1. Nervous System Overview
The nervous system is the body’s electrical system
It is broadly divided into two parts:
Central nervous system = CNS (brain and spinal cord)
Peripheral nervous system = PNS (nerves and ganglia)
The nervous system performs three important functions:
Sensory input
Integration
Motor output

2. Nervous System Organization
The PNS is divided into three parts:
The somatic nervous system (SNS) = “voluntary” = “skeletal muscles”
The autonomic nervous system (ANS) = “involuntary” = “smooth muscles”
The enteric nervous system (ENS) = “brain of the gut” = “GI tract NS”
All communicate with the CNS

3. Nervous System Organization
The ANS is divided into two parts:
The sympathetic nervous system = “fight or flight”
The parasympathetic nervous system “rest and digest”

4. Nervous System Tissue: Neuron
The neuron is the basic cell of the nervous system
Neurons communicate with other neurons and tissues by action potentials
Neurons have special processes:
Dendrites receive input
Axons transmit action potentials
The axons of some neurons are coated with a fatty substance called myelin
Gaps in the myelin coatings are nodes of Ranvier

5. Nervous System Tissue: Neuron

6. Nervous System Tissue: Synapses
Axodendritic: axon synapses with dendrite
Axosomatic: axon synapses with cell body
Axoaxonic: axon synapses with another axon
A synapse is the junction between two neurons

7. Nervous System Tissue: Neuron Types
Multipolar neurons have many dendrites and one axon. Most CNS neurons are multipolar.
Bipolar neurons have one dendrite and one axon. They are found in many special sense organs.
Unipolar neurons have one process that diverges from the cell body and forms dendrites on one end and axon terminals on the other. They are found in many ganglia of the cranial and spinal nerves.

8. Nervous System Tissue: Neuron Types

9. Nervous System Tissue: Neuron Types

10. Nervous System Tissue: Neuron Types

11. Nervous System Tissue: Neuron Types

12. Nervous System Tissue: Glial Cells
Glial cells (neuroglia) are the “glue” of the NS
They also perform many functions
There are six basic types:
Astrocytes (CNS)
Microglia (CNS)
Oligodendrocytes (CNS)
Ependymal cells (CNS)
Schwann cells (PNS)
Satellite cells (PNS)
Glial cells outnumber neurons about 10 to 1
Astrocytes provide structural support, form the blood-brain barrier, and regulate ions
Microglia function as phagocytes
Ependymal cells line the ventricles and spinal canal, and produce cerebrospinal fluid
Oligodendrocytes form the myelin sheaths around axons in the CNS

13. Nervous System Tissue: Glial Cells

14. Nervous System Tissue: Myelin Sheaths
Schwann cells form the myelin sheaths around axons in the PNS
Myelin sheaths insulate neurons and increase the speed of the action potential
Schwann cells myelinate a single axon
Oligodendrocytes myelinate parts of many axons
Schwann cells have a neurolemma, which can guide axon regrowth

15. Nervous System Tissue: Myelin Sheaths
Gaps between myelin sheaths are nodes of Ranvier
Satellite cells provide structural support and regulate ions in the PNS

16. Nervous System Tissue: Myelin Sheaths
Schwann cells can guide axon regrowth in the PNS

17. Nervous System Tissue: Myelin Sheaths
Oligodendrocytes can myelinate more than one axon

18. Nervous System Tissue
Nodes of Ranvier are gaps in the myelin sheath

19. Nervous System Tissue: Gray & White Matter
Gray matter consists of cell bodies, unmyelinated axons, dendrites, and glial cells
White matter consists of myelinated axons

20. Overview of Neural Transmission
NT’s bind to receptors on the postsynaptic neuron, which often opens ion channels
The flow of ions causes a an electrical current in the membrane
These graded potentials can be either
Positive/Depolarization/Excitatory
Negative/Hyperpolarization/Inhibitory
The graded potentials are summed in the axon hillock
If the sum exceeds threshold, then the postsynaptic neuron will fire an action potential
When the action potential reaches the synaptic end bulb, NT’s are released, and the cycle begins again
Neurons receive input from other neurons, especially through their dendrites
Neurons send output in the form of an action potential (AP) along their axons
When an AP arrives at the synaptic end bulb of the presynaptic neuron, neurotransmitter (NT) is released

21. The Action Potential (AP)
The AP propagates along the axon
As the wave of depolarization moves along the axon, Na+ and K+ channels open in sequence
Eventually the AP reaches the synapse and neurotransmitters are released

22. Graded or Local Potentials (GP)
Small deviations from resting potential of -70mV due to the opening of ion channels
Depolarization = membrane has become more positive
Hyperpolarization = membrane has become more negative
The signals are graded, meaning they vary in amplitude (size), depending on the strength of the stimulus
The signals are localized
Graded potentials occur most often in the dendrites and cell body of a neuron

23. Graded (Local) Potentials: Excitatory & Inhibitory Signals

24. Threshold and the Trigger Zone
Graded potentials that arise in dendrites converge in the axon hillock
Graded potentials are summed
If the sum exceeds the threshold value of the neuron (usually about mV), then an action potential arises in the trigger zone
Unlike graded potentials, the action potential is all-or-none
When an action potential arises, it propagates along the axon
When it reaches the axon terminals it may synapse with another neuron and cause graded potentials in that neuron

25. Speed of Impulse Propagation
The propagation speed of a nerve impulse is not related to stimulus strength
Larger fibers conduct impulses faster due to size (less resistance)
Myelinated fibers conduct impulses faster due to saltatory conduction
Fiber types
A fibers largest (5-20 microns & 130 m/sec)
myelinated somatic
sensory & motor to skeletal muscle
B fibers medium (2-3 microns & 15 m/sec)
myelinated visceral
sensory & autonomic preganglionic
C fibers smallest ( microns & 2 m/sec)
unmyelinated
sensory & autonomic motor

26. Nervous System Tissue: Fiber Diameter and Conduction Velocity
The fatter the fiber, the faster it flies

27. Continuous versus Saltatory Conduction
Continuous Conduction (unmyelinated axons)
Step-by-step depolarization of each portion of the length of the axolemma
Saltatory Conduction (myelinated axons)
Depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels
The myelin sheath and nodes of Ranvier act like a capacitor, and the impulse “leaps” down the axon, regenerating itself at each node

28. Continuous versus Saltatory Conduction
From the Latin verb saltare, which means “to leap”
The saltarello was a dance of the Middle Ages

29. The Saltarello

30. Nerve Transmission: The Story So Far
An action potential (AP) propagates over the surface of the axon membrane
Na+ flows into the cell causing a dramatic depolarization
In response to depolarization, adjacent voltage-gated Na+ and K+ channels open, self-propagating along the membrane
K+ flows out of the cell causing a dramatic hyperpolarization, the resting potential of the membrane is gradually restored, following a refractory period
The traveling AP is called a nerve impulse
When the nerve impulse reaches the synaptic end bulbs of the axon terminals, neurotransmitters are released
When neurotransmitters bind to receptors on the dendrites and soma of the postsynaptic neuron, they produce graded potentials
The graded potentials are summed in the axon hillock (trigger zone)
If the sum exceeds threshold, then an action potential is produced

31. Comparison of Graded & Action Potentials
Origin
GPs arise on dendrites and cell bodies
APs arise only at the trigger zone on the axon hillock
Types of Channels
AP is produced by voltage-gated ion channels
GP is produced by ligand or mechanically-gated channels
Conduction
GPs are localized (not propagated)
APs conduct (propagate) over the surface of the axon
Amplitude
amplitude of the AP is constant (all-or-none)
graded potentials vary depending upon stimulus strength
Duration
The AP is always the same
The duration of the GP is as long as the stimulus lasts
Refractory period
The AP has a refractory period due to the nature of the voltage-gated channels, and the GP has none.