Biology ch4

Question 1

What are neurons?

Answer 1

Specialized cells capable of transmiting electrical impulses and then translating those electrical impulses into chemical signals.

Question 2

VIP NOTE:

Answer 2

Each neuron has a shape that matches its function, as dictated by the other cells with which that neuron interacts with.

They are a variety of different types of neurons in the body, but they all share some specific anatomical features.

Question 3

Explain the anatomical structures of a neuron?

Answer 3

• Cell body: Like all other cells, neurons have nuclei. The nucleus is located in the cell body also called the soma. The soma is also the location of the endoplasmic reticulum and the ribosomes.
• Dentrites + Axon Hillock: The cell has many appendages emanating direcly from the soma called dentrites, which recieve incoming messages from other cells. The information recieved from the dentries is transmitted through the cell body before it reaches the axon hillock, which integrates the incoming signals. The axon hillock plays an important role in actions potentials, which is the transmission of electrical impulses down the axon.
• Signals arriving from the dentrites can be either excitatroy or inhibitatory; the axon hillock sums up these signals, and if the result is excitatory enough (reaching the threshold) it will initatiate an action potential.
• Axon: A long appendage that terminates in close proximity to a target structure (a muscle, gland, or another neuron cell).

Question 4

What is myelin and myelin sheath in the axon?

Answer 4

Most mammalian nerve fibers (neurons) are insulated by myelin, a fatty membrane to prevent signal loss or crossing of singals.

Just like insulation prevents wires next to each other from accidentally discharging each other, the myelin sheath maintains the electrical signal within one neuron.

In addition, presence of myelin increases the speed of conduction in the axon.

Question 5

What produces myelin in central nervous system and in peripheral nervous system?

Answer 5

Myelin is produced by oligodendrocytes in the central nervous system and schwann cells in the peripheral nervous system.

Question 6

What are the Nodes of Ranvier? Why these are important?

Answer 6

At certain intervals along the axon, there are small breaks in the myelin sheath with exons areas oof axons membrane called the nodes of ranvier.

Nodes of Ranvier are important for rapid signal conduction.

Question 7

What is the nerve terminal or synaptic bouton(knob)?

Answer 7

At the end of the axon is the nerve terminal or synaptic bouton (knob). This structure is enlarged and flattened to maximize transmission of the signal to the next neuron and ensure proper release of NT, the chemicals that transmit information between neurons (chemical transmission of information).

Question 8

What is a synaptic cleft?

Answer 8

Neurons are not physically connected to each other.

Between the neurons, there is a small space into which the terminal portion of the axon releases NT, which then binds to the dentrites of the adjacent neuron (the postsynatpic neuron). This space is known as the synaptic cleft.

Question 9

What is the synapse?

Answer 9

Together the nerve terminal + synaptic cleft + postsynaptic membrane is known as the synapse.

Question 10

Differentiate between the neurons in the CNS and PNS?

Answer 10

• PNS*: Multiple neurons may be bundled together to form a nerve. These nerves can be sensory, motor, or mixed; which refers to the type of information they carry. There can also be mixed nerve that carry both sensory and motor information. The cell bodies of the neurons of the same type are clustered together into the ganglia.

CNS: Axons may be bundled together to form tracts. Unlike nerves, tracts only carry one type of information. The cell bodies of neurons in the same tract are grouped into the nuclei.

Question 11

Neurons are not the only cells in the nervous system. Neurons must be supported and myelinated by other cells. What are these cells?

Answer 11

The cells are often called glial cells or neuroglia.

Question 12

What are the four types and their functions?

Answer 12

1. Astrocytes: noutrish neurons and form the blood-brain barrier (which controls the transmission of solutes from the bloodstream into the nervous tissue)
2. Ependumal cells: line the ventricles of the brain and produce cerebrospinal fluid, which physically support the brain and serve as a shock absorber.
3. Microglia: phagocytic cells that ingest and break down waste products and pathogens in the CNS.
4. Oligodendrocytes (CNS) and Schwann cells (PNS) produce the myelin around the axons.

Question 13

What is the definition of an action potential?

Answer 13

Neurons use all-all-nothing messages, called action potentials, to relay electrical impulses down the axon to the synaptic bouton.

Question 14

What is meant by the cells resting membrane potential? What is that potential (value)?

Answer 14

It is the net electrical potential difference that exists across the cell membrane, created by the movement of charged molecules across that membrane.

For neurons, this potential is about -70mV, with the inside of the neuron being negative relative to the outside.

Question 15

What are the two most important ions involved in generating and maintaining this resting membrane potential?

Answer 15

Potassium (K+) and sodium (Na+)

Question 16

Explain the concentration of potassium (K+) inside the cell?

Answer 16

The potassium concentration inside the cell averages about 140mV, as compared to only 4mV outside the cell. This means it is more favorable for potassium ions to move outside of the cell - IE: down the concentration gradien (from high concentration to low concentration).

Question 17

What facilitate that outward movement of potassium ions?

Answer 17

The cell membrane has a specific type of transmembrane channel, called potassium leak channels, which allows for the slow leak of potassium out of the cell.

Question 18

What happens to the charges inside and outside the neuron as potassium move outside?

Answer 18

As potassium continually leaks out of the cell, the cell loses a small amout of positive charge and, thus, leaves behind a small amount of negative charge and makes the outside of the cell more positively charged relative to inside. That’s why the inside of the neuron is more negative than the outside.

Question 19

Explain the concept of equilibrium potential of potassium? What is value of potassium equilibrium potential?

Answer 19

As negative charge builds up inside the cell, some potassium will be drawn back into the cell due to the attraction between the positive potassium ions and the negative potential building inside the cell. As the potential difference continues to grow, potassium will be more strongly drawn back into the cell. At a certain potential, each potassium ion that is pushed out due to the concentration gradient will be matched by another potassium ion moving inside due to the electrical potential. When that happens, there is no more net movement of the ion as the cell is in equilibrium with respect to potassium.

Potassium’s equilibrium potential is around -90mV. The negative sign is assigned due to convention and because a positive ion is leaving the cell.

Question 20

Now explain the same thing for sodium (Na+)?

Answer 20

• Sodium cocnetration gradient is the reverse of potassium with a concentration of 12mV inside and 145mV outside of the cell, meaning there is a driving force pushing the ions inside the cell (concentration gradient).
• This movement is faciliated by the sodium leak channels.
• The slow leak of sodium into the cell causes a buildup of electrical potential outside the cell that causes the ions to move outside. This maintains an equilibrium potential of sodium around 60mv and it is postive because sodium is moving into the cell.

Question 21

READ THE LAST PARAGRAPH ON PG. 151.