Nervous System & Histology

Telencephalon

cerebral hemispheres

“end brain”

Encephalon

“brain”

Diencephalon

thalamus, subthalamus, hypothalamus, & epithalamus

“in between the brain”

Brain stem composed of

mesencephalon (midbrain), pons, & medulla oblangata

Insular lobe function

pain perception, taste, emotion

limbic lobe functtion

learning & memory

Meninges

1. Dura Mater

   1. tough, outer fibrous layer that provides protection

2. Arachnoid Mater

   1. middle, web-like layer

   2. beneath it, subarachnoid space contains CSF

3. Pia Mater

   1. thin, delicate layer that closely adheres to brain surface

Gray Matter

• contain neuron cell bodies, dendrites, & axon terminals

• appears pinkish-grey in fresh brain tissue

• forms cortex in brain

• site of processing & info integration

White Matter

• composed mainly of myelinated axons

• connects regions of gray matter

• specialized for rapid signal transmission

• lies beneath cortex in brain & surrounds gray matter in spinal cord

conus medullaris

• cone-shaped terminal end of spinal cord

• located at L1-L2 vertebral lvl

• from which cauda equina & filum terminale arise

Cauda equina

bundle of spinal nerve roots below conus medullaris

filum terminale

thin fibrous strand of pia mater that extends from conus medullaris to coccyx, anchoring spinal cord in place

Lumbar puncture

performed at L3-L4 or L4-L5 to collect CSF

Spinal cord Levels

Cervical [C1-C8]

Thoracic [T1-T12]

Lumbar [L1-L5]

Sacral [S1-S5]

Coccygeal [CO1]

Cervical enlargement

C4-T1

• supplies upper limbs

Lumbosacral enlargement

L1-S2

• supplies lower limbs

central canal

small ependymal-lined CSF-filled cavity

Posterior [dorsal] horn

sensory processing

anterior [ventral horn]

somatic motor neurons

intermediate zone

autonomic neurons & interneurons

somatic motor neurons are for

skeletal muscle

visceral motor neurons are for

smooth muscle, cardiac muscle, & glands

brainstem

• connects cerebrum & cerebellum to spinal cord

• contains ascending & descending tracts

• houses nuclei & root entry/exit zones of 9 of 12 cranial nerves [NOT I, II, & XI - spinal origin]

• regulates vital autonomic functions

  • breathing, HR, BP, consciousness

somatic sensory system

detects sensations from skin, skeletal muscle, joints, & connective tissue

visceral sensory system

detects sensations from internal organs [viscera]

visceral motor system is also known as

autonomic nervous system, ANS

frontal lobe function

motor planning & execution

parietal lobe function

processing somatosensory info

occipital lobe function

processing visual info

temporal lobe function

processing auditory info

PNS contains how many pairs of nerves

43

• 12 pairs of cranial nerves

• 31 pairs of spinal nerves

  • Cervical [8]

  • Thoracic [12]

  • Lumbar [5]

  • Sacral [5]

  • Coccygeal [1]

Metencephalon

pons

basilar pons

ventral [anterior] portion of pons

pontine tegmentum

dorsal [posterior] portion of pons

cervical nerves innervate

neck, shoulders, diaphragm [phrenic C3-C5], & upper limb [esp. brachial plexus]

thoracic nerves innervate

chest & upper abdomen wall, intercostal muscles, skin

lumbar nerves innervate

lower abdomen, pelvis, hip region, anterior & medial thigh, parts of lower limb

sacral nerves innervate

pelvis, genitals, lower digestive & urinary tracts, buttocks, posterior lower limb, & foot

brachial plexus

C5-T1

supplies upper limb [shoulder, arm, forearm, hand]

lumbar plexus

supplies lower abdominal wall, pelvic region, & anterior/medial thigh

spinal nerves

each one is a MIXED nerve [both sensory & motor fibers]

formed by union of 2 roots attached to spinal cord

posterior [dorsal] root

• carries sensory sginals

• contains dorsal root ganglion [DRG]

dorsal root ganglion [DRG]

contains cell bodies [somas] of sensory neurons [typically pseudounipolar neurons]

anterior [ventral] root

• formed by nerve rootlets emerging from anterior spinal cord that converge

• carries motor signals

posterior ramus

innervates muscles, joints of spine, & skin of back

anterior ramus

innervates anterior & lateral trunk & limbs

communicating rami

[T1-L2 only]

• connect spinal nerves to sympathetic chain [trunk] ganglia

• carry sympathetic autonomic fibers

meningeal branch

re-enters vertebral canal to innervate the vertebrae, spinal ligaments, BVs, & meninges

nerves

bundles of axons in PNS

ganglion

cluster of neuron cell bodies in PNS

nucleus

cluster of neuron cell bodies in CNS

epineurium

thick layer of dense, irregular connective tissue that encloses entire nerve

perineurium

layer of dense, irregular connective tissue that wraps the fascicle

endoneurium

delicate layer of areolar connective tissue that wraps an individual axon

multipolar neurons

• many dendrites

• 1 axon

• most common type

• spinal motor neurons

bipolar neurons

• 1 axon

• 1 dendrite

• sensory neurons of retina, olfactory epithelium, inner ear

unipolar [pseudounipolar] neurons

• 1 process extends from cell body & splits into 2

  • central process → synaptic knobs of CNS

  • peripheral process → dendrites

• all other sensory neurons

• DRG of spinal cord

anaxonic neurons

• many dendrites

• no true axon

• don’t produce APs [send signals via electronic conduction]

• regulate electrical changes of adjacent CNS neurons

interneurons

“association neurons”

• multipolar

• located within CNS

  • coordinate sensory & motor neurons

• make up 99% of our neurons

cell body [perikaryon or soma]

• acts as trophic center, producing cytoplasm for neuronal processes

• large, euchromatic nucleus w/ prominent nucleolus [high synthetic activity]

nissl bodies

chromatophilic substance

• basophilic regions rich in RER & polysomes

  • esp abundant in motor neurons

dendritic spines

small dynamic projections supported by actin filaments & change morphology continuously

dendritic spine plasticity crucial for

• embryonic brain development

• neural adaptation

• learning & memory

axolemma

plasma membrane of axon

wallerian degeneration

when axon is severed from cell body, this occurs to the disconnected portion & is removed by phagocytic cells

anterograde transoprt

cell body → axon terminal

FAST via kinesin motor proteins

retrograde transport

axon terminal → cell body

FAST, uses dynein motor proteins

*can transport pathogens to CNS [tetanus toxin, HSV, rabies, polio]

resting membrane potential

-70mV

Na+-K+ ATP pump maintains

low Na+ inside

high K+ inside

[resting potential]

threshold

~ -55 mV

depolarization caused by

rapid influx of Na+ via voltage gated channels

Repolarization caused by

opening of voltage-gated K+ channels

Mechanism of local anesthetics

Ex. Lidocaine or benzocaine

• bind to & block voltage-gated Na+ channels

  • prevents initiation & propagation of APs

    • loss of sensation

Glial cells [neuroglia]

• nonexcitable support cells in CNS & PNS

• same # of glial cells as neurons

• mitosis, protect & nourish neruons, provide physical scaffolding for nervous tissue

Astrocytes

• glial cells of CNS

• perivascular feet = protective seal of BBB

• >90% of tissue in some areas of brain

• regulate tissue fluid composition

• secrete nerve growth factors

*most brain tumors are astrocytomas: distinguished pathologically by their expression of glial fibrillary acidic protein [GFAP]

Ependymal cells

• glial cell in CNS

• ciliated cuboidal epithelium

• line ventricles of brain & central canal of spinal cord

• lack BM

• produces CSF

Microglia

• small macrophages in CNS [glial cell]

• aid in synaptic remodeling, changing connections between neurons

*from monocytes from bone-marrow

multiple sclerosis [MS]

• myelin sheaths damaged by autoimmune mech

  • T lymphocytes & microglia, which phagocytose & degrade myelin debris, play major roles in progression of MS

    • destruction actions of these cells > oligodendrocytes capacity to produce & repair myelin

• symptoms

  • numbness, tingling, blurred/double vision, balance/walking difficulties, memory problems, extreme fatigue

oligodendrocytes

• only in CNS

• main glial cells in white matter

• forms myelin

  • covers entire axon using multiple oligodendrocytes

*1 oligodendrocyte can myelinate multiple axons from diff neurons

satellite cells

• glial cells of PNS

• surround neuronal cell bodies in dorsal root ganglia

• provide electrical insulation & structural support

• regulate exchange of nutrients, ions, & wastes

Schwann cells

• glial cells of PNS

• produce myelin sheath

  • wrap around a single axon

  • enable saltatory conduction btwn nodes of Ranvier

Guillain-Barré syndrome [GBS]

loss of myelin disorder → loss of reflexes