A&P - Nervous Tissue & Neural Phsyiology

Communication + coordination: Sensory (send info about environment to brain/spinal cord), integration (process + store info; make decisions), motor (send messages to muscles/glands to take action)

Functions of Nervous System:

Brain and spinal cord

Central Nervous System

Nerves and ganglia (nerves to/from CNS), 12 pairs cranial nerves, 31 pairs spinal nerves

Peripheral Nervous System

Sensory (toward): somatic, visceral, special

Afferent Division

Motor (exit): somatic motor (voluntary), autonomic motor (involuntary)

Efferent Division

Excitability, conductivity, secretion (neurotransmitter), longevity (lifetime), no mitosis, high metabolic rate (need lots O2, only use glucose)

Characteristics of Neurons (nerve cells)

Little branches - receive info

Dendrites

One nucleus, no centrioles (b/c no cell division)

Cell Body

Only one per neuron, can branch, up to 3 ft long (axon=nerve fiber)

Axon

Site of action potential generation

Axon Hillock

Group of cell bodies (CNS)

Nucleus

Group of cell bodies (PNS)

Ganglia

Bundle of axons (CNS)

Tract

Bundle of axons (PNS)

Nerve

Conduct signals from receptors to the CNS - detect stimulus (touch, pain, temp)

PNS: Sensory (efferent) Neurons:

Conduct signals from CNS effectors (glands/muscles)

PNS: Motor (efferent) Neurons:

Integration info and determine how body will respond (>90% neurons=Interneurons)

CNS: Interneurons:

Nerve glue - Glial cells (glia (20):neurons (1))

Neuroglia

Support neurons (physically/chemically), mitotic - can divide (Ex. brain tumor=glioma)

Functions/Features of Neuroglia

Astrocytes, oligodendrocytes, microglia, ependymal cells

Neuroglia of CNS

Star cell, most abundant, from blood brain barrier (BBB), regulate nutrient/waste transfer

Astrocyte

Line ventricles in brain and spinal cord, produce cerebrospinal fluid (CSF)

Ependymal Cell

Clean up debris/bacteria, machrophages of CNS

Microglia

PNS, can wrap around axon about 100x to create myelin sheaths, increases conduction speed in PNS axons

Schwann Cells

Wrap around axons to form myelin sheath

Myelination/Oligodendrocytes

20% protein, 80% lipid - insulation to increase conduction speed

Myelin Sheath

Spaces between myelin=exact, nerve fibers (axons) are very long=many oligodendrocytes/Schwann cells to myelinate

Myelination: Nodes of Ranvier

Myelin of oligodendrocytes=replaced by scar tissue, nerve conduction decreased or blocked

Multiple Sclerosis

Cause: autoimmune, virus (?); Symptoms: speech defects, blindness, tremors, numbness

Cause and Symptoms of MS

Electrical force: separation of charges=has potential energy

Electrical Potential (voltage)

Inside cell=negative; outside cell=positive; RMP of neuron=-70mV (inside); membrane polarized (b/c not zero)

Resting Membrane Potential

Ion concentration gradient between ICF + ECF (ions=K, Na) & Permeability of membrane to ions (# of open channels)

All cells have a RMP, but magnitude of RMP depends on:

1 - all cells use Na/K ATPase to pump Na out and K in (creates concentration gradients for Na and K)

1st step of Resting Membrane Potential

2 - Membrane=highly permeable (leaky) to K @ rest (lots of K channels open), slightly permeable to Na @ rest

2nd step of Resting Membrane Potential

K would flow out b/c of concentration gradient and cause inside of cell to be more negative (electrical attraction back in)

Hypothetically, what would happen if the membrane was only permeable to K?

Na would flow in b/c of concentration gradient and cause inside to be more positive (electrical attraction outward)

Hypothetically, what would happen if the membrane was only permeable to Na?

Only K channels are open, RMP=-70mV; cell=permeable to both Na + K

Real Life Membrane Potential is:

Membrane is more permeable to K than Na @ rest & K and Na leak (channels=always open)

Why does K have a bigger impact in a real life membrane potential?

When Na channels open, Na move into cell - voltage more positive; When K channels open, K move out - voltage more negative

Movement of Ions in/out of Cell that Changes Membrane Potential

Neuron stimulated @ dendrite/cell body (chemical, light, heat) - Na channels open - Na flows in - membrane=more positive

Local Potential:

If local potential is enough to bring membrane to -55mV (if not, nothing else happens) & voltage change needed to trigger AP

Threshold (action potential)

Membrane potential becomes more positive (less negative) & neuron=more likely to have AP

Depolarizing

Membrane potential becomes more negative toward RMP

Repolarizing

Membrane potential more negative than RMP & neuron=less likely to have AP

Hyperpolarizing

All or none, always same size, only produced if threshold is reached

Action Potentials

Occur @ dendrites/cell body, produced by nonvoltage gated channels (Ex. ligand, light, mechanical)

Local (graded) Potentials

Can be depolarizing (excitatory) or hyperpolarizing (inhibitory), graded (vary in size), reversible (return to RMP when stimulus stops), local (decrimental - decrease in size with distance from stimulus), can summate (add together)

Local (graded) Potentials Characteristics

Always produced by voltage gated channels, self propagating (one generates the next one), travel from axon hillock to end of axon, irreversible, always begins with depolarization (same pattern every time), cannot summate, not graded (same magnitude every time)

Action Potential Characteristics:

Stimulus: nonvoltage gated channels open - local potentials summate (add up) - threshold reached - voltage gated channels open - these channels only open if threshold is reached

Action Potential Steps:

Period of AP where difficult/impossible to generate another AP

Refractory Period

No stimulus will trigger another AP (during de/repolarization Na channels already open or inactive (can’t open))

Absolute Refractory Period

Only very strong stimulus will trigger another AP (Na closed, K still open)

Relative Refractory Period

AP - self propagation (one produces next), chain reaction of AP (nerve signal) - AP travels from axon hillock to axon terminal

Signal Conduction

Diameter of axon (bigger neuron=faster), presence/absence of myelin (has myelin=faster)

Conduction speed depends on:

1 mph (Ex. regulation of digestion (no hurry)), axon hillock: trigger zone/has too many voltage gated channels, nerve signal travels in one direction b/c absolute refractory period prevents it from going backwards

Unmyelinated Fibers

Ions can’t flow in/out where there is myelin (insulation)

Myleninated Fibers

Myelinated fibers - nerve signal jumps from node to node, 225 mph, Ex. motor neurons or sensory for sharp pain

Saltatory Conduction

1 - local potentials, 2 - AP generated @ axon hillock, 3 - AP travels down axon, 4 - neurotransmitters released, 5 - responds to NT by generating local potentials

Synaptic Transmission

Many neurons

Each neuron receives input from…

Spinal cord motor neurons: 10000 synapses each; Cerebellum neurons: 100000 synapses each

Typical Neuron: Many Synapses

1 - arrival of AP (presynaptic neuron), 2 - opens voltage gated Ca channels, 3 - vesicles fuse w/ membrane + release NT (exocytosis), 4 - generates local post synaptic potential - can be excitatory or inhibitory - depends on NT + postsynaptic receptor (postsynaptic neuron)

Chemical Synapse Steps:

ACh, Norepinephrine (NE), GABA

Examples of Neurotransmitters

Neuron before the gap that sends the message

Presynaptic Neuron

Neuron after the gap that receives the message

Postsynaptic Neuron

NT diffuses away, NT reuptake back into presynaptic neuron, NT degraded by enzyme (AChE)

Signal Termination Options (chemical synapse)

Local potentials: depolarizing local potentials, neuron more likely to fire AP

Excitatory Postsynaptic Potentials (EPSPs)

Hyperpolarizing local potentials, neuron less likely to form AP

Inhibitory Postsynaptic Potentials (IPSPs)