Physiology Chapter 6

Central Nervous System

Brain and spinal cord

Peripheral Nervous System

Consist of the nerves that connect the brain

Cell Body (Soma)

Contains nucleus and ribosomes

Dendrites

Receive inputs from other neurons

Axon

Carries output to target cells

Axon Terminals

Responsible for releasing neurotransmitters from axon; diffuse across an extracellular gap, called a synapse

Glial Cells

Surround soma, axon, and dendrites of neurons and provide them with physical and metabolic support

Obligodendrocytes

Produce myelin sheath in CNS

Schwann Cells

Form myelin sheaths around the larger nerve fibers in PNS

Nodes of Ranvier

Spaces between adjacent sections of myelin where the axon's plasma membrane is exposed to extracellular fluid

Astrocytes

Support cells, control extracellular environment of neurons

Ependymal Cells

Ciliated, production and movement of Cerebrospinal fluid

Microglia

Immune system of CNS

Afferent Neurons

Convey information from the tissues and organs of the body towards the CNS; contain sensory receptors; propagate electrical signals from their receptors into brain and spinal cord; usually only contain an axon

Efferent Neurons

Convey information away from the CNS to effector cells like muscle, gland, or other nerve cells; their cell bodies and dendrites are within CNS, and axons are in periphery

Interneurons

Connect neurons within the CNS (majority of neurons); entirely in CNS; function as integrators and signal changers; integrate gouts of afferent and efferent neurons into reflex circuits; 99% of all neurons

Sensory Receptors

Respond to various physical or chemical changes in their environment by generating electrical signals in the neuron

Nerves

Groups of afferent and efferent neurons, together with connective tissue and blood vessels

Synapse

Anatomically specialized junction between two neurons where one neuron alters the electrical and chemical activity of another

Presynaptic Neuron

Neuron that conducts a signal toward the synapse

Postsynaptic Neuron

Neuron conducting signals away from synapse

Electric Potential

Potential energy associated with attractive charges of opposite sign

Potential Difference

Difference in the amount of charge between two points

Current

Movement of electrical charge

Resistance

Hinderance to current

Resting Membrane Potential

All cells, at resting conditions, have a potential difference across the plasma membranes, with the inside of the cell negatively charged with respect to the outside

Depolarized

When potential becomes less negative (closer to 0)

Repolarized

When membrane potential that has been depolarized returns toward resting value (-70)

Hyperpolarized

Potential is more negative than resting (-70)

Threshold Stimuli

Stimuli that is just strong enough to depolarize the membrane enough to cause the positive feedback of Na (usually 15mV lower than rest, -55mV)

Absolute Refractory Period

During depolarization, section cannot be stimulated, cannot trigger another AP, when Na+ gates open

Relative Refractory Period

During depolarization, membrane can be stimulated by a STRONG stimulus, AP won't be as strong

Electrical Synapse

Pre- and post-synaptic cells are connected by gap junctions; allow the local currents resulting from arriving action potentials to flow DIRECTLY across the junction through the connecting channels

Chemical Synapse

Pre-synaptic neurons release neurotransmitter from their axon terminals, neurotransmitter binds to receptors on post-synaptic neurons; contain synaptic cleft that prevents direct propagation of current

Axon Transport

1. It refers to the passage of materials from the cell body of neuron to the axon terminals
2. It refers to the passage of materials from axon terminals to the cell body of a neuron
3. It is especially important for maintaing the integrity of neurons with

Glial Cells

1. They form myelin
2. They deliver fuel molecules to neurons and remove the waste products of metabolism
3. They are important for the growth and development of the nervous system
4. They regulate the composition of the extracellular fluid in the CNS

Membrane Potential

Requires very few ions to be distributed unevenly

Which is true about typical, resting neurons?

The permeability of the plasma membrane to potassium ions is much greater than sodium ions

The membrane potential of most neurons at rest is

More positive than the equilibrium potential for potassium

The diffusion potential due to the concentration gradient for Na+ across a nerve cell membrane

Favors its movement into the cell at the resting membrane potential

Which would result from an increase in the extracellular concentration of K+ above normal

Depolarization of resting nerve cells

Which is true about the Na+, K+, ATPase pump in neurons?

It generates a small electrical potential such that the inside is made negative with respect to the inside

Which of these would occur if the concentration of ATP were depleted in a typical nerve cell

Resting membrane potential would become less negative

Equilibrium Potential

1. A function of the concentration of that ion on both sides of the membrane
2. It's the potential at which there is no net movement of that ion across the membrane
3. A permeable ion will move in the direction that will tend to bring the membrane potenti

Which of the following statements concerning the permeability of a typical neuron membrane at rest is true

Most of the voltage-gated Na+ channels are in the closed state

A threshold stimulus applied to an excitable membrane is one that is just sufficient to

Trigger an action potential

What must happen in order for an action potential to begin?

Na+ influx must exceed K+ efflux

Which describes the response of the voltage-gated channels when axon is stimulated to threshold?

Na+ channels are activated and then inactivated

During the rising (depolarizing) phase of a neuronal action potential

Pna+ becomes much greater than Pk+

Why are action potentials sometimes described as being "all-or-none" in character?

An action potential occurs whenever a supra threshold stimulus occurs, and its amplitude does not very with the size of a stimulus

Neuronal action potential

During the after-hyperpolarization phase, the permeability of the membrane to potassium ions is greater than its permeability at rest

The relative refractory period of an axon coincides with the period of

Increased K+ permeability of the cell

How is strength of a stimulus encoded by neurons?

By the frequency of action potentials

Interneurons

1. They receive synaptic input from other neurons in the CNS
2. They sum excitatory and inhibitory synaptic inputs
3. They deliver synaptic input on other neurons
4. They can transmit information between afferent neurons and efferent neurons

Exocytosis of neurotransmitter into the synaptic cleft is triggered by an influx of ____ in response to the arrival of an action potential in the axon terminal

Ca2+

The main role of calcium ions at chemical synapses is to

Cause fusion of synaptic vesicles with the plasma membrane of the axon terminal

At an excitatory chemical synapse between two neurons

There is increased permeability of the post-synaptic cell to both Na+ and K+

An inhibitory postsynaptic potential

Occurs when a ligand-gated ion channel increases its permeability to K+

EPSP's

1. They are produced by the opening of chemically-gated sodium channels
2. They transmit signals over relatively short distances
3. They depolarize postsynaptic cell membranes
4. They are able to summate

Temporal Summation

A synapse is stimulated a second time before the effect of a first stimulus at the synapse has terminated

Presynaptic Synapse

A synapse between an axon terminal and another axon's terminal that can be either excitatory or inhibitory

Acetylcholine is the main neurotransmitter released by

Pre-ganglionic sympathetic neurons and motor neurons

Serotonine

Acts as a neuromodulator

Alzheimer's disease is thought to involve primarily

Loss of cholinergic neurons

The portion of the peripheral nervous system that is composed of nerve fibers that innervate skeletal muscle is called the

Somatic motor nervous system

The region of the brain that is the most important control area for homeostatic regulation of the internal environment

Hypothalamus

Which best describes the reticular formation of the brain

It integrates information from all regions of the CNS, and incorporates the mechanisms that regulate sleep and wakefulness

Limbic System

An interconnected group of brain structures including parts of the frontal lobe-cortex, temporal lobe, thalamus, and hypothalamus, that is associated with learning, emotional experience, and behavior

Post-ganglionic neuron cell bodies of the autonomic nervous system have which category of neurotransmitter receptor

Nicotinic acetylcholine receptors