Biomedical Engineering Reference
In-Depth Information
CHAPTER 1
Neural Anatomy
The brain may be studied at many scales, ranging from molecular to gross anatomy. In this text, the focus
will be primarily on quantifying cellular behavior for two reasons. First, much more is known about the
function of individual neurons. Second, an underlying assumption of neuroscience is that a complete
understanding of brain function will require deep insights at the cell and molecular level. In this chapter,
the anatomy of the neuron is explored to the depth needed to understand future chapters. It is very
important, however, to not lose site of the goal of understanding the brain at the larger scales. Therefore,
this chapter also outlines some of the gross level structures of the brain.
1.1 THENEURON
The human brain is composed of more than 10
12
neurons. The table below is a summary of some types
of neurons.
Table 1.1:
Bipolar
Found in eye, transduction of light to neural impulse
Somatosensory
Found in skin, tactile senses of pain and proprioception
Motor
Found in spinal cord, projects to muscles
Pyramidal cell
Found in cortex, relay information within brain
Purkinje Cell
Found in cerebellum, motor skills
Association cell
Found in thalamus, connect neurons together
Although the anatomy and function of the individual neuron can vary throughout out the brain, Fig. 1.1
shows the features of a generic neuron. Like other cells, the neuron separates space inside from space
outside by a 5
nm
bi-lipid membrane that acts as an electrical insulator. Typically, the neuron is divided
into the dendritic tree, soma and axon.
The
dendritic tree
is a complex web of branching structures that range in diameter from 1
20
μm
.
One interesting feature of some dendrites is the presence of
spines
that play a somewhat unclear electrical
or chemical role in neuronal function.
The
soma
, or cell body, is approximately 20
μm
in diameter and contains most of the organelles,
including the nucleus, Golgi apparatus, mitochondria, microtubuals, and endoplasmic reticulum. It is
here that the cell generates ATP, packages neurotransmitters, houses the genetic material, and assembles
proteins for the cell.
The
axon
branches off of the soma at the
axon hillock
. Axons can vary greatly in length (1
mm
−
−
1
m
)
and diameter (1
25
μm
). The intracellular space of the axons are covered in small specialized proteins
called
legs kinesin
which mechanically transport ATP, vesicles filled with neurotransmitter and enzymes
to and from the soma. Some axons have offshoots called
axon collaterals
.
The
synapse
is the site where the dendrite of on neuron interfaces with the axon of another neuron.
It was first described in 1897 by Sherrington who coined the term from the Greek, Syn (together) +
−
