Chp 12 (Nervous Tissue)

Overall Functions of Nervous System

Coordinates all systems functions, maintain homeostasis

Responsible for all perceptions, behaviours, memories, and voluntary movements

3 Different Functions of Nervous System

Sensory: detect changes in environment (stimulus), relay info to CNS

Integrative: analyze info and make decisions (interneurons)

Motor: respond to stimuli by initiating action

Organization of Nervous System (The 2 Systems)

Central Nervous System (CNS): brain and spinal cord

Peripheral Nervous System (PNS): all nervous tissue outside of CNS, including nerves and sensory receptors

Neurons

-cells that transmit signals

-dont divide

-electrically excitable

Neuroglia

-supportive cells that protect n nourish neurons

-can divide

-not electrically excitable

-4 kinds in CNS, 2 in PNS

Cell Body

-contains nucleus

-nissl bodies (Rough ER)

-neurofibrils give shape and support

-microtubules move material inside/along axon

Cell Processes (Nerve Fibers / Neurites)

-dendrites: conduct signals TO cell body (receiving)

-highly branched

-unmyelinated

-axons: conduct signals AWAY from cell body → other body cells, starting at trigger zone

-ends in thin axon terminals (with synaptic end bulbs at tips)

• filled with neurotransmitters

Explain the process of signal communication btw Dendrites, Cell Body, and Axons

-Dendrites receive the signal

-send it towards the cell body, where its processed

-sent down long axon.

At the end bulbs, Ach is released into a synapse, thus triggering a muscle/nerve activation. Ach-ase breaks down the Ach to be recycled

What Neuron is this?

Multipolar

• several dendrites, one long axon

• Majority of neurons in CNS (interneurons), all motor neurons

What Neuron is this?

Bipolar

• one dendrite and axon

• Eye retina, inner ear, olfactory area of brain

What Neuron is this?

Pseudounipolar

• Single fused axon and dendrite (which acts as sensory receptors)

• Sensory neurons

Functional Neuron Classifications + How do they all work tgt in carrying out a function?

(Classified whether they transmit AP’s towards or away from CNS)

• sensory / afferent (away)

  • Relay sensory info TO CNS through cranial or spinal nerves

• Motor / efferent

  • Send commands FROM CNS to muscles and glands (effectors) through cranial or spinal nerves

• Interneurons / association

  • Connect sensory to motor neurons in CNS, most numerous

Sensory neurons receive info from inside n outside the body n sends it to the CNS, Interneurons connect sensory and motor neurons tgt for the signal to flow through, the motor neurons initiate the command to the gland, muscle etc.

Eg; i pick up a hot coal (sensory tells CNS its hot), signal flows through (interneurons) my body, then i let go (motor)

Astrocytes (4 Neuroglial Cell Types in CNS)

-star shape

-cover blood cap’s, forms blood-brain barrier

-regulate proper ion + neurotransmitter concentrations in inter. Fluid

-provide structural support for neurons

-regulate growth in embryos

-influence synapses formation

-similar to satellite cells

Oligodendrocytes (4 Neuroglial Cell Types in CNS)

-form n maintain myelin sheath (lipid n protein covering) around axons in CNS

• myelin insulates axons and speeds up signals btw neurons

-similar to Schwann cells

Microglial Cells (4 Neuroglial Cell Types in CNS)

-phagocytic

• clear microbes n dead cells

Ependymal Cells (4 Neuroglial Cell Types in CNS)

-form epithelial membrane lining ventricles of brain and central canal

-make n circulates cerebrospinal fluid (CSF)

• forms blood-CSF barrier

Satellite Cells (2 Neuroglial Cell Types in PNS)

-surround cell body of PNS neurons

-structurally support neurons

-regulate exchange of mtaerial btw cell body of neurons and inter. Fluid

-similar to astrocytes

Schwann Cells (2 Neuroglial Cell Types in PNS)

-encircle + maintain axons in PNS, ensures neuronal survival

-produce myelin sheaths that surrounds axon in PNS

-similar to oligo’s

PNS Myelination

-myelin is made from Schwann cells

-outer plasma membrane of a schwann forms nuerolemma

-PNS can regenerate

• IF cell body is intact, schwann’s are functional, n scar tissue formation isnt too rapid

  • Distal axon degenerates, regen. Tube is formed by Schwann’s

CNS Myelination

-myelin is made from Oligo’s

-cell processes wrap around CNS axons, but cell bodies dont surround axons, so no neurolemma (= little regen.)

Clusters of Cell Body + Bundles of Axons in PNS n CNS

PNS

• cell body clusters: ganglion

• Axon bundles: nerves

CNS

• celll body clusters: nucelus

• Axon bundles: tracts

White v Gray Matter

White: myelinated axons

Gray: cell bodies, dendrites, axon terminals, unmyelinated axons, neuroglia

-gray is typicall found in the middle or around the edges of the brain n spinal cord

Electrical Signals in Neurons

-excitable cells communicate via AP’s n GP’s

-AP’s communicate through short/long distances, but GP’s are for short only

• Production of an AP/GP depends on a resting membrane potential (RMP) and certain ion channels

Channels are needed so ions can diffuse through cell membrane

Leak Channels (Types of Channels in Neurons)

• alt btw open/close randomly, K channels more numerous than Na channels

• dendrites, cell bodies, axons of all neurons

Ligand-gated Channels (Types of Channels in Neurons)

• need specific chemical binding to open/close (eg; neurotransmitter, hormone)

• Dendrites of SOME sensory neurons, dendrites + cell bodies of inter + motor neurons

Mechanically Gated Channels (Types of Channels in Neurons)

• open/close in response to mechanical stimuli (eg; vibration, pressure)

• Dendrites of SOME sensory neurons (eg; touch, pressure like skin, and auditory like ear)

Voltage-Gated Chanels (Types of Channels in Neurons)

• open/close in response to a membrane potential change, generates/conducts AP’s

• In axons of all neurons

Graded Potentials / GP’s (local membrane changes)

-in response to a stimulus (eg; change in MP) can be tiny/large and in any direction (inc. or dec. = graded)

-mainly in dendrites and cell body, uses mechanically + ligand channels

-if sufficient (reach threshold), triggers an AP (can travel long distances)

GP’s can vary in size (depending on stimulus) but all AP’s are the same

Resting Membrane Potential (RMP)

-nerve cells are excitable because

• RMP is polarized (conc. Of ions n net charge is diff from inside to outside cell membrane)

• Specific ion channels

-more - ions inside and + ions outside

-voltage across membrane is -70mV for a neuron

-Na and Cl are high in ECF

-K, proteins, PO4- are high in cytosol

What transporter is responsible for the conc. Gradient of Na+ and K+ ions?

Na/K ATPase

OR

Na/K Pump

Flow of Ions

-ions must flow in/out of cell for signals (changes in potential) to take place

-flow of ions occurs through channels in membrane

• ions flow from high to low conc., and towards opposite charge

Hyperpolarization and Depolarization

Hyper: membrane becomes more - than MRP (-70 → -80)

• taking in anion (-) or losing cation (+)

• Further from threshold, less likely to trigger AP

Depolarization: membrane becomes more + than MRP (-70 → -60)

• taking in cation

• Closer to threshold, more likely to trigger AP

Action Potentials (AP’s)

-rapid depolarization of MP followed by restoration to MRP (-70 mV)

-strength of an AP is ALWAYS the same

-involved voltage channels (Na n K)

-travels whole axon, releases neurotransmitter at synapse

Phases of AP

-chemical or mechanical stimulus causes GP to reach at least -55mV (threshold), then 2 phases

• depolarizing: voltage Na channels open, Na rushes in, MP becomes +30mV

• Repolarizing: Na channels inactive (Na inflow stops), K channels open to let K out, MP back to -70mV

Refractory Period (period where a neuron CANT generate another AP), 2 types

• absolute: inactivated Na channels must return to resting state before reopened

• Relative: suprathreshold stimulus can start an AP, K channels still open, but Na channels closed, hyperpolarization

Stimulus Strength of AP’s

Subthreshold stimulus: GP insufficient in reaching threshold = no AP

Threshold stimulus: sufficient depolarzation = 1 AP

Suprathreshold stimulus: multiple rounds of depolarization = many AP’s

Propagation of AP’s

-communication through ur body only happens when AP travels from trigger zone → axon terminals

• travel = propagation, 2 kinds

Continous Conduction (Unmyelinated Axons):

-travels down ENTIRE axon, overall slower

Saltatory Conduction (Myelinated Axons):

-myelin sheaths pass Na down quickly along axon, since they dont interact with them (so just skips past all myelin parts), overall faster

• lipid portion of myelin is hydrophobic (no like Na)

-so, myelin = faster signal conduction

Factors Affecting Propagation Speed

• Axon diameter (larger = faster)

• Myelination Amount (more myelin = faster)

• temperature (hotter = faster)

Clinical Manipulation of AP’s

-anesthetics (eg; procaine, lidocaine) act by blocking the opening of voltage Na channels

• no gen. Of AP’s = pain sensations arent transmitted to CNS

Signal Transmission In Synapses

-synapse: junction btw neurons and another cell (2 kinds)

• Electrical: ionic current spreads to next cell through gap junctions, faster, can synchronize groups of cells/neurons

• Chemical: one way info transfer from a PRE-synaptic neuron → POST-synaptic cell

Chemical Synapses

1. AP reaches end bulb

2. Voltage channels open

3. Ca flows inward triggering release of neurotransmitter

4. Neuro. Crosses synaptic cleft (space btw neurons), binds to receptor on post-cell, causes ion channels to open

5. Ions rush in post-cell

6. Triggers depolarization or hyperpolarization (depends on what channels open)

7. Post-synaptic potential is generated

-postsynaptic potential = temporary graded change in MP of the post-synaptic cell

Excitatory and Inhibitory Potentials

-a neurotransmitter can be either inhibitory or excitatory

excitatory:

• when NT Causes opening of ligand channels, depolarization happens (Post-cell more likely to reach threshold = start AP)

• Depolarizing postsynaptic potential = EPSP (Excitatory Postsynaptic Potential)

Inhibitory:

• when NT Causes opening of ligand Cl or K channels, hyperpolarization happens (post-cell less likely to reach threshold = no AP)

• Hyperpolarizing postsynaptic potential = IPSP (Inhibitory Postsynaptic Potential)

-EPSP and IPSP are both GP’s and present in PNS and CNS

-one NT can be excitatory and inhibitory in diff locations (depends on receptors and channels)

Summation

Spatial: NT’s released from several end bulbs onto one neuron

Temporal: NT’s released from 2 or more firings of same end bulbs onto in rapid succession

Summation of all postsynaptic potentials (EPSP+IPSP) from all pre-synaptic NT’s will determine whether an AP is triggered + how frequent

Small Molecule NT’s

Ach: released by many PNS + some CNS neurons, excitatory on NMJ but inhibitory at other synapses

Amino Acids:

• Glutamate: released by most excitatory neurons in brain, inactivated by reuptake by neurons and by neuroglia

• GABA: inhibitory NT for 1/3 of brain’s synapses

Biogenic Amines (modified amino acids)

• norepinephrine, seratonin (happiness), dopamine (mood)

CO1 + Nitric Oxide: Vasodilation

Neuropeptides

-3-40 amino acids, excitatory/inhibitory effects in CNS and PNS

• Endorphins: natural painkiller (blocks release of substance P

• Substance P: enhances pain perception

Removal of NT’s

-once NT are released into cleft, removed by

• Diffusion: NT’s move away from cleft (down gradient)

• Enzymatic Degradation: enzymes degrade NT (eg; Ach-esterase)

• Uptake by Glial Cells / Reuptake by Neurons: NT get into neurons or glial cells by transporters