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1. NEURONS:
STRUCTURE AND
FUNCTION
Katie Siengsukon, PT, PhD
PTRS 852 Neurologic Physical Therapy & Rehabilitation I

2. Objectives
1. Describe the four main parts of neuron
2. Explain how the resting membrane potential of a
neuron is maintained
3. Discuss the development and propagation of an action
potential
4. Describe synaptic transmission
5. List the major neurotransmitters

3. Overview
• 21 BILLION neurons in cerebral cortex!
• 150,000 kilometers (93,205.6 miles) of nerve fibers
• Constantly changing (neural plasticity)
• Developing new connections
• Modifying output

4. Structure
• Cell like any other cell
• Has organelles including nucleus, Golgi bodies, mitochondria,
lysosomes, and endoplasmic reticulum
• Produce neurotransmitters

5. Structure
Four components:
1. Dendrites-receive information
from other cells
2. Axon-carries output
information to presynaptic
terminal
• Length varies
3. Presynaptic terminals-transmit
information to other cells
• Neurotransmitter released
4. Soma-cell body
• Where neurotransmitter is produced
Fig 2-1

6. Types
• Two Groups
• Based on number of processes
that directly arise from cell body
• Bipolar-two primary processes
(dendritic root and axon)
• Pseudounipolar-subclass
• Have single projection from soma
that divides into two axons
• Ex. Sensory neurons
• Peripheral axon-carries information
from periphery to soma
• Central axon-carries information from
soma to spinal cord
• Multipolar-multiple dendrites and
single axon Figure 2-4

7. Types
• Two Groups
• Multipolar-multiple dendrites
and single axon
• Most common
• ex. motor neuron,
interneurons and Purkinje
cells

8. Electrical Potentials
• Essential for transmission of information
• Three types:
1. Resting membrane potential
2. Local potential
3. Action potential

9. Membrane Channels
• Openings in membrane allow ions to
move from inside to outside of cell and
vice versa
• Two types of channels:
• Leak-always open; allow diffusion of ions
from high concentration to low
concentration; no energy used to maintain;
“open door”
• Gated-open in response to stimulus and
close when stimulus is removed
• Modality-gated: open in response to specific
sensory information
• Ex. Mechanical forces (stretch, touch,
pressure), temperature, chemicals
• Ligand-gated: open in response to
neurotransmitter binding to channel receptor
• Voltage-gated: open in response to change
in electrical potential
• Important for release of neurotransmitter and
propagation of action potential
Fig 2-5

10. Electrical Potentials
1. Resting Membrane Potential
• Difference in electrical charge across the cell membrane at rest
• No net flow of ions across membrane
• Must be capable of PRODUCING change in ion flow (be excitable) so
membrane maintains an unequal distribution of ions across membrane
• Typically -70 mV
• Maintained by:
• Na+-K+ pump—pumps out 3 Na+ ions for every 2 K+ pumped in
• Negatively charged anions inside—too large diffuse out
• Passive diffusion of ions
Fig 2.7b Lundy-Ekman

11. Electrical Potentials
• Change in resting membrane potential results in
transmission of information (depolarization) or lack of
transmission/more difficult to transmit information
(hyperpolarization)
• Depolarization-less negative; excitatory
• Hyperpolarization-more negative; inhibitory
• Results from flow of ions through gated channels

12. Electrical Potentials
2. Local Potential
• Initial change in membrane potential
• Only spreads short distance
• If sufficient depolarization (threshold level is reached),
action potential will develop
• Either depolarizes (excites) or hyperpolarizes (inhibits)
• Produced by information at modality-gated channel of
sensory neuron receptor (receptor potentials) or ligand-
gated channel at postsynaptic membrane (synaptic
potentials)

13. Local Potentials
• Strength of local potentials can
be combined
• Temporal summation: combined
effect of local potentials occurring
in rapidly
• Spatial summation: combined
effect of several small local
potentials occurring at same time
• Combined local potentials can
either promote or inhibit
generation of action potential

14. Electrical Potentials
3. Action Potential
• Larger change in electrical potential
• Large depolarization that repeatedly regenerated along
length of axon
• All-of-none—every time a minimally sufficient stimuli is
provided, an action potential occurs
• Spreads long distances by activation of voltage-gated
channels
• Transmits information down axon to presynaptic terminal
and causes release of neurotransmitter

15. Action Potentials
• A 15 mV depolarization (change in membrane potential from -70 mV
to -55 mV) produces action potential—threshold stimulus
• Three events of action potential:
1. Voltage-gated Na+ channels open and Na+ flows rapidly into cell
• Due to Na+ concentration gradient and attracted to – charge inside cell
2. Voltage-gated Na+ channels close
3. Voltage-gated K+ channels open and K+ leaves cell
• Due to repelled by + charge inside cell and by K+ concentration gradient
• Hyperpolarizes neuron temporarily
• More difficult initiate subsequent action potential (refractory period)
• Absolute refractory period: no amount of stimulus will result in action potential
• Relative refractory period: stronger than usual stimulus will produce action potential
• Promotes forward propagation of action potential toward presynaptic terminal
• Resting membrane potential restored by leak channels and then Na+-
K+ pump

17. Action Potentials
• Once initiated, propagated entire length of axon via
saltatory conduction at nodes of Ranvier
• For faster conduction:
1. Larger diameter axon
2. Myelinated axon
Fig 2.13 Lundy-Ekman

18. Action Potential
• http://youtu.be/U0NpTdge3aw

19. Functional Classification
Three functional groups of neurons:
1. Afferent
2. Efferent
3. Interneuron

20. Neuronal Interactions
Convergence
• Multiple inputs from
several neurons terminate
on single neuron
Divergence
• A single neuron branches
and synapses on multiple
neurons
Hearing Vision Touch
Sensory
Association
Area in
cortex
Pinprick
stimulates
sensory neuron
Motor response
Conscious pain
Unconscious pain

21. Glial Cells
Function
• Provide structure
• transmit information
• Involved in neural development
• repair following brain damage
Types
1. Macroglia
• Astrocytes—provide structure; regulate neuronal signaling; blood-brain
barrier; neural development; recovery from injury
• Oligodendrocytes—produce myelin sheath in CNS
• Schwann cells—produce myelin sheath in PNS; ONLY glial cell in PNS
2. Microglia
• Phagocytes
• Abnormal activation involved in several disease
Fig 2.16 Lundy-
Ekman

22. Glial Cells

23. Demyelination Diseases
PNS
• Guillain-Barre
• Antibodies attack Schwann cells
CNS
• Multiple Sclerosis
• Antibodies attack oligodendrocytes

24. Synapse: Structure
• Presynaptic terminal
• end of axon
• releases neurotransmitter
• Postsynaptic terminal
• receiving cell
• contains receptors
• gland, muscle cell,
another neuron
• Synaptic cleft
• space between

25. Synaptic Transmission
Fig 3.2 Lundy-Ekman

26. Synaptic Transmission
• http://youtu.be/Dzvy5UB2Xv4

27. Postsynaptic Potentials
• Local change in ion
concentration (local
potential)
• Excitatory postsynaptic
potential (EPSP)
• Depolarization
• Summation can lead to action
potential
• Inhibitory postsynaptic
potential (IPSP)
• Hyperpolarization
• Decreases chance of action
potential
Figs 3.4 and 3.5 Lundy-

28. Presynaptic Influence
• Presynaptic facilitation
• More neurotransmitter released
• Presynaptic inhibition
• Less neurotransmitter released
Fig 3.6 Lundy-Ekman

29. Neurotransmitters
• Acetylcholine—prevalent in PNS
• Amino acids
• Glutamate (Glu)
• Aspartate
• Glycine (Gly)
• Gamma aminobutyric acid (GABA)
• Amines
• Dopamine (DS)
• Norepinephrine (NE)
• Serotonin (5-HT)
• Histamine
• Peptides
• Substance P
• Endorphins
• Enkephalins
• Nitric Oxide

30. Neurotransmitters
Receptors
• Neurotransmitter must bind to receptor to have an effect
• Either direct or indirect effect
• Directly open ligand-gated ion channel
• Indirectly open ion channel through G-protein
• Activates cascade of intracellular events through second
messenger
• Can be internalized or inactivated

31. Neural Stem Cells
• Mature neurons cannot reproduce
• Neural stems cells discovered in developing brain and adult brain
• Undifferentiated precursors to both neurons and glial cells
• Two areas of the brain are most essential in producing new neurons
in an adult.
• subgranular zone of the hippocampal dentate gyrus and the subventricular
zone of the lateral ventricle wall (Landgren & Curtis, 2011)
• Possible role of stem cells as brain cell implants for rehabilitation after
injury or disease
• Treat ALS, MS, or spinal cord injury
• The transplantation of stem cells into the brain of mice resulted in myelination
of axons and reduced the activity of glia cells (Cristofanilli et al, 2011)
• Several phase 1 studies in humans
• Difficult to obtain from the brain
• Umbilical cord and bone marrow can serve as sources of neural stem cells
(Venkataramana et al., 2010)
• Advances in engineering of stem cells
• Reprogrammed human skin cells into pluripotetent embryonic stem cells (Masahito et al.,
2013)

32. Fig 2.8 Lundy-Ekman