Neurohistology

Neuron Components• Research by Purpura

– Mental retardation (MR) is from failure of normal circuits to form in brain

– Dendritic spines of children with MR look like those of human fetus

– Impoverished environment in critical period can lead to less dendritic spine formation

– deprivation-induced changes in brain can be reversed if intervention occurs early

• Two parts of the neuron– Cell body (soma)– Neurites

• Axons (one)• Dendrites (many)

• Dendrites– Some have dendritic spines

that receive some types of synaptic input

– Sensitive to type and amount of synaptic activity

– Sensitive to quality of early environment

From: Bear, Connors, Paradiso. Neuroscience Exploring the Brain, 2007.

Classification of Neurons

• Based on dendrites, e.g. in cerebral cortexo Stellate cells (star shaped)o Pyramidal cells (pyramid shaped)

• Based on number or neurites extending from somao Unipolar (1)o Bipolar (2)o Multipolar (most neurons)

From: Bear, Connors, Paradiso. Neuroscience Exploring the Brain, 2007.

Classification of Neurons

• Based on neurotransmitter– Cholinergic, i.e. cells that

release acetylcholine• Used in the motor system

to command voluntary movements

• Based on connections– Primary sensory– Primary motor– Interneurons (connect only

with other neurons)

• Based on axon length– Golgi type I extend from one

part of brain to another (e.g. pyramidal cells)

– Golgi type II are more local (e.g. stellate cells that never extend beyond cerebral cortex)

Glia (support cells)• Role of astrocytes

– Regulate chemical content of extracellular space around neurons• E.g. envelop synaptic

junctions thereby limiting spread of neurotransmitter

• Can remove neurotransmitter from synaptic cleft

– Possess neurotransmitter receptors and can trigger electrical and biochemical events (new discovery – not sure of exact function yet)

• Astrocytes– Most common glia– Fill most of brain space not

occupied by neurons

From: Bear, Connors, Paradiso. Neuroscience Exploring the Brain, 2007.

Myelinated Glia• Myelin spiral around

axon to form the myelin sheath

• Myelin sheath is interrupted periodically by nodes of Ranvier

• Oligodendrocytes (in CNS)– Can myelinate several

neurons

• Schwann cells (in PNS)– Only myelinates one neuron

From: Bear, Connors, Paradiso. Neuroscience Exploring the Brain, 2007.

Cross section of optic nerve myelinated fibers

Other Non-Neuronal Cells

• Microgliao Function as phagocytes to

remove debris from degenerating neurons and glia

• Ependymal cellso Line the fluid filled

ventricleso Play role in directing cell

migration during brain development