section 1, chapter 10

Nervous System IBasic Structure and Function

ivyanatomy.com

The nervous system is divided into 2 subdivisions

The Central Nervous System (CNS) consists of the brain and spinal cord.

The Peripheral Nervous System (PNS)• Consists of 12 pairs of cranial

nerves and 31 pairs of spinal nerves

• Nerves may be motor (efferent), sensory (afferent), or both (mixed)

Divisions of the PNS

The Somatic Nervous System is under voluntary control Somatic motor controls skeletal muscles

Somatic sensory relays info regarding touch, pressure, and pain to the brain

The Autonomic Nervous System is under involuntary control autonomic motor controls smooth muscles, cardiac muscles,

and glands

autonomic sensory relays visceral info regarding pH, blood gasses, etc. to the brain

Figure 10.2. (a) overview of nervous system. CNS is grey, PNS is yellow. (b) CNS receives sensory input from PNS, and sends motor output to PNS. Somatic division of PNS is under voluntary control, while the autonomic division is under involuntary control.

The Autonomic Nervous System (ANS) is further divided into two branches.

The Sympathetic branch • prepares the body to respond to a stressful situation.• “Fight or Flight” Response

The Parasympathetic branch • Maintains normal body activities at rest• “Resting and Digesting”

Cells of the Nervous SystemNeurons• Integrate, regulate, and coordinate body functions

• Functions• Receive information - sensory • Conduct impulses - motor• Connect neurons - integrative

Neuroglia (glia = “glue”)• Neuroglia provide neurons with

nutritional, structural, and functional support

Neurons

Neurons vary in shape and size

3 Components of a neuron1. Dendrites receive impulse

2. Call body (soma)

3. Axon – transmits the impulse away from the cell body

Dendrites conduct information to the soma.

A cell may have a few dendrites, many dendrites, or no dendrites.

Dendritic Spines are additional contact points on some dendrites that increase the number of synapses possible by a neuron

Cell Body – SomaContains organelles such as the nucleus Mitochondria, Golgi Apparatus, etc.

The rough ER is often called chromatophilic substance (Nissl Bodies)

Dendrites

Axon

Axon Hillock is a specialized part of the soma that connects to the axon.

The axon hillock is often called the Trigger Zone because action potentials begin here.

Each neuron has only 1 axon, but it may divide into several branches, called collaterals

The end of the axon is called the axon terminal and it enlarges into a synaptic knob (bouton)

Microtubules called neurofibrils support long axons and aid in axonal transport (transport of biochemicals between the soma and the axon terminal)

Axon

Schwann Cells form the myelin sheath in the PNS. They wrap around the axons in a jelly-roll fashion to form a thick layer of fatty insulation.

The cytoplasm and the nucleus are pushed to the outermost layer, forming the neurolemma

Schwann cells are separated by gaps, called Nodes of Ranvier.

Myelination of axons occurs differently in the PNS than in the CNS.

Myelination of Axons

The myelin sheath is a thick fatty coating of insulation surrounding the axon that greatly enhances the speed of impulses.

Myelination of axons in the PNS.

Schwann cells still surround the axons of unmyelinated neurons, but they do not form the myelin sheath.

Unmyelinated axons with a Schwann cell.

A myelinated axon with a Schwann Cell.

Myelination of Axons in CNS

end of section 1, chapter 10

Within the CNS the myelin sheath is formed by Oligodendrocytes.

1 Oligodendrocyte may form the myelin sheath of several axons.

A mass of myelinated axons in the CNS forms the white matter.

A mass of cell bodies (which are unmyelinated) along with unmyelinated axons form the gray matter.

Gray and White Matter

Chapter 10, Section 2

Neurons and Neuroglia

ivyanatomy.com

Structural Classification of Neurons

A multipolar neuron contains many dendrites and 1 axon. • Includes most neurons in the brain and motor neurons

A bipolar neuron contains 1 dendrite and 1 axon• Includes some sensory neurons such as

photoreceptors and olfactory neurons

A unipolar neuron contains a single process extending from the soma• Example includes the cells of the dorsal root ganglion

Peripheral Process – conducts information from PNS Central Process – conducts information to CNS

structural classifications of neurons

Functional Classification of Neurons

An afferent or sensory neuron conducts information from the PNS to CNS• Dendrites may act as receptors (eyes, ears, touch)• Most afferent neurons are unipolar, and some are bipolar

An efferent or motor neuron conducts impulses from CNS to PNS

Voluntary Control – in somatic nervous systemInvoluntary control – in autonomic nervous system

An interneuron or association neuron is located completely within the CNS. Interneurons link neurons together in the CNS, and they also connect sensory neurons to motor neurons.

Functional Classification of Neurons

Figure 10.7. Neurons classified by their functions. Sensory, Motor, and Interneurons.

Neuroglia in the CNS are different from those in the PNS

Astrocytes “star-shaped” attach blood vessels to neurons.

Astrocytes aid in metabolism, strengthen synapses, and participate in the blood-brain-barrier

Ependymal cells line the central canal of the spinal cord and the ventricles of the brain.

Ependyma regulate the composition of cerebral spinal fluid (CSF)

Neuroglia in CNS

Neuroglia in CNS

Microglia are normally small cells, but they enlarge into macrophages during an infection.

• Phagocytize bacteria and cell debris

Oligodendrocytes form the myelin sheath in the CNS

Figure 10.8. Types of neuroglia in the CNS. Neuroglia compose half of the brain’s volume

Neuroglia of the PNSSchwann Cells form the myelin sheath in the PNS.

Satellite Cells support clusters of cell bodies, called ganglia in PNS.

Multiple Sclerosis (MS) The immune system attacks neurons in the CNS, destroying the

myelin sheath of neurons.

The damaged myelin sheath is replaced with Connective tissue, leaving behind scars (scleroses)

Scars block the transmission of underlying neurons, so muscles no

longer receive stimulation and begin to whither (atrophy).

Disorders of Neuroglia

End Section 2, Chapter 10

section 3, chapter 10

The Synapse And

Membrane Potential

ivyanatomy.com

Synaptic transmission is the mechanism that transmits a signal from the pre-synaptic neuron to the post-synaptic neuron.

Synaptic Transmission

An action potential causes the release of neurotransmitters from the presynaptic cell that diffuse across the synapse and bind to the postsynaptic cell.

Steps involved in Synaptic Transmission

1. A nerve impulse (action potential) travels down the axon to the axon terminal.

2. The action potential opens calcium channels causing calcium to diffuse into the synaptic knob.

3. The calcium influx triggers the release of neurotransmitters from synaptic vesicles into the synapse.

4. The neurotransmitters diffuse across the synapse and bind to receptors on the post-synaptic cell

Some neurotransmitters are inhibitory whereas others are excitatory, so the post-synaptic cell may be stimulated or it may be inhibited depending on the neurotransmitter.

Cell Membrane Potential

The cell membrane is usually polarized (charged)

• Inside the membrane is negatively charged relative to outside the membrane

• Polarization is due to unequal distribution of ions across the membrane

• Polarization is maintained by a series of ion pumps and channels

The sodium/potassium pump actively transports 3Na+ out of the cell, and 2K+ into the cell.• It creates a high extracellular [Na+] and a high intracellular [K+]

• requires ATP

• The Na+/K+ pump only contributes a small amount (-5mV) to the membrane potential.

Factors that maintain the cell membrane potential

1. Sodium/Potassium (Na+/K+) pump

Factors that maintain the cell membrane potential

2. Non-gated potassium channels “K+ leak channels”• The cell membrane has many K+ leak channels, but

only a few Na+ leak channels

• K+ continually leaks out of the cell, making the inside of the cell more negative.

Figure A. The sodium-potassium pumps transports sodium out of the cell, while transporting potassium into the cell.

Figure B. Leak channels allow some of the potassium to leak out of the cell, contributing to the positively charged extracellular fluid.

Factors that maintain the cell membrane potential

Factors that maintain the cell membrane potentialThe distribution of ions across the membrane creates a membrane potential (electrical gradient).

For a neuron at rest the membrane potential is -70mV inside the cell. This is the Resting Membrane Potential

RMP = -70mV inside the cell.

RMP = -70 mV inside the cell.

Factors that change the cell membrane potentialGated ion channels open and close in response to a stimulus.

Gated Ion Channels

1. Mechanically-Gated Channels• Open or close in response to physical stress.• Touch, hearing, vibrations, ect.

2. Ligand-Gated Ion Channels• Open or close in response to a ligand

(neurotransmitter, hormone, or other molecule)• Includes ACh receptors on motor endplates

3. Voltage-Gated Ion Channels• Open or close in response to small changes in the

membrane potential (millivolts = mV)• Voltage-gated Na+ channels open when membrane

potential reaches -55mV.

Figure 10.15b. Ligand-gated Na+ channels (blue) open in response to neurotransmitters. Voltage-gated Na+ channels (pink) open in response to changes in membrane potential.

Gated Ion Channels

End of section 3, chapter 10

Chapter 10, Section 4

Graded and Action Potentials

Changes in Membrane Potential

Resting Membrane Potential (RMP) for a neuron = -70mV• Membrane potential of a cell at rest

Environmental stimuli cause changes in membrane potential by opening gated ion channels• Ligand-gated ion channels• Voltage-gated ion channels• Other-gated ion channels

(respond to mechanical, temperature, or other stimulus)

If membrane potential becomes more negative, it has hyperpolarized e.g. A membrane potential of -100mV is hyperpolarized

If membrane potential becomes less negative, it has depolarizede.g. A membrane potential of -60mV is depolarized

Local Potential Changes

Graded Potentials• Local changes in membrane potential (usually occurs at dendrites)• Magnitude of response is proportional to stimulus • Graded potentials summate (add together)• Graded potentials generate action potentials

If a graded potential reaches threshold stimulus (-55mV), it results in an action potential

Summation of Graded Potentials

Summation of graded potentials my occur by:1. Spatial Summation – stimulating multiple dendrites2. Temporal Summation – Stimulating a dendrite at a high frequency3. Combined – stimulating multiple dendrites at a high frequency

Graded Potentials are summed together at the Axon Hillock “Trigger Zone”• If summation of graded potentials reaches threshold stimulus (-55mV), an action

potential is initiated at the axon hillock.

Figure 10.15. (a) a subthreshold depolarization will not result in an action potential. (b) Summation of graded potentials may reach threshold stimulus, initiating an action potential at the trigger zone. The action potential begins when voltage-gated Na+ channels open at the trigger zone.

3 Phases of an Action Potential

1. Depolarization Phase• Voltage-gated Na+ channels open at

-55mV (threshold stimulus)• Na+ diffuses into cell

2. Repolarization Phase• Voltage-gated K+ channels open at

+30mV• K+ rushes out of the cell repolarizing

the membrane• Na+ channels close

3. Hyperpolarization Phase• The slower voltage-gated K+ channels

remain open briefly, resulting in a slight hyperpolarization (-90mV).

3

21

Figure 10.17. An oscilloscope records and action potential

Action Potential

Figure 10.16(a) At rest, the membrane is polarized (RMP = -70mV). Sodium is mostly outside the cell and potassium is within the cell.

Figure 10.16(b) When a stimulus reaches threshold stimulus (-55mV), voltage-gated Na+ channels open. With Na+ channels open, sodium rapidly diffuses into the cell, depolarizing the membrane up to +30mV.

Action Potential

Figure 10.16(c) When the membrane reaches +30mV, voltage-gated K+ channels open an quickly repolarize the membrane. Sodium channels also close at this point.

Following an action potential, Na+/K+ pumps work to actively reestablish the Na+ and K+ concentration gradients.

Action Potential PropagationOnce initiated an action potential is propagated along the entire axon at full strength. It does not weaken.

Figure 10.18An action potential in one region, depolarizes the adjacent region to threshold stimulus (-55mV).

Once the adjacent region reaches threshold stimulus, it triggers another action potential.

The second action potential causes depolarization in its adjacent region, triggering yet another action potential.

This sequence continues all the way to the end of the axon at full strength.

All-Or-None Response

All-or-none response• Action potentials occur completely, or they do not occur at all.

• An action potential occurs whenever a stimulus of threshold intensity or above is applied to a neuron.

• Greater stimulation does not produce a stronger impulse (although a greater stimulation will produce more impulses per second)

Refractory Period

Refractory Period: For a brief period following an action potential, a threshold stimulus will not trigger another action potential.

Absolute Refractory Period• no new action potentials can be produced• Occurs while the membrane is changing in sodium permeability• Between the depolarization and repolarization phases

Relative Refractory Period• Action potential can be generated with a high intensity stimulus• Occurs while membrane is reestablishing its resting membrane potential• Lasts from the hyperpolarization phase, until RMP is reestablished

End of Chapter 10, Section 4

section 5, chapter 10

Impulse Conduction

&Neurotransmitters

ivyanatomy.com

Impulse ConductionMyelinated axons conduct impulses differently than unmyelinated axons.

Unmeylinated Axon generates a series of action potentials along the entire axon The impulse is slow (travels at 1 mile/hour)

Myelinated AxonMyelin is an electrical insulator and prevents action potentials along myelinated portions of the axon.

Action potentials are generated only at the Nodes of Ranvier

The impulse travels through the myelinated portions by electrical conduction.

The impulse is fast (travels at 285 miles/hour!)

Saltatory Conduction – Action potentials appear to jump from node to node on myelinated axons

Figure 10.19. On a myelinated axon, a nerve impulse appears to jump from node to node.

Myelinated Vs. Unmeylinated neurons

Myelinated neurons transmit impulses rapidly whereas unmyelinated neurons transmit impulses slowly.

Example: Think when you cut yourself with a knife.

The sharp instant pain travels on myelinated neurons. Shortly after, the slow throbbing pain travels on unmyelinated neurons.

The summary of events leading to the release of neurotransmitters. These events are also outlined in chapter 10, section 3

Synaptic Transmission

Synaptic Transmission

Neurotransmitters diffuse across the synapse and bind to receptors (ligand-gated ion channels) on postsynaptic dendrites.

The neurotransmitters cause changes in local (graded) membrane potential on postsynaptic neuron = synaptic potentials

The neurotransmitters may either excite the post-synaptic cell or it may inhibit the post-synaptic cell.

Synaptic Potentials

• EPSP = Excitatory postsynaptic potential EPSPs depolarize the local membrane of the postsynaptic neuron

EPSPs increase the likelihood of generating an action potential.

• IPSP = Inhibitory postsynaptic potential IPSPs hyperpolarize the local membrane of the postsynaptic neuron

IPSPs decrease the likelihood of generating an action potential

Summation of EPSPs and IPSPs

EPSPs and IPSPs are added together in a process called summation

Summation occurs at axon hillock

The integrated sum of EPSPs and IPSPs determines if an action potential occurs

If threshold stimulus is reached an action potential is triggered.

Figure 10.20 The synaptic knobs of many axons may communicate with the cell body of a neuron.

NeurotransmittersThe nervous system produces at least thirty different types of neurotransmitters.

Examples:1. Acetylcholine – skeletal muscle contractions

2. Monoamines• Norepinephrine

- in CNS it creates a sense of well-being- in PNS it may stimulate or inhibit autonomic nervous system

• Dopamine- in CNS it creates a sense of well-being- Amphetamines increase the levels of norepinephrine and dopamine

3. Amino Acids• GABA – inhibitory neurotransmitter of the CNS

• Many sedatives and anesthesia enhances GABA secretions• Schizophrenia is associated with a deficiency of GABA

4. Gases• Nitric Oxide

• Vasodilation in PNS

Impulse ProcessingNerve impulses are processed by the CNS in a way that reflects the organization of neurons in the brain and spinal cord.

Neuronal Pools – organized group of interneurons within the CNS.

Pools are organized as neural circuits that perform a common function, even though they may be in different parts of the CNS

May have either excitatory or inhibitory effects on effectors or other pools

Convergence – several neurons synapse onto one post-synaptic neuron• A neuron may sum impulses from different

sources

e.g. Information from various sensory receptors may converge onto a single processing center

Divergence – impulse spreads from one axon to several post-synaptic neurons.

• A single neuron may ultimately stimulate many neurons - Amplifies an impulse

Neuronal Pools

End of chapter 10