Chapter 11: Functional Organization of Nervous Tissue

Exam 2 material

Two main cell types in nervous tissue

neurons and glial cells

Nerve

bundle of axons outside the brain and spinal cord

Cranial nerves

orginate from the brain; 12 pairs

Spinal Nerves

originate from the spinal cord; 31 pairs

Ganglion

collection of neuron cell bodies (holds nucleus) outside the brain and spinal cord (dorsal root ganglion)

Plexus

extensive network of axons and sometimes neuron cell bodies; located outside the CNS

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ex. brachial plexus

Glial Cells

Supportive cells with many functions

Functions of the nervous system

maintain homeostasis

receive sensory input (monitor internal/external stimuli)

Integrating information (CNS)

controlling muscles and glands (effector organs)

establishing and maintaining mental activity

Central nervous system

brain and spinal cord

peripheral nervous system

sensory receptors and nerves

Sensory division of PNS

transmits AP from receptors to CNS (afferent)

Sensory receptors

can be neuron endings or specialized cells that detect external and internal stimuli; send input along nerves to brain or spinal cord

motor division of PNS

transmits AP from CNS to effectors (efferent)

somatic nervous system

from CNS to skeletal muscles

voluntary; single neuron system

autonomic nervous system

from CNS to smooth muscle, cardiac muscle or glands

subconscious or involuntary control

two neuron system

* first from CNS to ganglion
* second from ganglion to effector

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divisions of ANS

sympathetic, parasympathetic, or enteric

sympathetic ANS

prepares body for physical activity

Parasympathetic ANS

regulates resting functions

Enteric ANS

plexuses within the wall of the digestive tract

multipolar/motor neuron

receive information through dendrites

integrate information in cell body

send signals to cause action in effectors through axon

Interneurons

within the CNS, connect 1 neuron to another

\*sometimes not seen in very simple neural pathways

Multipolar

most neurons in CNS; motor neurons

Bipolar Neurons

sensory in retina of the retina and nasal cavity

psuedo-unipolar

single process that divides into two branches.

part that extends into the periphery has dendrite-like sensory receptors

anaxonic

no axons, only dendrites

found in brain and retina where they only communicate using graded potentials

Astrocytes

glial cell

prevents blood composition fluctuations from affecting the brain function

forms blood-brain barrier (keep some thing in and some things out)

promote development of synapses

reactive astrocytosis

CNS injury sites are walled off to limit inflammation

can have scar forming astrocytes which limits axon regeneration in damaged neurons

Ependymal cells

glial cell of CNS

line brain ventricles and central canal of spinal cord

specialized versions form choroid plexus

**forms cerebrospinal fluid**

has some astrocyte-like functions

Microglia

glial cell in CNS

specialized CNS macrophage

respond to inflammation, phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS

can be used in autopsy to detect CNS damage

Oligodendrocytes

glial cell of CNS

form myelin sheaths by wrapping cytoplasmic extensions around axons

can form myelin sheaths around portions of several axons

Shwann Cells

glial cell of PNS

wrap only one axon to form myelin sheath

outer layer is neurilemma (most cytoplasm, nucleus, and organelles)

Satellite cells

glial cell of PNS

surround neuron cell bodies in sensory and autonomic ganglia

provide support, nutrients, and protection from heavy metal poisons

Characteristics responsible for the resting membrane potential

graded potentials result from

ligands bind to receptors

change in charge across membrane

mechanical stimulation

temp change

spontaneous change in permeability

graded potential occurs in ….

areas of synaptic contact

cell body and dendrites

action potential occurs in ….

Axons

graded potential

small change in membrane potential in a localized area

magnitude varies based on stimuli and frequency

hyperpolarizing (Cl+ in/K+ out) or depolarizing

can summate/add to reach threshold

conducted over the plasma membrane in a decremental fashion (decrease in magnitude as the spread)

graded potential steps

1. small act of Na+ entering → slight depolarization
2. and 3. more Na+ enters → greater depolarization


4. stimulus is applied to cell causing a small depolarization
5. summation → larger depolarization

Characteristic of action potentials

produced when graded potential reaches a threshold

all or none response

this is how neurons communicate with an effector

1st step of an action potential and a voltage-gated ion channel

resting membrane potential

voltage-gated = closed, inactivation gate = open

outside is + charged

2nd step of an action potential and a voltage-gated ion channel

Na+ activation channel = open quickly (graded potential)

Depolarization when threshold is reached

K+ start to open slowly

Inside becomes more +

open voltage channels increase open voltage channels until all open

positive feedback cycle

3rd step of an action potential and a voltage-gated ion channel

repolarization

inactivation gates of Na+ voltage channels close

More k+ channels open

\*no more Na in and increased k+ out →repolarization

4th step of an action potential and a voltage-gated ion channel

End of repolarization

reestablishes resting condition

Activation v inactivation in Na+ and K+ channels

1. rest: Na+ activated closed, Na+ inactivated open
2. graded: Na+ activated open quickly, Na+ inactivated open, K+ open slowly
3. depolarization: Na+ open until all are open, K+ opening
4. Repolarization: Na+ inactivated close, K+ still open
5. End of repol: Na+ activated close, inactivated open, k+ still open
6. Afterpotential: Na+ all closed, K+ still open


1. Rest: K+ and Na+ closed

Afterpotential

Hyperpolarization

allows k+ to leave cell

Refractory Period

sensitivity of area to further stimulation decreases for a time

absolute (complete insensitivity, beginning go AP to \~end of repol.)

relative (stronger than threshold stimulus can initiate another AP)

stimulus strength and AP frequency

Strength of Ap doesn’t change w increase in strength of stimulus BUT frequency of action potential can inc. to a point

target of AP will interpret the higher frequency as a stronger stimulus which could lead to a stronger contraction/secretion

AP Propagation in a unmyelinated axon

AP is propagated more slowly

Na+ diffuses into membrane and creates a local current that travels down the axon

saltatory conductions

AP at a node of Ranvier generates local currents

myelin sheath insulates and forces current from node to node

Na+ channels are highly concentrated

The synapse

junction between two cells

where action potentials in one cell causes an action potential in another cell

Types of cells in a synapse

presynaptic (signal to synapse)

postsynaptic (receiving signal)

types of synapses

electrical (gap junction→ cardiac/smooth)

chemical (neurotransmitters→ skeletal)

Electrical Synapses

connected by gap junctions

graded current flow between adjacent cells

cardiac and smooth muscle (where contractility is important)

Connexons

protein tubes in cell membrane

chemical synapses

components: presynaptic terminal, synaptic cleft, postsynaptic membrane

neurotransmitters released by AP

AP causes Ca2+ to enter and neurotrasmitter to be released from vesicles (voltage-gated)

neurotransmitter binds to receptor on ligand gated ion channels

Steps in a chemical synapse

1. AP opens V-gated Ca channels
2. Ca into cell and bind to vesicles and they release neurotransmitter
3. neurotransmitter diffuse across cleft


1. nt. bind to receptors and creates a graded potential in membrane

Removal of neurotransmitter from synaptic cleft (ACh)

1. Act released from receptors
2. Ach-esterase splits ACh into choline and acetic acid
3. choline is taken up by presynaptic terminal and binds with acetyl-CoA to reform acetylcholine


1. other molecule diffuse into ECF

Removal of Neurotransmitter from Synaptic Cleft (norepinephrine; NE)

1. NE released from cleft
2. taken up by presynaptic terminal
3. repackaged into synaptic vesicles


1. enzyme monoamine oxidase (MAO) breaks down some molecules of NE

Specificity of receptor molecules in synapses

neurotransmitter only fits one receptor

only effects cells with receptors specific to them

some neurostransmitters are excitatory and/or are inhibitory

some can also attach to the presynaptic terminal and modulate its own release

Neuromodulators

released from neurons to pre/post synaptic membrane to synaptically influence the likelyhood of an action potential

ex. decrease the chance of excitatory NT release and prevent the generation of a AP

Neurotransmitters and Neuromodulators

chemical messengers secreted by neurons

neurons can secrete more than 1 type (100 diff ones)

criteria to be a neurotransmitter

synthesized by neuron and stored in synaptic vesicles in presynaptic terminals

AP must stimulate its exocytosis

must bind to receptor in postsynaptic membrane

must evoke a response

classification of neurotransmitter is based on

chemical structure

effect on postsynaptic membrane

mechanism of action at the targy

Ionotropic effect of neurotransmitters

bind to ion channels

metabotropic effect on neurotransmitters

binding to G-protein-linked receptors

Excitatory postsynaptic potential

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* Depolarization occurs and response is stimulatory.
* Depolarization might reach threshold producing an action potential and cell response.

Inhibitory postsynaptic potential (IPSP)

hyper polarization and response is inhibitory

decrease likelihood of action potential by moving membrane potential farther from threshold

Axoaxonic synapses

Neuromodulator

axon of one near synapses with the presynaptic terminal (axon) of another

common in CNS

Presynaptic inhibition

neuromodulator

reduction in amount of neurotransmitter released from presynaptic terminal

presynaptic facilitation

neuromodulator

amount of neurotransmitter released from presynaptic terminal increases

Spatial summation

AP 1 and 2 cause the production of graded potential at 2 different dendrites

summate at the trigger sone to produce a graded potential that exceeds threshold resulting in an action potential

Temporal Summation

two AP arrive in close succession at the presynaptic membrane

1st causes production of a graded potential that does not reach threshold at the trigger zone. The second action potential results in the production of a second graded potential that summates with the first to reach threshold, resulting in the production of an action potential.