- receive information about external & internal surroundings
- transmitted to brain & spinal cord via sensory neurons

- process & store information
- make decisions
- in spinal cord and brain
- via interneurons: w/in brain & spinal cord that take incoming info and may decide to store info for later or induce a motor function

- send commands to effectors
- via motor neurons

- brain and spinal cord
- integration centers

- cranial (12 pairs) & spinal (31 pairs) nerves part of neuron that sends signal

- ganglia

- sensory receptors

- signals to the CNS

i.  Somatic Sensory Division

ii.  Visceral Sensory Division: associated with organs; ie stomach is full and brain reacts with secretions etc

- signals from CNS to effectors

i.  Somatic Nervous System (SNS)
- to skeletal muscles
- voluntary control & somatic reflexes

ii.  Autonomic Nervous System (ANS)

- to glands, cardiac muscle, & smooth muscle
- involuntary/visceral reflexes

a.  Sympathetic Division
- arouses body for action
- “ fight or flight”

b.  Parasympathetic Division

                   - calms the body
                   - “rest & digest”

1.  Electrical Excitability: High degree of ability to respond to some kind of stimulus (external to the cell)

2.  Conductivity: convert response into an electrical signal; generate action potential

3.  Secretion: associate with synapse (place where signal is transferred from one cell to the next)

- control center

- nucleus and other organelles

- Nissl bodies (rough ER): centers of protein synthesis; show up darkly

- neurofibrils

- cytoplasmic inclusions

- no mitosis

-  branched processes: a lot of surface area

-  processes that emerge from cell body

-  primary sites for receiving signals

- conducting zone

- mostly unbranched process
- emerges from cell body
- propagates nerve impulse: special ion channels that are not in the soma

- wide range in length
- no rough ER/no protein synthesis
- axoplasm & axolemma

- axon hillock where joins cell body
- initial segment & trigger zone

- axon collaterals may be present: branch offs at a 90 degree angle to activate more than one neuron

- axon terminals at distal end

one axon and multiple dendrites

- motor neurons & interneurons

a single process leading away from cell body

- process branches
- both branches function as axon

- branches into dendrites at distal end

- axon terminals/synaptic knobs in CNS
- sensory neurons

supportive cells

Astrocytes in CNS

Neuroglia 

Oligodendrocytes in CNS

Neuroglia 

- forms myelin sheath around multiple neurons – cell wraps around neuron

- insulates axon

Microglia in CNS

Neuroglia 

Epindymal Cells in CNS

Neuroglia 

- lining of internal cavities of brain & canal of spinal cord

- produce & circulate cerebrospinal fluid – they secrete this

- cilia & microvilli on apical surface
- blood-cerebrospinal fluid barrier

Schwann Cells in PNS

Neuroglia 

- surround cell bodies in ganglia
- may regulate exchange of materials with extracellular fluid

- electrically insulates axon

- consists of p.m. of glial cells

-  many Schwann cells per axon
- spiral repeatedly around the axon – like squeezing up a toothpaste tube, you wrap the outside as the inside stuff moves along

- neurilemma is thicker, outer layer
- nodes of Ranvier are gaps – regions of axon that are not insulated

- internodes are myelin-covered segments – are insulated

- electrically insulates axon

- consists of p.m. of glial cells

- each oligodendrocyte partially myelinates several axons

          - no neurilemma

- Nodes of Ranvier are present

degenerative disorder of myelin sheath in CNS

Oligodendrocytes are destroyed and replaced with sclerotic (scar) tissue, could be autoimmune

- electrical voltage – degree of separation from + and - charge

 - enables electrical current/ion flow to occur

- some gated
- electrical currents can be turned on and off
- current flows across membrane thru ion channels

- electrochemical gradient

- can change membrane potential

Ion Channel?

- more K+ leakage channels than Na+ leakage channels in p.m.
- membrane more permeability to K+ 

Ion Channel?

-Typically associated with axon

- respond to change in membrane potential

Ion Channel?

- respond to chemical stimuli

- respond to mechanical stimulation (ie sound waves, feelings in skin)
- typically associated with sensory receptors

- typical in a neuron is -70 mv

- due to unequal distribution of various ions in EC fluid & the cytosol:
- EC fluid rich in Na+ and Cl-
- cytosol rich in K+ and other anions (other ions that can’t pass through membrane à too large, negative charge)

- K+ tends to diffuse out of the cell along concentration gradient (anions don’t go with)

↓

- inside of membrane becomes increasingly negative

↓

- K+ is attracted back in along its electrical gradient & reaches equilibrium

- inward leakage of Na+ is slow, but counteracts negative voltage

- Na+/K+ pumps offset the inward leakage of Na+ -- 3+’s out 2+’s in

- Na+/K+ pump has electrogenic effect

- net effect of ion flows & pump action produces a RMP of about –70 mV

- small deviation from membrane potential

- response to ligand or mechanically-gated channels

- vary in amplitude & strength

- produces a localized flow of current that dies out quickly

- may be excitatory or inhibitory – hyperpolarize the cell; excitatory is something that depolarizes neuron

- sequence of rapidly occurring events
- 2 phases:  depolarizing and repolarizing
- during depolarizing phase: membrane potential becomes positive
- during repolarizing phase: resting membrane potential is restored -70 à +30, then back to -70
- involves 2 types of voltage-gated channels in p.m. of axolemma and axon terminals
- Na+ channels open first & allow Na+ to flow in - depolarize
- K+ channels open next & allow K+ to flow out - repolarize
- all-or-none principle applies – amplitude/strength is always the same
- travel long distances without dying out

i.  Depolarizing graded potential causes membrane to depolarize to threshold (-55 mV)

ii.  Activation gates of voltage-gated Na+ channels open rapidly

iii.  Na+ flows into cell along favorable electrical and chemical gradient à  causes further depolarization à more Na+ channels open (positive feedback)

iv.  Inactivation gates of voltage-gated Na+ channels close

i.  Voltage-gated K+ channels begin to open at threshold but open slowly à become active when voltage-gated Na+ channels are closing

ii.  Na+ inflow slows and K+ outflow accelerates

Effect: Membrane potential changes to -70 mV

iv.  Outflow of K+ may lead to hyperpolarizing phase

Effect: Membrane potential drops to -90 mV

- Na+ voltage-gated channels return to resting state during repolarization

-  time during which excitable cell cannot generate an AP
- absolute refractory period:

- no stimulus can initiate a second AP
- when voltage-gated Na+ channels are activated (both gates open)  and inactivated (inactivation gate closed)

- relative refractory period:
- stronger stimulus needed to initiate a second AP
- when voltage-gated K+ channels still open but voltage-gated Na+ channels are in resting state (inactivation gate open & activation gate closed)

- nerve impulses arise at trigger zone
- propagated/conducted to axon terminal – like domino effect

- involves positive feedback:

depolarization in one region à triggers opening of voltage-regulated Na+ channels in adjacent segments à that segment depolarizes and produces AP à triggers opening of Na+ channels in adjacent segment à that segment depolarizes & produces AP à process repeated to distal end of the axon

-ion flow is not across the membrane, but along membrane

- refractory period prevents travel backwards

Continuous propagation:

- you trigger a.p.’s at very short distances away from previous a.p.

- occurs in myelinated axons – covered with myelin sheath w/occasional gaps
- voltage-gated channels unevenly distributed
- few in regions covered by myelin sheath

- many at nodes of Ranvier
- current flows across membrane & APs produced at nodes
- electrical current flows thru EC fluid & cytosol from node to node

- effects of saltatory conduction:

1.       faster conduction of signal – current is much faster than a.p.’s

- enzymatic degradation
- uptake by presynaptic neuron or neuroglia

- integration of inputs to postsynaptic neuron

- additive effect
- occurs at trigger zone
- spatial summation
- temporal summation

- usually small, organic compounds such as

1.  Acetylcholine – at neuromuscular junctions, and in brain
2.  Amino acids
3.  Modified amino acids – Epinephrine, Norepinephrine, Adrenaline, Noradrenaline
4.  Neuropeptides – Chain of amino acids (2-40)

- characteristic features:

1.  synthesized by presynaptic neuron

2.  released in response to stimulation

3.  bind to specific receptors on postsynaptic cell

4.  alter physiology of postsynaptic cell

Modifying the Effects of Neurotransmitters

- various chemicals may stimulate or inhibit

            i.    neurotransmitter synthesis

ii.                  neurotransmitter release

iii.                neurotransmitter receptors

iv.                 neurotransmitter removal

- functional groups of neurons
- organized into types of networks

1.  Simple series circuit: linear

2.  Diverging circuit: single presynaptic neuron synapses with several postsynaptic neurons

3.  Converging circuit:  several presynaptic neurons synapse with a single postsynaptic neuron

4.  Reverbrating circuit:  simple series circuit with branches that synapse back to earlier neurons in circuit

5.  Parallel after-discharge circuit:  divergence followed by asynchronous convergence
- single presynaptic neuron synapses with several postsynaptic neurons
- this produces multiple separate circuits of differing length
- each circuit arrives at common postsynaptic neuron at a different time