Chapter 12

Encoding of stimulus intensity

1) A light touch generates a low frequency of action potentials. A firmer grip elicits action potentials that pass down the axon at a higher frequency. 2) The number of sensory neurons recruited by the stimulus- a firm grip stimulates a larger number of pressure-sensitive neurons than does a light touch.

How information is transferred from one neuron to another across a chemical synapse

1) Nerve impulse in presynaptic axon opens Ca+ channels in synaptic end bulb 2) Ca+ stimulates release of neurotransmitter 3) Neurotransmitter crosses synaptic cleft and binds to receptors on postsynaptic neuron 4) Generates a postsynaptic potential

Difference with graded and action potentials

1) Origin: Graded potentials arise mainly in dendrites and the cell body. Action potential arise at trigger zones and propagate along the axon. 2) Types of channels: Graded potentials have ligand-gates or mechanically gated channels. Action potentials have voltage-gated channels for Na+ and K+. 3) Conduction: Graded potentials are not propagated, they permit communication over short distances. Action potentials are propagated, and permit communication over longer distances. 4) Amplitude: Graded potentials depend on the strength of the stimulus- <1mV to >50mV. Action potentials are all or none (constant), typically about 100mV. 5) Duration: Graded potentials are typically longer, range from several milliseconds to several minutes. Action potentials are shorter, range from .5 to 2msec. 6) Polarity: Graded potentials may be hyper polarizing or depolarizing. Action potentials always consist of depolarizing phase followed by repolarizing phase and return to resting membrane potential. 7) Refractory period: Graded potentials don't have a refractory period, therefore summation can occur. Action potentials have a refractory period therefore summation cannot occur.

functions of the nervous system

1) Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood acidity, and external stimuli, such as a raindrop landing on your arm. This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves. 2) Integrative function. The nervous system processes sensory information by analyzing and storing some of it, and by making decisions for appropriate responses-an activity known as integration. 3) Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.

Electrochemical gradients

A concentration (chemical) difference plus an electrical difference. Ions move from where they are more concentrated to where they are less concentrated.

Inhibitory postsynaptic potential

A neurotransmitter that causes hyper polarization of the postsynaptic membrane is inhibitory.

Excitatory postsynaptic potential

A neurotransmitter that depolarizes the postsynaptic membrane is excitatory because it brings the membrane closer to the threshold.

Node of Ranvier

A space along a myelinated axon between the individual Schwann cells that form the myelin sheath and the neurolemma.

suprathreshold stimulus

A stimulus that is strong enough to depolarize the membrane above threshold.

sub-threshold stimulus

A weak polarization that cannot depolarize the membrane to threshold.

Factors affecting impulse speed

Amount of myelination, axon diameter, and temperature (occurs slower when temperatures are cooled).

All-or-none principle

An action potential that either occurs completely or it does not occur at all.

Threshold stimulus

An action potential will occur- the stimulus is just strong enough to depolarize the membrane to threshold.

resting membrane potential

An electrical potential difference (voltage) across the plasma membrane in excitable cells.

Chemical synapse

An impulse in a presynaptic neuron causes the release of neurotransmitter molecules that produce an impulse in a postsynaptic neuron.

Neuron repair in the CNS versus the PNS

CNS- little or no repair of damage to neurons occurs in the brain and spinal cord. Damage to the axon results in the formation of scar tissue that is a physical barrier to regeneration. PNS- Damaged dendrites and axons may be repaired if the cell body is intact, if the Schwann cells are functional, and if the scar tissue formation does not occur too rapidly. PNS neuron repair begins with Wallerian degeneration of the distal region of the axon and myelin sheath. A regeneration tube formed by Schwann cells directs passage of the regenerating axon to previously contacted receptors and effectors.

Polarized

Cell that exhibits a membrane potential. Condition in which the inside of a cell is more negatively charged than the outside of the cell. -70 (resting plasma potential)

Organization of the nervous system

Central Nervous System (CNS)-Consists of brain and spinal cord. Peripheral Nervous System (PNS)- Consists of all nervous tissue outside CNS: cranial nerves and spinal nerves.

white matter composition

Consists primarily of the myelinated axons. The whitish color of myelin gives white matter its name.

Gray matter composition

Contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It looks grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas.

Continuous conduction impulse propagation

Involves step by step depolarization and repolarization of each adjacent segment of the plasma membrane as ions flow through each voltage-gated channel along the membrane

why nerve impulse propagation is compromised in patients with multiple sclerosis

Myelin sheaths in CNS destroyed- Immune system attacks myelin, and turns it to hardened lesions called scleroses. Impulse conduction slows and eventually ceases.

neuron communication- receptor stimulation to the response of an effector

Neurons create two types of electrical signals 1)Graded potentials: short-distance communication 2)Action potentials: longer distance communication Action potential in neuron is a nerve impulse which travels along axon. Neurotransmitter release at synapse is triggered by action potential arriving at axon terminal. Neurotransmitter can stimulate graded potential in next cell. In response to the graded potential, the axon of the interneuron forms a nerve action potential, which travels along the axon, which results in neurotransmitter release at the next synapse with another interneuron. This occurs over and over.

cranial nerves

One of 12 pairs of nerves that leave the brain; pass through foramina in the skull; and supply sensory and motor neurons to the head, neck, part of the trunk, and viscera of the thorax and abdomen. Each is designated by a Roman numeral and a name.

Summation of postsynaptic potentials

Process by which graded potentials add together- postsynaptic potentials that form in the postsynaptic membrane. The sum of all the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron.

Action potential

Sequence of rapidly occurring events that briefly reverses the membrane potential and then eventually restores it to the resting state.

Microglia

Small cells with slender processes that give off numerous spinelike projections. Microglia phagocytize microbes and damaged nervous tissue.

Graded potentials

Small deviation from the membrane potential that makes the membrane either more polarized or less polarized.

Saltatory conduction impulse propagation

Special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels.

how the myelin sheath is formed around a peripheral nervous system neuron

The axons of most neurons are surrounded by a multilayered lipid and protein covering called the myelin sheath that electrically insulates them and increases the speed of impulse conduction. Schwann cells begin to form myelin sheaths around axons during fetal development. Each Schwann cell wraps about 1 millimeter (0.04 in.) of a single axon's length by spiraling many times around the axon. Eventually, multiple layers of Schwann cell plasma membrane surround the axon, with the Schwann cell's cytoplasm and nucleus forming the outermost layer. The inner portion, consisting of up to 100 layers of Schwann cell plasma membrane, is the myelin sheath. The outer, nucleated cytoplasmic layer of the Schwann cell enclosing the myelin sheath is the neurolemma.

Threshold

The generation of an action potential depends on whether a stimulus is able to bring the membrane potential to a certain level.

Repolarizing phase

The membrane potential is restored to the resting state of -70mV

Depolarizing phase

The negative membrane potential becomes less negative, reaches zero, and then becomes positive.

Postsynaptic neuron

The neuron that carries an impulse away from a synapse

Presynaptic neuron

The neuron that carries an impulse toward a synapse

Neurolemma

The outer, nucleated cytoplasmic layer of the Schwann cell enclosing the myelin sheath. When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon.

Refractory period

The period of time after an action potential begins, during which an excitable cell cannot generate another action potential

Astrocytes

The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord). Astrocytes cling to and support neurons. They help to maintain the appropriate chemical environment for the generation of impulses by providing nutrients to neurons, removing excess neurotransmitters, and regulating the concentration of important ions. Astrocyte processes wrap around blood capillaries to inhibit movement of potentially harmful substances in blood, creating a blood-brain barrier that restricts the movement of substances between the blood and neurons of the CNS.

Dendrites

The receiving or input portions of a neuron. They usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body.

Electrochemical basis of the resting membrane potential

The resting membrane potential exists because of a small buildup of negative ions in the cytosol along the inside of the plasma membrane, and an equal buildup of positive ions in the extracellular fluid along the outside surface of the plasma membrane. The greater the difference in charge across a plasma membrane, the larger the membrane potential.

Synapse

The site of communication between 2 neurons or between a neuron and an effector cell (muscle or glandular cell).

Function of the sodium-potassium pump in maintaining the resting membrane potential

The small inward Na+ leak and outward K+ leak are offset by sodium potassium pumps, which help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in and returning K+ to the cell interior. The sodium potassium pump expels 3 Na+ for each 2 K+ that come in. Because the pumps remove more positive charges from the cell they contribute to the negativity of the resting membrane potential.

Neurotransmitter removal from the synaptic cleft

This is essential for normal synaptic function. It is removed through 1) Diffusion: Some of the released neurotransmitter molecules diffuse away from the synaptic cleft. Once a transmitter molecule is out of reach of its receptors, it can no longer exert an effect. 2) Enzymatic degradation: Certain neurotransmitters are inactivated through enzymatic degradation- i.e. acetylcholinesterase breaks down acetylcholine in the synaptic cleft. 3) Uptake by cells: Many neurotransmitters are actively transported back into the neuron that released them, or are transported into neighboring neuroglia.

Nerve impulse propagation

When an action potential keeps its strength as it spreads along the membrane- the movement. This is dependent on positive feedback. The action potential regenerates over and over at adjacent regions of the membrane from the trigger zone to the axon terminals.

hyperpolarizing graded potential

When the response makes the membrane more polarized (inside more negative)

Cell body of a neuron

contains a nucleus surrounded by cytoplasm that contains typical organelles such as lysosomes, mitochondria, and a Golgi complex. Neuronal cell bodies also contain prominent clusters of rough endoplasmic reticulum, termed Nissl bodies, where protein synthesis occurs.

Function of motor neurons

convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves. Motor neurons are multipolar in structure.

Function of sensory neurons

either contain sensory receptors at their distal ends (dendrites) or synapse with sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure.

Function of neurons

form the complex processing networks within the brain and spinal cord and also connect all regions of the body to the brain and spinal cord. Neurons carry out most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions.

Schwann cell

form the myelin sheath around axons in the PNS. A Schwann cell can myelinate a single axon or enclose multiple unmyelinated axons (axons that lack a myelin sheath). Schwann cells participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.

Function of interneurons

mainly located within the CNS between sensory and motor neurons. Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure.

neurotransmitter

A chemical released by a neuron that excites or inhibits other neurons or effector cells.

Synaptic cleft

A tiny space filled with interstitial fluid.

Satellite cells

Flat cells surrounding the cell bodies of neurons of PNS ganglia. In addition to providing structural support, satellite cells regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.

current

Flow of charged particles, flow of ions is the electrical current.

Axoaxomic

From axon to axon

Axosomatic

From axon to cell body

Axodendritic

From axon to dendrite

Myelin sheath

Multilayered lipid and protein covering, formed by Schwann cells and oligodendrocytes, around axons of many peripheral and central nervous system neurons. Electrically insulates axons and increases the speed of impulse conduction.

Oligodendrocyte

Oligodendrocyte processes are responsible for forming and maintaining the myelin sheath around CNS axons.

Nissl bodies

Rough ER, where protein synthesis occurs

Graded

The electrical signals vary in amplitude (size), depending on the strength of the stimulus.

depolarizing graded potential

When the response makes the membrane less polarized

peripheral nerves

all nervous tissue outside the CNS

sensory receptors

nervous system structures that monitor changes in the external or internal environment.

ganglia

small clusters of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord.

Function of neuroglia

support, nourish, and protect neurons, and maintain homeostasis in the interstitial fluid that bathes them.