BI231 Test 2

Nervous System fundamentals

Major integrator of body function, works with endocrine system to regulate homeostasis, specialized for communication, rapid via electrical signals, private via chemical messengers released at synaptic site

Neurons

The excitable nerve cells that transmit electrical signals, extreme longevity, amitotic, lose the ability to undergo mitosis, high metabolic rate, require a continuous supply of O₂ and glucose

Structural classification of neurons

Terminology based on cell body and extensions from cell body - dendrite and axon structure

• multipolar - most common

• bipolar - rare

• unipolar - sensory neurons in dorsal root ganglia

Neuron Structure

• cell body

  • contains the nucleus

  • biosynthetic center of a neuron

  • one of the most active areas of any cell in the body

  • contains all the usual organelles with the exception of the mitotic spindle

• dendrites

  • thin, highly branched cytoplasmic extensions

  • serve as a receptive area that receives graded potentials

  • the highly branched dendrites provide an enormous surface area for information collection

• axon

  • transmits electrical impulses from the cell body to another cell

  • each neuron has only one axon (nerve fiber) which usually branches profusely at its terminal end

  • axon hillock - cone shaped end of soma - where threshold is reached

• myelin sheath

  • protects and electrically insulates fibers (axons) from one another, and increases the speed of transmission

  • myelinated nerve fibers may conduct impulses up to 150x faster than unmyelinated axons

  • in the PNS myelin sheath is made from Schwann cells - individual wraps around axon - outer layer of Schwann cell is called neurolemma, insulates axon and increases conduction from 1m/sec without to 150m/sec with

    • nodes of ranvier - gaps every 1mm between Schwann cells

• axon terminals

  • also called a button (individually), terminal branches, or terminal arborization

  • often 10,000 or more per axon

  • contain neurotransmitters

  • part of synapses

Supporting cells

6 types of non-excitable cells that surround and support the nervous system called neuroglia or glial cells

PNS: schwann cells, satellite cells

CNS: astrocytes, microglia, ependymal, oligodendrocytes

Neuroglia of the CNS

1. astrocytes - perivascular foot processes help maintain the chemical environment in the brain, regulate passage of molecules, buffer excess K+ ions and neurotransmitters in extracellular space

2. microglia - phagocytic cells within the CNS, remove foreign microorganisms and dead neural tissue

3. ependymal - forms the epithelial lining of the brain cavities (ventricles) and central part of the spinal cord

4. oligodendrocytes - form the myelin sheaths around axons in the CNS

nerves

• cord-like organ

• contain axons and blood vessels

• contain all four types of neurons: somatic afferent, somatic efferent, visceral afferent, autonomic efferent

• nerves - connective tissue

  • endoneurium - loose connective tissue covering around each axon (encases myelin)

  • perineurium - denser connective tissue that surrounds bundles of axons call fascicles

  • epineurium - dense regular connective tissue surrounds nerve

ganglia

cluster of neuron cell bodies outside of the central nervous system, dorsal root ganglia, paravertebral ganglia, prevertebral ganglia, terminal ganglia

organization of the nervous system

central nervous system, peripheral nervous system

Central nervous system (CNS)

includes brain and spinal cord, interneurons (neurons completely within CNS)

CNS glial cells - motor neuron cell bodies and dendrites, fiber tracts - collection of axons going from spinal cord to brain, nuclei - collection of cell bodies in the CNS

Peripheral nervous system (PNS)

includes all nerves and ganglia (clusters of neuron cell bodies outside of the CNS)

PNS divisions - somatic, autonomic

nerve - collections of axons and support cells outside of the CNS wrapped in connective tissue

afferent - some axons “run into” CNS

efferent - some “run out” of CNS to effectors

effectors - muscles, glands, or other neurons

Somatic Nervous System

voluntary nervous system, nerve fibers from brain and spinal cord, afferent and efferent fibers, acetylcholine is the only neurotransmitter

Autonomic Nervous System

Involuntary, nerve fibers from the brain and spinal cord, may inhibit or excite smooth, cardiac muscle, glands or neurons, 2/3 subdivisions - sympathetic, parasympathetic, enteric

Sympathetic nervous system organization

• mobilized under emergency conditions, exercise, embarrassment or excitement

• thoracolumbar in reference to the origins of the nerves

• fight or flight response

• both afferent and efferent

• increase in heart rate, blood pressure, blood sugar, and sweating

• decrease in digestive process

• acetylcholine and norepinephrine are neurotransmitters

Parasympathetic nervous system organization

• conservation and restoration - rest and digest

• digestion, defecation, diuresis

• craniosacral origin of innervation

• both afferent and efferent

• decrease in heart rate and blood pressure

• increase in digestive processes

• acetylcholine is the neurotransmitter

enteric nervous system organization

• as many neurons as the spinal cord (millions)

• confined to the digestive system

• can work with or without the ANS and CNS

• many different neurotransmitters - serotonin is the primary one

Electrical disequilibrium

while the body as a whole is electrically neutral individual cells can create an electrical disequilibrium through the separation of ions

for every positive ion in the body there is a matching electron, typically part of a negative ion

cells can separate ions creating different concentrations on either side of the membrane, this produces both a chemical and electrical disequilibrium

the difference in electrical gradients between the ECF and ICF is referred to as the resting membrane potential

resting membrane potential

the potential difference across the cell membrane of the neuron is average/approximately -70mV, the resting membrane potential is a result of the ionic concentration differences, potential represents a source of stored or potential energy, relative charge scale, the resting membrane potential is measured by assigning the ECF a value of zero than measuring the difference between the two sides of the membrane

concentration differences are a result of

• electrical attraction by fixed anions

  • proteins within the cell tend to have negative charges

  • denoted Pr

• greater permeability of the resting cell membrane to K+ than to Na+

  • potassium is able to leak across the cell membrane

  • a typical cell is about 40x more permeable to K+ than Na+

• active transport by the Na+/K+ ATPase pump

  • Na+ is actively pumped out of the cell while K+ is actively brought back in

    • this maintains the resting membrane potential

  • effect of membrane electrical potential on ion diffusion

  • ions may exhibit a net movement across a membrane even when a concentration gradient does not exist

  • this is because ions are charged and can be either attracted or repelled across a membrane

  • even though a typical cell demonstrates permeability to K+, the K+ ions do not reach equilibrium, for as chemical gradients favor K+ leaking from the cell, electrical forces favor attracting K+ into the cell

  • the membrane is said to be polarized at the resting membrane potential

Depolarization

• a reduction in the membrane potential

• the inside of the membrane becomes less negative

• the potential approaches zero

Hyperpolarization

• an increase in membrane potential

• the inside of the membrane becomes more negative

• voltage increases

• graded potentials

Action Potentials

• a local change in the membrane potential that varies directly with the intensity of the stimulus

  • the more intense the stimulus the further the current flows

• they will decline over distance

  • as ions migrate along the membrane they can be pumped out

  • action potentials

• generation of an action potential

  • begins as depolarizing graded potentials summate to open voltage regulated Na+ gates at the axon hillock

• threshold

  • the point at which voltage regulated gates open

  • typically 15-20 mV above RMP (i.e. RMP -70mV, then threshold is -55 mV)

• propagation of an action potential

  • a positive feedback loop

  • as voltage regulated Na+ channels open, the interior of the cell becomes depolarized which opens more voltage regulated Na+ gates

  • this further depolarized the cell

  • to change membrane potential by 100mV only 1 out of every 100,000 Na+ ions needs to enter or 1 out of every 100,000 K+ ions is required to leave

• absolute or relative refractory period

Absolute refractory period

Ensured that each action potential is a separate all-or-none event, Na+ gates are open and cannot respond to a new stimulus

Relative refractory period

Na+ gates are closed or closing and a strong enough stimulus can open them depolarizing the membrane and producing a second action potential

intensity is coded in the frequency of the action potential not the amplitude

Conduction velocities of axons

depend upon

• axon diameter - the larger the axons diameter the faster the rate of conduction, resistance to electrical current decreases as the diameter increases

• myelin sheath - myelin electrically insulates fibers and increases conduction rates, it serves to prevent leakage of the charge from the axon, a thin unmyelinated nerve fiber has a conduction rate of about 1 m/sec, whereas impulses along thick myelinated fiber can travel about 100 m/sec

multiple sclerosis

autoimmune disease - the immune system attacks myelin in the CNS - leads to decrease of conduction

oligodendrocytes

leads to symptoms like clumsiness, visual problems, muscle atrophy and other neuromuscular issues

nerve fibers

• group A fibers

  • large myelinated fibers

  • conduct impulses at a rate of 15-130 m/sec

  • includes the somatic sensory and somatic motor fibers

• group B fibers

  • intermediate diameter, lightly myelinated

  • conduct impulses at a rate of 3-15 m/sec

  • includes the visceral sensory and motor and some somatic afferent pain and touch receptors

• group C fibers

  • small unmyelinated fibers

  • conduct impulses at a rate of 1 m/sec or less

  • includes the thermoreceptors and pain receptors of the skin

functional classification of neurons

• afferent - sensory

  • cell bodies lie outside the CNS

  • axons travel toward the CNS

• efferent - motor

  • axons travel away from the CNS

• interneurons

  • lie between sensory and motor pathways

Synapse

junction that mediates the transfer of information

• axodendritic - synapse between the axonal ending of one neuron and the dendrites of another neuron

• axosomatic - synapse between the axonal endings of one neuron and the cell body of another neuron

• presynaptic neuron - send information

• postsynaptic neuron - receive information

electrical synapses

provide a means of synchronizing the activity of all the interconnected neurons, neurons have electrically coupled junctions through which ions can flow directly from one neuron to the next, found primarily in the developing embryo, most are eventually replaced by chemical synapses as the nervous system develops, some electrical synapses do persist in the adult brain, may allow for two-way transmission of impulses

chemical synapses

allow for release and reception of neurotransmitters, it consists of an axonal terminal and a receptor region divided by a synaptic cleft

information transfer across chemical synapse

1. Ca²⁺ gates open in the pre-synaptic axonal terminal

2. initiate binding between V-SNARE and T-SNARE

3. V-Snare and T-Snare interaction results in the docking of the vesicle to the inside of the plasma membrane

4. the docked V-Snare and T-Snare complexes move away from each other

5. Neurotransmitter is released via exocytosis

6. binds the postsynaptic receptors

7. opens ion channels on post-synaptic membrane

chemical synapses are either excitatory or inhibitory

binding of neurotransmitter either drives them toward threshold, or away from threshold

opening of Na+ drives toward threshold

opening of K+ or Cl- channels hyperpolarizes membrane

termination of neurotransmitter

three options depending on neurotransmitter

1. degraded by enzymes on receptor protein

2. re-sequestered into pre-synaptic terminal

3. diffuses away from synapse and is destroyed systematically

Excitatory synapses

neurotransmitter binding drives the resting membrane potential toward threshold, chemically gated not voltage gated, opens gates that allows Na+ and K+ to diffuse simultaneously, influx of Na+ greater than the outflow of K+, Na+ follows electrochemical gradient K+ follows chemical gradient, this results in local depolarization called excitatory postsynaptic potentials (EPSPs), these graded potentials may vary in amplitude according to the amount of neurotransmitter bound, if the EPSPs reaching the hillock are strong enough to reach threshold then an AP will result

inhibitory synapse

reduces resting membrane potential resulting in hyperpolarization, inhibitory neurotransmitters open K+ and/or Cl- ion channels, Na+ channels are not affected, this results in a local hyperpolarization of the postsynaptic membrane, termed an inhibitory postsynaptic potential (IPSP)

integration and modification of synaptic events

a single EPSP cannot induce an AP, but they can summate

temporal summation - neurons transmit impulses in rapid order

spatial summation - a large number of terminal stimulate the post synaptic membrane

synaptic potentiation

repeated use of a synapse enhances the ability of postsynaptic excitation producing larger postsynaptic potentials

R/T increase concentration of Ca²⁺ in the presynaptic neuron

presynaptic inhibition

excitatory neurotransmitter release is inhibited by the activity of another neuron via the axonic synapse, leads to a decrease amount of neurotransmitter releases

cell to cell communication

• gap junctions

• autocrines and paracrines

• hormones

• neurotransmitters

• neurohormones

gap junctions

allow ion flow directly from one cell to another

autocrines and paracrines

active in the organ in which they are produced

local hormones

autocrines - a chemical that acts directly upon the cell that secreted it - ex: IL2

paracrines - a chemical produced by a specific tissue in an organ and acts upon a different tissue - ex: gastrin, histamine

hormones

used by the endocrine system, these chemical mediators are secreted into the interstitial fluid and then diffuse into the blood to act on cells that display protein receptor specificity for that hormone, protein receptors may either be on the cell membrane or inside the cell, we will cover second messenger systems and gene activation in detail in the endocrine system

second messenger systems

allow for a cascade effect, happen rapidly

gene activation

takes more time to effect a response since proteins are being manufactured

neurotransmitters

released from axonal endings and diffuse across a synapse to influence a target cell, may exhibit direct effects or indirect effects on membrane channels, also determines the length of the effect

direct effects of neurotransmitters

directly open a channel or results in rapid, short-acting fast synaptic potentials

ex: ACh

indirect effects of neurotransmitters

depends upon a second messenger to either open a channel or change metabolic activity inside the cell, results in slow, long-term effect on synaptic potentials

mediated by

1. G-proteins

   1. ex: NE

2. intracellular enzymes

   1. ex: nitric oxide

neurohormone

released from neuronal tissue and diffuse into the blood

Endosteum

Areolar connective tissue

Yellow marrow

In and around arthroses, adipose connective tissue

red marrow

reticular connective tissue

ligaments

dense regular connective tissue

periosteum, intramembranous ossification, joint capsules

dense irregular connective tissue

intervertebral discs

elastic connective tissue

articular surfaces, epiphyseal plates, endochondral ossification, costal cartilage, framework of nose

hyaline cartilage

elastic cartilage

not found in skeleton

intervertebral discs, pubic symphysis, large arthroses (meniscus or labrum)

fibrocartilage

composition of bone

organic - cells, collagen fibers, ground substance - provides tensile strength and plasticity

inorganic - 65% of mass - provides hardness and rigidity - hydroxyapatite (mineral salts) calcium phosphates

bone cells

osteogenic cells, osteoblasts, osteocytes, osteoclasts

osteogenic cells

mesenchyme - found in periosteum and endosteum

osteoblasts

bone forming cells that secrete bone matrix (osteoid) - 90% collagen and calcium binding proteins - unmineralized

osteocytes

mature cells that are locked in lacunae - stress and strain sensors

osteoclasts

derived from hematopoietic stem cells - similar to macrophages, giant multinucleated cells, reabsorb bone matrix for remodeling

functions of bone

protection of internal organs, support structure, provide leverage for muscles, mineral and growth factor storage, hematopoiesis, triglyceride (fat) storage, hormone production (osteocalcin)

classification of bones

types of osseous tissue, types of bones

types of osseous tissue

1. compact - cortical - dense and homogeneous

   1. it makes up the outer surface of every bone

2. spongy - cancellous - projections called trabeculae

   1. it makes up the inside of bones

   2. called diploe inside of the cranial bones

types of bones

1. long bones - shaft with two ends

   1. longer than it is wide

   2. primarily compact bone

   3. limbs (except patella, wrist and ankle)

2. short bones - cubelike bones

   1. mostly spongy surrounded by a thin layer of compact bone (patella, carpals, and tarsals)

3. flat bones

   1. thin, flattened, usually curved

   2. parallel layers of compact bone with spongy in between (sandwich)

   3. i.e. sternum, ribs, neurocranium

4. irregular

   1. mainly spongy surrounded by compact bones

   2. does not meet criteria to be other types

   3. i.e. vertebrae, splanchnocranium, parts of the pelvis, scapula

gross anatomy of long bones

1. diaphysis (shaft) - thick collar of compact bone surrounding the medullary cavity which is filled with yellow marrow

2. epiphysis/es - ends of the bones - compact bone encapsulates spongy bones, red marrow = hematopoiesis

3. epiphyseal line - remnant of epiphyseal plate (hyaline cartilage) - junction of diaphysis and epiphysis, can only be seen on the inside of adult bones

4. periosteum - covers the outer surface of all bones except on the articular cartilage - doubled layered membrane, inner layer is the osteogenic layer (osteoblasts), outer layer is the dense irregular connective tissue - point of attachment for tendons and ligaments - entry for blood vessels and nerves, sharpey’s fibers (perforating fibers) connect periosteum to the compact outer layer of bone

5. endosteum - lines the internal structure of bone - spongy bone, medullary cavity, and canals for vessels, contains osteoblasts and osteoclasts

6. articular cartilage - hyaline cartilage - found where bones articulate - replaces the periosteum

7. red marrow - hematopoietic tissue - found in trabecular cavities in long bones and diploe of flat bones of adults, also in medullary cavity in newborns, anemia - can cause yellow marrow to revert to red marrow

Microscopic anatomy of bones

1. compact bone

   1. Haversian system or osteon - Haversian canal (central canal)

   2. Volkmann’s canal (perforating canal)

   3. lamellae - lacunae, osteocytes, canaliculi

2. spongy bone

   1. consists of trabeculae

   2. function to resist stress on the bone

   3. no osteons present

   4. osteocytes are present

bone identification

bone - usually one word, side - R/L, feature, paired/unpaired

markings on bones (features)

articulations - places where one bone touches another

perforations - openings in bone for nerves and/or blood vessels to pass through

depressions - flat or concave areas on bone

projections - areas that bulge outward from the surface and represent areas of attachment (ligaments/tendons) and stress

articulations

suture, facets, symphysis, condyle

bony perforations

fontanelle, foramen, canal, meatus, sinus, alveolus, fissure

bony depressions

fossa, fovea, groove, sulcus

bony projections

process, eminence, spine, tuberosity, tubercle, trochanter, malleolus, boss, condyle, epicondyle, head, neck, torus, ridge, crest, line

axial skeleton

skull, vertebrae, ribs, sternum

bones of the skull

22 bones, 8 cranial - neurocranium, 14 facial - splanchnocranium

named structures of the cranium

sagittal - connects the 2 parietals

coronal - connects the frontal to the parietals

squamosal - connects the temporal to the parietals

lambdoidal - connects the occipital to the parietals

metopic - connects the two frontal bones in a child

basilar - connects the occipital to the sphenoid

all other structures are named for the two bones they connect

cranium

aka brain case or neurocranium, encloses and protects the brain - each bone touches the brain

frontal bone

frontal sinus, glabella, supraorbital foramen/notch, lacrimal fossa

temporal bone

mastoid process, external auditory meatus, mandibular fossa (glenoid fossa), zygomatic process, styloid process, petrous portion, internal acoustic meatus, jugular foramen, carotid canal

occipital bone

• foramen magnum

• occipital condyles

• hypoglossal canal

• superior nuchal lines

• external occipital protuberance

sphenoid

• greater and lesser wings

• superior orbital fissue

• sella turcica

• optic foramen/canal

• foramen rotundum

• foramen ovale

• foramen spinosum

• pterygoid process

• foramen lacerum - actually at the junction of the sphenoid and temporal (filled with cartilage in real life)

ethmoid

• crista galli

• cribriform plate

• superior nasal concha (turbinate)

• middle nasal concha (turbinate)

• perpendicular plate

Cranium bones

frontal, temporal (2), parietal (2), occipital, sphenoid, ethmoid

facial/splanchnocranium bones

mandible, maxillae (2), palantine (2), zygomatic (2), lacrimal (2), nasal (2), vomer, inferior nasal conchae (2)

mandible bone

mandibular condyle, alveoli, coronoid process, ramus, body, mandibular foramen, mental foramen

maxillae bones

alveoli, maxillary sinuses, palatine process, infraorbital foramen, incisive foramen

zygomatic bone

temporal process

lacrimal bones

lacrimal duct

associated bones of the skull

• hyoid

• ossicles

  • malleus

  • incus

  • stapes

bony orbit bones

frontal, sphenoid, zygomatic, maxillary, palantine, ethmoid, and lacrimal

bones that contain sinuses

maxillary, frontal, ethmoid, sphenoid

vertebral column divisions

cervical (7)

thoracic (12)

lumbar (5)

sacral (5)

coccygeal (4)

spinal curvature

thoracic and sacral curvatures are convex posteriorly (bulging out) - form during fetal development

cervical and lumbar curvatures are concave posteriorly (cupping in) - cervical curvature forms when infant begins to hold their head erect, the lumbar curvature forms later when child begins to sit up, stand and walk

spine supported by ligaments and numerous muscles, like straps

major supporting ligaments are

• anterior longitudinal ligament - runs as a continuous band down the vertebral column; firmly attached to both the bony vertebrae and the discs; prevents hyperextension

• posterior longitudinal ligament - narrow and not nearly as strong; attaches only to the discs; prevents hyperflexion

Intervertebral discs

located between the vertebral bodies, cushion - like pads that act as shock absorbers, allow the spine to flex and extend laterally, thickest in the lumbar and cervical regions, account for about 25% of vertebral column height

• nucleus pulposus - inner gelatinous substance gives the disc its compressibility and elasticity

• annulus fibrosus - surrounds the nucleus; composed of a strong collar of collagen fibers superficially and fibrocartilage internally

• herniated disc - involves the rupture of the annulus fibrosus and protrusion of the nucleus pulposus

abnormal spinal curvatures

• scoliosis

  • lateral curvature of the spine

  • leads to unopposed force

• lordosis

  • accentuated lumbar curvature

• kyphosis

  • exaggerated thoracic curvature

typical features of vertebrae

can be identified on all classes of true vertebrae

• body

• pedicle

• vertebral foramen

• lamina

• spinous process

• transverse processes

• articular processes (superior and inferior)

• intervertebral discs

cervical vertebra (C1-C7)

special features - transverse foramen

atlas - dens facet

axis - dens process (odontoid process)