The Nervous System Is Composed of Cells

• Types of cells in the nervous system– Neurons, or nerve cells

• are the most important part of the nervous system• There are many types of neurons

– Glial cells provide support for neurons• Microglia• Astrocytes • Oligodendrocytes • Schwann cells

• Neuron doctrine states that:– The brain is composed of independent cells.– Information is transmitted from cell to cell across synapses.

Scale of Things

Figure 2.2 The Major Parts of the Neuron

Figure 2.5 A Classification of Neurons into Three Principal Types

Figure 2.7 Glial Cells

Neuron Classification• By Function into three groups, which is all but useless

– Sensory Neurons – Interneurons – Motor Neurons

• By Structure into three groups, not much better– Unipolar: dendrite and axon emerging from same process. – Bipolar: axon and single dendrite on opposite ends of the

soma. – Multipolar: more than two dendrites:

• Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.

• Golgi II: neurons whose axonal process projects locally; the best example is the granule cell.

• Most neurons are named as they are discovered which is not classification and results in a very long list

• There currently is no useful classification system for neurons

Box 2.1 Neuroanatomical Methods Provide Ways to Make Sense of the Brain

Figure 2.1 Nineteenth-Century Drawings of Neurons

Retinal Circuits

http://webvision.med.utah.edu/book/part-vi-development-of-cell-types-and-synaptic-connections-in-the-retina/development-of-cell-types-and-synaptic-connections-in-the-retina/

Retinal Circuits

Figure 2.3 Variety in the Form of Nerve Cells

Diversity of Neurons• Thousands of different types of neurons• Improves information processing• Perception under a range of circumstances• Flexible processing to assess unexpected things• Circuit diagram of sensory neocortex

– Yuste lab composite diagram of circuits in the sensory neocortex

Synapses• Neurons are interconnected through synapses

– Individual neurons do not have specific behavioral or cognitive functions

– neurons connected into a circuit have function

• The neuronal cell body and dendrites receive information across synapses.

• Dendrites have a branched arborization pattern to facilitate contacts.

• Information is transmitted from the presynaptic neuron to the postsynaptic neuron. (Mostly)

– There are chemical signals from postsynaptic neuron to presynaptic neuron called “retrograde” which is some sort of feedback signal

Figure 2.6 Synapses

Figure 5.3, Bear, 2001

EM of Chemical synapse

Active Zone

mitochondria

EM of synapses on cell body

A segment of pyramidal cell dendrite from stratum radiatum (CA1) with thin, stubby, and mushroom-shaped spines from rat hippocampus. Found at Synapse Web http://synapses.clm.utexas.edu/anatomy/compare/compare.stm

Neurophysiology

• The electrical and chemical processes within neurons

– An action potential is a rapid electrical signal that travels along the axon of a neuron.

– A neurotransmitter is a chemical messenger between neurons.

– Electrical and chemical processes work together• Chemistry produces the electrical activity

• Electrical activity modulates the chemistry

Figure 3.3 The Ionic Basis of the Resting Potential

Fig 3.4

Fig 3.4 Distribution of Ions

Fig 3.6 Action Potential Mediated by Voltage-Gated Na Channels

Ion Channels• The cell membrane is a lipid bilayer, with two layers of

lipid molecules.• Ion channels are proteins that span the membrane and

allow ions to pass.– There are many types of ion channels – Usually classified based on gating and type of ion

• Gating– Voltage-Gated ion channels open and close in response to membrane

potential. – Ligand-gated ion channels open or close depending on binding of ligands

to the channel

• Type of Ion– Sodium (Na)– Potassium (K)– Chloride (Cl)– Calcium (Ca)

FIGURE 2 | General structural topology of voltage-gated ion channels. The distribution and targeting of neuronal voltage-gated ion channels Helen C. Lai and Lily Y. Jan Nature Reviews Neuroscience 7, 548-562 (July 2006)

Voltage-gated sodium channel Function

Animation of voltage gated Na channel, when the outside is depolarized the gate on the inside of the channel opens.

Potassium Channels

Potassium channel in membrane You Tube video

Action Potential Animations

• Action Potential Animation You Tube video

• Action Potential animation, advanced with detailed explanation You Tube video

Fig 3.8a Conduction along Axons

Fig 3.8b Conduction along Axons