Lesson 3: Nerve Cells and Nerve Impulses

Nerve Cells

Responsible for direct communications from different groups of cells.

1. Gather information from sensory receptors

2. Process information and provide a memory.

3. Generate appropriate signals to effector cells

Neurons form a network of highly specific connections between different groups of cells to:

Cell body

contains the nucleus and most of the organelles responsible for maintaining the cell (ribosomes and mitochondria)

Axon

A long cell process stretching from the cell, often over a long distance, which is responsible for transmitting signals from the neuron to other cells.

Dendrites

Numerous short cell processes that increase the surface area available for connecting with axons of other neurons.

Synapses

Specialized cell junctions between its axons and other neurons to allow direct cell communication.

Myelin Sheath

Insulating material covering many vertebrate axons. Important for message conduction to reach effector cells faster.

Nodes of Ranvier

Interruptions or gaps in the myelin sheath.

1. Bipolar Neuron

2. Unipolar Neuron

3. Multipolar Neuron

3 Types of Neurons

Bipolar Neuron

Neuron with 2 axons

Unipolar Neuron

Neuron with 1 axon; common in sensory neurons

Multipolar Neuron

Multiple axons from the cell body

Bipolar Neuron

Name the type of neuron

Unipolar Neuron

Name the type of neuron

Multipolar Neuron

Name the type of neuron

Motor Neurons

Has its soma in the spinal cord and receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle. Have a large cell body for metabolic support for the large axon.

Betz Cells (Cerebral Motor Cortex) and Anterior Horn Cells (Spinal Cord)

Example of Motor Neurons

Sensory Neurons

Commonly unipolar cells, characterized by the possession of one major process. Specialized at one end to be highly sensitive to a particular type of stimulation, such as light, sound, or touch.

Interneurons

Generally small, simple cells with short processes that provide local connections within the CNS. Mostly bipolar (one dendritic and one axonal).

Basket Cells

Multiple cell processes and wrap around the cell body of a target nerve cell to create multiple connections.

Pyramidal Cells

Nerve cells in the cerebral cortex because of the roughly triangular shape.

Invertebrate Axons

Do not have myelin sheaths.

End Bulb/Bouton

A presynaptic terminal where axons release chemicals that cross through the junction between one neuron and the next.

Afferent Axon

Admit; brings information toward or into a structure.

Efferent Axon

Exit; brings information away from a structure

afferent

Every sensory neuron is an ___ to the rest of the nervous system

efferent

Every motor neuron is an ___ from the nervous system

neuron; efferent; afferent

In a nervous system, a given ___ is an ___ from one structure and ___ to the other.

Intrinsic Neuron

If a cell’s dendrites and axon are entirely contained with a single structure.

Shape

____ of a given neuron determines its connections with other neurons and its contribution to the nervous system.

Glia (Glial Cells)

also known as neuroglia and derived from Greek work meaning “glue”; the other major components of the nervous system, do not transmit information over long distances as neurons do, although they perform many other functions.

1. Schwann Cells

2. Oligodendrocyte

3. Microglial Cell

4. Ependymal Cells

5. Astrocyte

Identify the following kinds of glial cells

Ependymal Cells

For productions of cerebrospinal fluid

Astrocytes

Remove waste material created when neurons die and control the amount of blood flow to each brain area and synchronizes associated axons; star-shaped

Microglia

Key players in the immune response within the CNS. Involved in phagocytosis (engulfing and digesting cellular debris). Particularly concentrated in regions where there is a higher likelihood of injury or infection.

Oligodendrocytes

Produces and maintain myeline. Found in the CNS, can extend its processes to myelinate multiple axons from different neurons.

Schwann Cells

Found in the PNS. Produce and maintain the myelin sheath around axons in the PNS essential for efficient conduction of nerve impulses in peripheral nerves.

1. Myelination

2. Axonal Support and Regeneration

3. Nerve Signal Conduction

Key functions of schwann cells

Myelination

schwann cells wrap around and provide insulation for individual axons in the peripheral nerves by forming the myelin sheath

Axonal Support and Regeneration

Play a role in supporting the health and regeneration of peripheral nervous axons. Involved in the repair process after nerve injuries, assisting in the regeneration of damaged axons.

Nerve Signal Conduction

By providing insulation through myelination, schwann cells contribute to the saltatory conduction of nerve impulses.

Blood-Brain Barrier

Mechanism that excludes most chemicals from the vertebrate brain.

Endothelial Cells

Form the walls of the capillaries; such cells are separated by small gaps, but in the brain, they are joined so tightly that they block viruses, bacteria, and other harmful chemicals from passage.

The barrier keeps out useful and harmful chemicals. The useful chemicals include all fuels and amino acids. For the brain to function, it needs special mechanism to get these chemicals across the blood-brain barrier.

Why don’t we have similar walls around our other organs?

1. Oxygen

2. Carbondioxide

3. Water through special protein channels in the wall of the endothelial cells

4. Molecules that dissolve in the fats of the membrane

Chemicals that can cross blood-brain barrier

1. Vitamin D

2. Vitamin A

3. Antidepressants and other Psychiatric drugs to illegal drugs

Types of molecules that dissolve in the fats of the membrane

Active-transport

Protein-mediated process that expends energy to pump chemicals from the blood into the brain.

1. Glucose

2. Amino Acids

3. Purines

4. Choline

5. Few Vitamins

6. Iron

7. Certain Hormones

Chemicals actively transported into the brain

Glucose

Brain’s main fuel; important because the metabolic pathway that uses this requires oxygen, neurons need a steady supply of oxygen.

20%

Percentage of the amount the brain uses oxygen consumed in the body.

Amino Acids

Building blocks of proteins

Glycerol

Breakdown product from fats.

Vitamin B1 (Thiamine)

Essential to use glucose. The continued deficiency of this, common in chronic alcholism, leads to death of neurons and a condition called Korsakoff’s Syndrome.

Korsakoff’s Syndrome

Syndrome marked by severe memory impairments.

Action potential

Brief reversal of electric polarity across the cell membrane. Messages sent by axons

Polarization

Difference in electrical charge between the inside and outside of the cell.

Hyperpolarization

Increased polarization

Voltage-gated Channels

Axon channels regulating sodium and potassium

Saltatory Conduction

The jumping of action potentials from node to node

Sodium-Potassium Pump

A protein complex, repeatedly transports three sodium ions out of the cell while drawing 2 potassium ions into it.

Propagation of the action potential

Describes the transmission of an action potential down an axon

Refractory Period

Resists the production of further action potentials. Lasts from start to point the voltage first returns to resting membrane value.

Absolute Refractory Period

The membrane cannot produce another action potential, regardless of the stimulation. Lasts until end of hyperpolarization.

Relative Refractory Period

Stronger-than-usual stimulus is necessary to initiate an action potential.

-55 mV threshold

amount of mV threshold to open the voltage-gated ion channels.