Anatomy and Physiology Exam 4

Dr. Lovelace

nerves

bundles of axons

neuron

functional unit of the nervous system

sensory input

receptors detect changes and carry to brain

integration

to process/interpret sensory input

perception

being consciously aware of something

peripheral nervous system

portion of the nervous system outside of the CNS (everything but brain and spinal cord)

what 2 fiber types are in the sensory (afferent) division of PNS

somatic sensory fibers and visceral sensort fibers

somatic sensory fibers

convey impulses from skin, skeletal muscle, and joints

visceral sensory fibers

convey impulses from visceral organs to CNS

what does the motor (efferent) division of the PNS do

transmit impulses from CNS to effector organs

3 subdivisions of peripheral nervous system

somatic, autonomic, and enteric nervous system

somatic nervous system

convey impulses from only skeletal muscle, voluntary

autonomic nervous system conveys impulses from...

mostly visceral organs (smooth and cardiac muscle, glands- involuntary)

enteric nervous system

brain of the gut, sensory receptors monitor and communicate with conditions of the GI tract, secretes hormones (involuntary), enteric interneurons may process sensory info and decide to modify GI muscle contractions and secretions

interneurons

process information and make decisions about what needs to happen

nervous tissue characteristics

cells often have high cell density with little extracellular space

two main nervous cell types

neurons and neuroglia

neuron characteristics

excitable, transmit electrical and chemical signals

neuroglia

small cells, support, surround, wrap delicate neurons

dendrites

receiving end of neuron, receive neurotransmitter signals and conduct impulses to cell body

cell body (perikaryon/soma)

contains nucleus surrounded by cytoplasm, contain organelles

axons

conduct impulses towards another neuron/effector cell

axon hillock

where axon joins cell body

initial segment

beginning of axon

trigger zone

the junction between the axon hillock and the initial segment

axon functions

transmits APs along axolemma to axon terminal, NTs are released into extracellular space

What do neurotransmitters do?

excite/inhibit cells that axon terminals form synapses with

can a single neuron communicate with multiple axons at once?

yes

what organelles do axons lack

Rough ER and Golgi, they rely on cell body to renew proteins and membranes

what happens if axons are cut or damaged

they quickly decay

axon terminals (telodendria)

tips swell into bulb shape synaptic end bulbs

synapse

communication between 2 neurons or a neuron and an effector cell

synaptic cleft

the gap between the pre and post-synaptic cells

chemical signal

APs can't propagate across a synaptic cleft so NTs are used to modify electrical activity

electrical synapse

electrical impulse modifies electrical activity in post-synaptic cell using gap junctions

2 types of transport systems to carry materials from cell body to axon terminals

slow axonal transport and fast axonal transport

slow axonal transport

conveys axoplasm in one direction only - from the cell body toward the axon terminals.

fast axonal transport

moves materials in both directions (anterograde & retrograde)

sensory neurons

Transmit impulses from sensory receptors toward the CNS, almost all unipolar, cell bodies in ganglia in PNS

motor neurons

carry impulses from the CNS to effectors, multipolar, most cell bodies in CNS

Where are interneurons located?

between sensory and motor neurons, most of CNS neurons are this type

astrocytes

in CNS, most abundant neuroglia, support and brace neurons, respond to nerve impulses, influence functioning

microglial cells

small, ovoid, monitor neuron health, migrate to injured neurons, can turn into phagocytes

ependymal cells

may be ciliated, line central cavities of CNS, secrete CSF

oligodendrocytes

Processes wrap CNS nerve fibers forming myelin sheath

satellite cells

surround neuron cell bodies in PNS

Schwann cells (neurolemmocytes)

form myelin sheath around axon in PNS for one axon at a time

myelin sheath

protects and electrically insulates axon, increases speed and distance of impulse transmission

Outer collar of perinuclear cytoplasm

Peripheral bulge of schwann cell, contains nucleus and cytoplasm

plasma membrane of myelinating cells

less protein, no ion channels, good insulator, interlacing proteins bind membrane layers together (PNS)

myelin sheath gaps

site where axon collaterals can emerge, high density of ion channels

nonmyelinated fibers

axons not wrapped in myelin

excitable cells resting voltage

-70mV in cell and 0mV outside of cell

electrical potential

electrical charges of opposite signs have the potential to do work if they come together

membrane potential

voltage of intracellular region of membrane compared to extracellular (higher potential excites cell (+) and lower potential inhibits cell (-))

potential difference

difference in potential (charge) between outside and inside of cell (increasing difference at rest makes it more negative and decreasing difference at rest makes it more positive)

conductance

represented by g, permeability of the membrane of an ion, more ion channels open=greater conductance

current (flux)

represented by I, actual movement (flow) of ions through membrane channels, high current=high flow, positive current-ions flowing out of cell, negative current-ions flowing into cell

sodium ionic concentration gradient

higher concentration outside of cell, cation

chloride ionic concentration gradient

anion, higher outside of cell

calcium ionic concentration gradient

cation, higher outside of cell

potassium ionic concentration gradient

cation, higher inside of cell

phosphate ionic concentration gradient

polyatomic ion, higher inside cell, majority is bonded to proteins and ATP

diffusion

movement of an atom/molecule from high to low concentration

equilibrium potential (nernst equation)

voltage at which electrical gradient cancels out chemical gradient (specific for each ion)

Nernst for sodium and potassium

(ENa)=+55mV (EK)=-78mV

driving force

how badly something wants to cross the membrane (depends on concentration and electrical gradient)

4 types of ion channels

1. leak channels
2. ligand-gated channels
3. mechanically-gated channels
4. voltage-gated channels

ligand-gated ion channel

respond to specific chemical stimuli, mainly concentrated at synapse

voltage-gated ion channels

respond to changes in transmembrane electrical potential, mainly located along axon, participate in generation and conduction of AP

mechanically gated channels

respond to mechanical deformation (applying pressure to a receptor)

leakage channels

randomly alternate between open and closed

what are graded potentials used for

short distance communication

what are action potentials used for

allow communication over long distances within the body

If the leak permeability for Na and K were equal...

RMP would lie exactly between the 2 Nernst values

a cell with RMP is...

polarized and primed- ready to produce APs

what must happen for AP to happen from RMP?

graded potentials must occur to depolarize the cell to threshold

graded potential occurs when...

ion flow in mechanical or ligand gated channels produce a localized current, it spreads and dies out within a few mm

when ACH binds to nicotinic ACh receptor letting some Na flow through, what is this an example of?

this is a graded potential

depolarizing graded potential (excitatory)

stimulus causes less negative voltage

hyperpolarizing graded potential (inhibitory)

stimulus causes intracellular membrane voltage to be more negative

Where do graded potentials occur most often?

dendrites and cell body

Action potential/impulse

signal that travels the length of the axon, during AP membrane potential reverses then goes to rest

threshold value

minimum voltage required to trigger an action potential

threshold stimulus

AP produced then travels through axon then axon terminals

subthreshold

no nerve impulse, membrane goes back to resting value

depolarization

Na flows into cell

repolarization

K leaves cell

hyperpolarization

K continues leaving the cell after repolarization

3 states of voltage-gated Na+ channels

resting state, activation state, inactivation state

deactivated state Na

closed, can open if stimulated

activated state Na

open

inactivated state Na

closed, channel is inactive and cannot reopen until it resets to deactivated state which takes time

voltage gated K channels

Slow to open and slow to close

absolute refractory period

when you absolutely cannot generate another AP despite stimulus

what causes the refractory period

depolarization requires enough VG sodium channels to open but too many of the channels are inactive

relative refractory period

follows absolute refractory period, when a second AP can be initiated but only by a larger than normal (suprathreshold) stimulus

continuous conduction

happens in unmyelinated axons, VG channels regenerate AP at each point along the axon so voltage doesn't decay, very slow

saltatory conduction

happens in myelinated axons, myelin keeps current in axon so voltage doesn't decay and Ap is regenerated only at gaps, aka "looping" conduction, very fast

what affects the speed of APs

axon diameter, degree of myelination, temperature