Ch 12 Tissues and Cells of the Nervous system

Nervous system

-Employs electrical and chemical means to send messages from cell to cell

-Foundation of our conscious experience, personality, and behavior

-Vary complex

Neurobiology

-Combines the behavioral and life sciences

Endocrine system

-Communicates by means of chemical messengers (hormones) secreted into the blood

Receive information

-Sense organs receive information about changes in the body and external environment, and transmit coded messages to the brain and spinal cord (CNS)

Processes information

-CNS processes this information, relates it to past experiences, and determines appropriate response

Sending out commands

-CNS issues commands to muscles and gland cells to carry out such a response

Central Nervous System

-CNS

-Brain and spinal cord enclosed by cranium and vertebral column

Peripheral Nervous System

-PNS

-All nervous system except the brain and spinal cord; composed of nerves and ganglia

-Contains sensory and motor divisions each with somatic and visceral subdivisions

Nerve

-A bundle of nerve fibers (axons) wrapped in fibrous connective tissue that

Ganglion

-Knot-like swelling in a nerve where neuron cell bodies are concentrated

Sensory division

-Afferent

-Carriers signals from receptors to CNS

Somatic sensory division

-Carrier signals from receptors in the skin, muscles, bones, and joints

Visceral sensory division

-Carriers signals from the viscera (heart, lungs, stomach, and urinary bladder)

Motor division

-Efferent

-Carriers signals from CNS to effectors (Glands and muscles that carry out the body's response)

Somatic motor division

-Carrier signals to skeletal muscles

-Output produces voluntary muscular contraction as well as somatic reflexes (involuntary muscle contractions

Visceral motor division

-Autonomic nervous system

-Carries signals to glands, cardiac and smooth muscle

-Its involuntary responses are visceral reflexes

Sympathetic dividion

-Tends to arouse body for action

-Accelerating heartbeat and respiration, while inhibition digestive and urinary systems

-Fight or flight

Parasympathetic division

-Tends to have calming effect

-Slows heart rate and breathing

-Stimulates digestive and urinary system

-Rest and digest

Excitability

-Irritability

-Respond to environmental changes in stimuli

Conductivity

-Respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations

Secretion

-When an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell

Sensory neruons

-Afferent

-Detect stimuli and transmit information about them toward the CNS

Interneuron location

-Lie entirely within CNS connecting motor and sensory pathways

-About 90% of all neurons

Interneuron function

-Receive signals from many neurons and carry out integrative functions

-Make decisions on responses

Motor neuron

-Efferent

-Sends signals out to muscles and gland cells (the effectors)

Neurosoma

-Control center of neuron

-Soma or Cell body of neuron

-Has single, centrally located nucleus with large nucleolus

Cytoplasm of neuron

-Contains mitochondria, lysosomes, Golgi complex, inclusions, extensive rough ER and cytoskeleton

-Dense mesh of microtubules and neurofibrils that compartmentalize rough ER into dark-staining chromatophilic substance (Nissl bodies)

Cytoplasm Inclusions of neuron

-Glycogen, lipid droplets, melanin, and lipofuscin pigment

-No centrioles, no mitosis

Dendrites

-Branches that come off the neurosoma

-Primary sight for receiving signals from other neurons

-More dendrites means it can receive more information

-Precise pathways for the reception and processing of information

Axon

-Nerve fiber

-Originates from a mound on the neurosoma called axon hillock

-Cylindrical, relatively unbranched for most of its length

-One axon per neuron

Axon collaterals

-Branches of axon

-Branch extensively on distal end

-Specialized for rapid conduction of signals to distant points

Axoplasm

-Cytoplasm of axon

Axolemma

-Plasma membrane of axon

Terminal arborization

-Extensive complex of branches

-Distal end of axon

Axon terminal

-Little swelling that forms a junction (synapse) with the next cell

-Contains synaptic vesicles full of neurotransmitters

-many vesicles are docked on plasma membrane ready to release neurotransmitter

-A reserve pool of synaptic vesicles is located further away from membrane

Unipolar neruon

-Single process leading away from neurosoma

-Sensory cells from skin and organs to spinal cord

Anaxonic neuron

-Many dendrites but no axon

-Retina, brain, and adrenal gland

Multipolar neuron

-One axon and multiple dendrites

-Most common neurons in CNS

Bipolar neurons

-One axon and one dendrite

-Olfactory cells, retina, inner ear

Axonal transport

-Two-way passage of proteins, organelles, and other material along with axon

-Microtubules guide materials along axon

-Motor proteins carry materials "on their backs" while they "crawl" along microtubules

Anterograde transport

-Toward the terminal

-Movement down the axon away from neurosoma

Retrograde transport

-Away from the terminal

-Movement up the axon toward the neurosoma

Axonal transport function

-Many proteins made in neurosoma must be transported to axon and axon terminal

-To repair axolemma, serve as gated ion channels, enzymes or neurotransmitters

Kinesin

-Motor proteins in anterograde transport

Dynein

-Motor proteins in retrograde transport

Fast axonal transport

-rate of 20 to 400 mm/day

-Anterograde

-Retrograde

Fast anterograde transport

-Organelles enzymes, synaptic vesicles, and small molecules

Fast retrograde transport

-For recycled materials and pathogens - rabies, herpes simplex, tetanus, polio viruses

-Delay between infection and symptoms is time needed for transport up the axon

Slow axonal transport

-0.5 to 10 mm/day

-Always retrograde

-Damaged nerve fibers regenerate at a speed governed by slow axonal transport

Slow axonal transport function

-Always anterograde

-Moves enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons

-Speed of nerve fiber regeneration

Neuroglia

-Out # neurons 10 to 1

-Neuron protection

-Binds neurons to create nervous framework

-Covers "dead ends"

-Prevents neurons from touching each other

-Gives precise conduction pathways

Oligodendrocytes

-Glia in CNS

-Form myelin sheaths in CNS that speed signal conduction

-Arm-like processes wrap around nerve fibers

Ependymal cells

-Glia of CNS

-Line internal cavities of the brain

-Secrete and circulate cerebrospinal fluid (CSF)

-Cuboidal epithelium with cilia on apical surface

Microglia

-Wander through CNS looking for debris and damage

-Develop from WBC and become concentrated in areas of damage

Astrocytes

-Most abundant Glial cell in CNS

-Covering brain surface and most nonsynaptic regions of neurons in the gray matter

Astrocyte function

-Form supportive framework

-Forms extensions that surround capillaries to form the Blood Brain Barrier

-Monitor neuron activity and regulate blood flow based on metabolic need

-Convert glucose to lactate and supply this to neurons

-Regulate chemical composition of tissue fluid by absorbing excess NT and ions

Perivascular feet

-Extensions of astrocytes

Schwann cells

-Envelop nerve fibers in PNS (>100 layers)

-Wind repeatedly around a nerve fiber

-Produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS

-Assist in regeneration of damaged fibers

Satellite cells

-Surround the neruosomas in ganglia of the PNS

-Provide electrical insulation around the neurosoma

-Regulate the chemical environment of the neurons

Myelin sheath

-Insulation around a nerve fiber

-Oligodendrocytes in CNS

-Schwann cells in PSN

-Consists of the plasma membrane of glial cells

-20% protein 80% lipid

Myelination

-Production of the myeline sheath

-Begins at 14 weeks of fetal development

-Proceeds rapidly during infancy

-Completed in late adolescence

-Dietary fat is important to CNS development

Neurolemma

-Thick, outermost coil of myelin sheath

-Contains nucleus and most of its cytoplasm

-Surrounded by basal lamina

Endoneurium

-Thin layer of fibrous connective tissue that surrounds neurolemma and basal lamina

Oligodendrocyte function

-Anchors to multiple nerve fibers

-Cannot migrate around

-Must push new layers of myelin underneath old ones

-Nerve fibers in CNS have no neurolemma or endoneurium

Nodes of ranvier

-Gaps between segments of myelin

-Contain many voltage-gated ion channels, while myelin-covered internodes contain few

Internodes

-Myelin-covered segments from one gap to the next

Initial segement

-Short section of nerve fiber between the axon hillock and the first glial cell

Trigger zone

-The axon hillock and the initial segment

-Play an important role in initiating a nerve signal

-350 to 500 gates per um^2

-If excitatory local potential reaches trigger zone and is still strong enough, it can open these gates and generate an AP

Unmyelinated nerve fibers

-Many CNS and PNS fibers are unmyelinated

-In PNS Schwann cells hold 1-12 small nerve fivers in surface grooves

-Membrane folds once around each fiber

Diameter of fiber

-Larger fibers have more surface are and conduct signals more rapidly

Presence of myelin

-Myelin speeds up signal conduction by minimizing leakage of Na+ out of the cell and further separation the inner positive ions from attraction of negative ions outside of cell

-Signal strength does start to fade in the internode

Small unmyelinated fibers

-0.5 to 2.0 m/s

Small myelinated fibers

-3 to 15.0 m/s

Large myelinated fibers

-up to 120m/s

Slow signals

-Gastrointestinal tract speed is less of an issue

Fast signals

-Skeletal muscle speed improves balance and coordinated body movements

Regeneration criteria

-If neurosoma is intact

-At least some neurilemma remains

Step one of regeneration

-Fiber distal to the injury cannot survive and degenerates

-Macrophages clean up tissue debris at point of injury and beyond

Step two of regeneration

-Neurosoma swells, ER breaks up, and nucleus moves off center

-Due to loss of nerve growth factors from neuron's target cell

Step three of regeneration

-Axon stum sprouts multiple growth processes as severed distal end continues to degenerate

Step four of regeneration

-Schwann cells, basal lamina and neurilemma form regeneration tube

-Enables neuron to regrow to original destination and reestablish synaptic contact

Step five of regeneration

-Once contact is reestablished with original target, neurosoma shrinks and returns to its original appearance

-Nucleus returns to normal shape

-Atrophied muscle fibers regrow

Regeneration speed

-Not fast, perfect, or always possible

-Process could take 2 years

-Some nerve fibers connect with the wrong muscle fibers; some die

-Regeneration can not occur in the CNS

Nerve growth factor

-NGF

-protein secreted by gland, muscle, or glial cells and picked up by the axon terminal of neurons

-Prevents apoptosis in growing neurons

-Enables growing neurons to make contact with their targets

Electrophysiology

-Study of cellular mechanisms for producing electrical potentials and currents

-Basis for neural communication and muscles contraction

Electrical potential

-A difference in concentration of charged particles between one point and another

-Living cells are polarized and have RMP (neuron - (-)70mV

-Cells have more negative particles inside of membrane that outside

Electrical current

-A flow of charged particles from one point to another

-Movement of Na+ or K+ through channels

-gated channels opened or closed by various stimuli

-Enables cells to turn electrical currents on and off

Resting membrane potential

-RMP

-Exists because of unequal electrolyte distribution between ECF and ICF

RMP results from ...

-Ions diffuse down their concentration gradient though the membrane

-Plasma membrane is selectively permeable and allows some ion to pass easier that others

-Electrical attraction of cations and anions to each other

Potassium

-K+ has the greatest influence on RMP

-Plasma membrane is more permeable to K+ than any other ion

-Leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached

-K+ is about 40x as concentrated in ICF as in ECF

Cytoplasmic anions

-Cannot escape due to size or charge

-Phosphates, sulfates, small organic acids, proteins, ATP, and RNA

Sodium

-Membrane is not vary permeable to Na+

-RMP is slightly influenced by Na+

-12x as concentrated in ECF as in ICF

-Some Na+ leaks into cell, diffusing down its concentration and electrical gradients

-Na+ leakage makes RMP slightly less negative than it would be if RMP were determined solely by K+

Sodium-potassium Pump

-Moves 3 Na+ out for every 2 K+ it brings in

-Works constantly to compensate for Na+ and K+ leakage and requires a great deal of ATP (one per exchange)

-70% of the energy requirement of the nervous system

Sodium-potassium function

-Necessitates glucose and oxygen be supplied to the nervous tissue (energy needed to create RMP)

-The exchange contributes about -3 mV to the cell's RMP of -70mV

Local potentials

-Changes in membrane potential of a neuron occurring at and nearby the part of the cell that is stimulated

-Different neurons can be stimulated by chemicals, light, heat, or mechanical disturbance

Stimulant binds to neuron receptor

-Opens Na+ gates and allows Na+ to enter cell

-Positive Ion entering causes Depolarization

-Na+ entry results in a current that travels toward the cell's trigger zone

Graded local potentials

-Vary in magnitude with stimulus strength

-Stronger stimuli open more Na+ gates

Decremental

-Get weaker the farther they spread from the point of stimulation

-Voltage shift caused by Na+ inflow diminishes with distance

Reversible

-If stimulation ceases, the cell quickly returns to its normal resting potential

Excitatory or inhibitory

-Some neurotransmitters make the membrane potential more negative - hyperpolarize it - so it becomes less likely to produce an AP