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

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

Nervous System Organization

• The ANS is divided into two parts: The sympathetic nervous system = “fight or flight” The parasympathetic nervous system “rest and digest”

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

Nervous System Tissue: Neuron

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

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.

Nervous System Tissue: Neuron Types

Nervous System Tissue: Neuron Types

Nervous System Tissue: Neuron Types

Nervous System Tissue: Neuron Types

Nervous System Tissue: Glial Cells

• 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

• 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

Nervous System Tissue: Glial Cells

Nervous System Tissue: Myelin Sheaths

• Schwann cells myelinate a single axon

• Oligodendrocytes myelinate parts of many axons

• Schwann cells have a neurolemma, which can guide axon regrowth

• Schwann cells form the myelin sheaths around axons in the PNS

• Myelin sheaths insulate neurons and increase the speed of the action potential

Nervous System Tissue: Myelin Sheaths

• Gaps between myelin sheaths are nodes of Ranvier

• Satellite cells provide structural support and regulate ions in the PNS

Nervous System Tissue: Myelin Sheaths

• Schwann cells can guide axon regrowth in the PNS

Nervous System Tissue: Myelin Sheaths

• Oligodendrocytes can myelinate more than one axon

Nervous System Tissue

• Nodes of Ranvier are gaps in the myelin sheath

Nervous System Tissue: Gray & White Matter

• Gray matter consists of cell bodies, unmyelinated axons, dendrites, and glial cells

• White matter consists of myelinated axons

Overview of Neural Transmission

• 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

• 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

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

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

Graded (Local) Potentials: Excitatory & Inhibitory Signals

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 -55 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

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 (0.5-1.5 microns & 2 m/sec) unmyelinated

sensory & autonomic motor

Nervous System Tissue: Fiber Diameter and Conduction Velocity

The fatter the fiber, the faster it flies

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

Continuous versus Saltatory Conduction

• From the Latin verb saltare, which means “to leap”• The saltarello was a dance of the Middle Ages

The Saltarello

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

• 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.

Comparison of Graded & Action Potentials