The Brain: No Moving Parts

Neurons

Neurons are similar to other cells in the body in some ways such as:

1. Neurons are surrounded by a cell membrane. 2. Neurons have a nucleus that contains genes. 3. Neurons contain cytoplasm, mitochondria and other "organelles". 4. Neurons carry out basic cellular processes like protein synthesis

and energy production. However, neurons differ from other cells in the body in that:1. Neurons have specialized extensions called dendrites and axons.

Dendrites bring information to the cell body and axons take information away from the cell body.

2. Neurons communicate with each other through an electrochemical process.

3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

Neurons come in different shapes and sizes. Some of the smallest neurons have cell bodies that are only 4 microns wide, while some of the biggest neurons have cell bodies that are 100 microns wide.

What Neurons Look Like

Concept of Polarization: Direction of Information Flow

A Morphological Classification:

One way to classify neurons is according to the number of extensions that originate from the neuron's cell body (soma).

•Pseudounipolar cells (example: dorsal root ganglion cells). Actually, these cells have 2 axons rather than an axon and dendrite. One axon extends toward the spinal cord, the other axon extends toward the skin or muscle.•Multipolar Neurons have many processes extending from the cell body, although only one of these is the axon (examples: spinal motor neurons, pyramidal neurons, Purkinje cells).

•Bipolar Neurons have two processes extending from the cell body (examples: retinal cells, olfactory epithelium cells).

What’s Inside?

A neuron has many of the same "organelles", as other cells in

the body. •Nucleus - Contains genetic material (chromosomes) including information for cell development and synthesis of proteins necessary for cell maintenance and survival. Covered by a membrane.

•Nucleolus - Produces ribosomes necessary for translation of genetic information into proteins

•Nissle Bodies - groups of ribosomes used for protein synthesis.

•Endoplasmic reticulum (ER) - system of tubes for transport of materials within cytoplasm. Can have ribosomes (rough ER) or no ribosomes (smooth ER). With ribosomes, the ER isimportant for protein synthesis.

•Golgi Apparatus - membrane-bound structure important in packaging peptides and proteins (including neurotransmitters) into vesicles.

•Microfilaments/Neurotubules - system of transport for materials within a neuron and may be used for structural support.

•Mitochondria - Produce energy to fuel cellular activities.

Axons & Dendrites:

Axons

•Take information away from the cell body

• Smooth Surface

• Generally only 1 axon per cell

• No ribosomes

• Can have myelin

• Branch further from the cell body

Dendrites

• Bring information to the cell body

• Rough Surface (dendritic spines)

• Usually many dendrites per cell

• Have ribosomes, mitochondria

• No myelin insulation

• Branch near the cell body

There are several differences between axons and dendrites:

What neurons Really look like:

What neurons Really look like:

What neurons Really look like:The cytoskeleton

Principle of dynamic polarization:

Information (generally) flows in a single direction

Pre-synaptic neuron:

• Generates an action potential which arrives at the pre-synaptic terminal, causing neurotransmitter release.

Post –synaptic neuron:

• Neurotransmitter activates the postsynaptic termninal on dendrites, which transmit a signal to the cell body.

Terminology:

Afferent = “going to”

Refers to signals that reach the neuronal cell body (e.g., vie the dendrites).

Efferent = “leaving from”

Refers to signals that leave the cell body (e.g., via the axon).

The Axon: Efferent component

For communication between neurons to occur, an electrical impulse must first travel down an axon to the synaptic terminal.

Dendrites and their spines:

•The integrative unit of the neuron

•Contain the majority of Synapses

•Contain high concentrations of ion channels and cell membrane receptors.

Spines are dynamic objects

Dendrites &Synapses

Fluorescence staining of a hippocampal dendrite .

A. After filling with Lucifer yellow.

B.After staining against synaptophysin.

The arrows in both images point to the sites of spines, and the arrowheads point to the site shaft synapses.

The Chemical Synapse

The Synapse

Electron Micrographs have shown that synapses can be either asymmetrical(red arrow) or symmetrical (green arrow).

•The Red arrow is pointing to a synapse that has one dark band and one lighter band.

•The green arrow is pointing to a synapse that has two dark bands. Asymmetrical synapses are thought to be excitatory synapses and symmetrical synapse are thought to be inhibitory synapses.

•The yellow lineyellow line outlines the dendrite (D).

Ion ChannelsNa+ Channel K+ Channel

The Knee Jerk Reflex

The TaylorHammer

The Krauss Hammer

The Troemner Hammer

The Berliner Hammer

The Babinski /Rabiner Hammer

The Queen’s Square Hammer

The Knee Jerk Reflex

The Knee Jerk Reflex

A somatic reflex arc in which the final element in the chain is

skeletal muscle.

1 Is some sensory transducer in the periphery, for example, a Golgi tendon organ, a Pacinian corpuscle or other tactile sensor in the skin.

2 The pseudounipolar sensory neuron in the circuit. Its soma is physically located in a craniospinal ganglion (pictured here as a dorsal root ganglion, but it could also be on a cranial nerve).

3 An interconnector neuron, whose soma is found in the CNS.

4 Is a motor neuron whose soma is in the ventral horn of the gray H of the spinal cord.

5 The effector organ, which in the case of this type of arc, will always be skeletal muscle.

Spinal cord injury & spasticity

Spinal cord injury & spasticity

Although the knee jerk reflex is classically considered to be “monosynaptic”, it actually is not solely controlled at the spinal level.

Descending pathways exert excitatory or inhibitory effects on motor neurons (each of which has over 100,000 synaptic contacts) either directly, or via interneurones.

Disrupting these descending pathways removes inhibition, causing spasticity.