Chapter 10

Nervous System I

PowerPoint Presentation to accompany Hole’s Human Anatomy and Physiology, 10th edition, edited by S.C. Wache for Biol2064.01

 You are responsible for the following figures and tables:

Tab. 1.2 - Nervous System Function.Fig. 10.1 - Typical neuron of the nervous system.Fig. 10.2 Organization of the nervous system and division into CNS and PNS.Fig. 10.3, 10.4 - Common neuron of the PNS.Fig. 10.6 - Structural classification of neurons.Fig. 10.7 - Functional classification of neurons.Fig. 10.8 Tab. 10.2 - Neuroglial cells of the CNS. Fig. 10.10 - Regeneration of nerve cell axons.Read TB, pp.348-354, on cell membrane potentials.Fig. 10.14 - Action potential characteristics.Fig. 10.18 - Synapses connecting nerve cells.Compare p.283, Fig. 9.8 - NMJ or neuromuscular junction.Tab. 10.4 - Neurotransmitters.  Tab. 10.6 - Drugs that affect levels of neurotransmitters. [see table in the attached lecture handout]

• CNS: brain, spinal cord – Functions: integrative• PNS (Peripheral Nervous System): cranial and spinal nerves - Functions: sensory, motor• Note: The PNS has somatic and autonomic branches:

Somatic: sensing and conscious activitiesANS (Autonomic Nervous System): controls unconscious activities

Fig. 10.7

neurons neuron

neuron

• Sensory (afferent) neurons carry impulses from the body to brain or spinal cord.

• Interneurons (association neurons) lie within brain and spinal cord. They form links with other neurons.

• Motor (efferent) neurons carry nerve impulses out of the brain or spinal cord to effectors, muscles or glands.

Functions of Nervous System Neurons

Functions of the Nervous System

• Nervous tissue is composed of nerve cells or neurons and neuroglia.• Neurons react to their environment.• Dendrites receive input.• Axons carry information away from the cell.• Information is carried across the synapse (the space between neurons) by

neurotransmitters.

Neuron

Fig. 10.1

Parts of the Neuron• The cell body contains granular cytoplasm,

organelles, and microtubules

• Neurofibrils support fibers in the soma

• Chromatophilic substance in cytoplasm

• Dendrites are branched receptive structures

• One axon arises from the axon hillock and has branches, collaterals. Axons end in a synaptic knob at the terminal, separated from another receptive surface by the synaptic cleft

Neuron

Schwann Cell

Fig. 10.3

Neuri-lemma

Nucleus

Neurofibrils

Neuron Structure

• Schwann cells are neuroglial cells that produce the myelin sheath, a lipid-rich covering of the axon.

• A neurilemma surrounds the myelin sheath.

• Nodes of Ranvier are gaps in the sheath.

• Fibers with myelin sheaths are myelinated nerve fibers (white matter).

• Fibers without myelin are unmyelinated nerve fibers (gray matter).

Neuron Classification

• Bipolar neurons: two processes from the cell body. These are found in sense organs.

• Unipolar neurons: one process from the cell body. These are found in sense organs.

• Multipolar neurons: many processes extend from the cell body, one axon and many dendrites. These are found in the brain and spinal cord.

Neuroglia Classification

Neuroglia Classification

• Astrocytes: star-shaped cells that regulate environment around neurons and aid in healing

brain injury.

• Oligodendrocytes: form myelin in the brain and spinal cord.

• Microglia: support neurons and phagocytize bacteria and cell debris.

• Ependyma: regulate the composition of cerebrospinal fluid.

Fig. 10.10

Nerve Fiber Regeneration

Problem: Mature neurons do not divide so neurons are not replaced in response to injury.However: Damaged axons may regenerate.• The distal portion deteriorates and is

phagocytosed. The proximal end, an extension of the nerve cell membrane, grows in response to nerve growth factors.

• Schwann cells will regenerate the myelin sheath.

+

-

+

Cell Membrane Potential (Fig. 10.12)

Observation: The cell membrane is polarized:

the inside is overall negatively charged with

respect to the outside which is positively charged.

This is due to ion distribution and concentration.

• K+ is the major intracellular cation, Na+ is the major extracellular cation.

• The Na+/K+ pump actively pumps Na+ out of and K+ into the cell.

RMP (Resting Membrane Potential)

Observation: A resting nerve cell is one at rest, that is, it has a negative membrane potential of -70mV because the membrane is not stimulated and no nerve impulse or action potential is developing.

• The resting cell membrane is slightly more permeable to K+ than Na+.

• The nerve cytoplasm contains anions which cannot diffuse through the membrane.

Local Potential Changes along a NerveStimulation Phase of the Action Potential:• Depolarization: the membrane potential becomes less negative.

Relaxation Phase of the Action Potential:• Hyperpolarization: membrane potential becomes more negative than the RMP.

Stimulation depends on Graded Local Potentials: • The amount of change in potential is proportional to the intensity of

the stimulation.• Threshold potential: level of stimulation to generate an action

potential.• Summation: several stimuli can add up if close together to reach

threshold.

• The action potential causes an electrical current that stimulates adjacent regions.

• A series of action potentials occur sequentially along the length of the nerve.

Steps in the Formation of an Action Potential

• Voltage-gated Na+ channels open in the trigger zone of the axon hillock. (Fig. 10.13)

• Na+ diffuses inward, depolarizing the membrane, reaching +30 mV. (Fig. 10.13).

Steps in the Formation of an Action Potential

• Voltage-gated K+ channels open, K+ diffuses outward, repolarizing the membrane. (Fig. 10.13).

Steps in the Formation of an Action Potential

End of Action Potential

• Refractory period: short time following a nerve impulse that a threshold stimulus will not trigger another impulse.

• Absolute refractory period: Na+ channels are still open.

• Relative refractory period: membrane is reestablishing its potential.

Impulse Conduction

• If a nerve fiber responds, it responds completely which is known as an all-or-none response.

• Unmyelinated fibers conduct an impulse over their entire surface.

• Myelinated fibers conduct action potentials at the nodes. This is saltatory conduction which allows faster transmission of the signal.

• Also: Thicker fibers conduct faster.

Saltatory Impulse Conduction on Myelinated Fibers

(Fig.10.16)

Synaptic Transmission

Definition: A presynaptic neuron brings the impulse / action potential to the synapse to stimulate or inhibit a postsynaptic neuron.• At the synapse, voltage-gated Ca++ channels in the cell

membrane open and Ca++ diffuses into the cytoplasm.• This induces exocytosis of neurotransmitters.

Synaptic potentials are graded. They can depolarize or hyperpolarize the receiving postsynaptic cell membrane.• Excitatory postsynaptic potentials (EPSP) open Na+

channels and depolarize the membrane.• Inhibitory postsynaptic potentials (IPSP) open K+ channels

and hyperpolarize the membrane.

Neurotransmitters

Definition: a substance (as norepinephrine or

acetylcholine) that transmits nerve

impulses across a synapse.

• Acetylcholine: stimulates muscle contractions

• Monoamines: epinephrine, norepinephrine, dopamine, serotonin

• Amino acids: glycine, glutamic acid, GABA

• Peptides: enkephalins, substance P

Neuropeptide Neurotransmitters that Determine Pain Levels Felt

• Substance P: helps perceive pain.

• Enkephalins: either of two pentapeptides with opiate and analgesic activity that occur naturally in the brain and have a marked affinity for opiate receptors: a : LEUCINE-ENKEPHALIN ; b : METHIONINE-ENKEPHALIN .

They bind to opiate receptors and relieve pain.

• Endorphins: any of a group of endogenous peptides (as enkephalin and dynorphin) found especially in the brain that bind chiefly to opiate receptors and produce some of the same pharmacological effects (as pain relief) as those of opiates.

• Beta-endorphin: binds opiate receptors and lasts longer.

Impulse Processing in the CNS

CNS Interneurons (integrative function) are

organized into neuronal pools, groups of

neurons that work together

• Convergence: a single neuron receives impulses from two or more fibers, allows collection and processing of information

• Divergence: an impulse from one neuron stimulates two or more fibers. This may amplify a response

Fig. 10.17

Fig. 10.17

Drugs that Alter Neurotransmitter (NT) Levels – Tab. 10.6, p358.

DRUGNT AFFECTED

MECHANISM OF ACTION EFFECT

Tryptophan Serotonin Stimulates NT synthesis sleepiness

Reserpine Norepinephrine packages NT vesicles limb tremors

Curare Ach decreases NT in NMJ muscle paralysis

Valium GABA enhances receptor binding decreased anxiety

Nicotine binds Ach receptors

ACh Increased ACh-esterase increased alertness

Cocaine Norepi blocks reuptake euphoria

Tricyclic Antidepressants

Norepinephrine blocks reuptake mood elevation

Prozac Serotonin blocks reuptake/induces new tissue synthesis of serotonin

mood elevation

Caffeine Epinephrine no interactions reported, but cannot be excluded

additive effect on heart rate