Neurobiology

Neurons

Main signaling units of the nervous system

• The neuron is an anatomical unit—the
  fundamental structural and functional
  unit of the nervous system.

• The neuron is composed of three parts:
  cell body, dendrites, and axons.

• Neurons are discrete cells that are not
  continuous with other cells.

• The points of connection between
  neurons are called synapses.

• The neuron is a physiological unit.
  Electrical activity flows through the
  neuron in one direction (from dendrites
  to the axon, via the cell body)

this is an image of a generic multipolar neuron

neurons have distinct neurological domains

neurons are an anatomical unit

fundamental structural unit of brain and broader nervous system

discrete cells, not continuous with other cells, contiguous —> form synapses

are physiological units

dentrites —> cell body —> axon —> axon terminals

this implies that the information is integrated within the soma and then the product of that integration in the form of an action potential is conducted across the axon

neurons are specialized for electrical signaling

Dendrite

neurite that conveys information toward
the cell body (receives information) recieves signals and then pass those signals on toward the cell body

Soma

Cell body (contains nucleus) integrates
information , also the primary metabolic center

integrative region that puts those inputs together

Axon

neurite that conveys signals and information away from
the soma, sometimes over short or long distances
(sending information) interneurons = short distance

projection neurons = long distances

conductor of signal away from neuron

action potential conducted across neuron

Synaptic terminal

site of output from a given neuron output, passed along to target

where neurotransmitters are released at a given synapse, passing it from one neuron to another forming a circuit

neurotransmitter acts as a chemical signal communicating the signal from one neuron to another

Neuronal morphology

*listen back in recording

larger dendritic arbor —> more inputs it can recieved, more synapses can form

large arbors = more convergence and computational power

Convergence

The number of inputs to a single
neuron; reflects the ability to integrate signals.

Divergence

The number of targets innervated by
any one neuron.

Non-neuronal Cells

Many cells in the nervous system are non-neuronal cells
– Glia
• Differences: no active electrical response, small, have symmetrical branches
• Why are glial cells so important to the nervous system?
– Provide physical support
– Regulate extracellular environment to maintain homeostasis
– Provide protection, have defensive roles (immune cells)
– Produce myelin

morphology of neurons

unipolar neuron

multipolar neuron

most common type found in brain

pseudounipolar neuron

Macroglia

Astrocytes and Oligodendrocytes

Astrocytes

• Provide structural support
• Act as glial guide wires for neurons during development
• Maintain ion balance around neurons
• Participate in reuptake of neurotransmitter adjacent to synapse
• Surround blood vessels (BBB)
• Migrate to site of neuronal injury and proliferate to aid in repairing damaged neuronal
tissue (specialized set of astrocytes called Reactive Astrocytes)
- Unfortunately if unsuccessful can lead to gliosis – glial scarring
- glial cells dominate injury site

Protoplasmic astrocytes

Delicate w/ many branched processes
Occur in gray matter

Fibrous astrocytes

More fibrous and robust
Radiate long processes in all directions from soma
Surround blood vessels (Blood Brain Barrier)
Occur in white matter
Cover exterior surface of CNS

Oligodendrocytes

Predominate in white matter
Extend mulitiple arms to myelinate multiple axons within the CNS ONLY
Insulating cells in the CNS
(in PNS – this function is done by the Schwann Cell)

Microglial Cells

Macrophages (scavengers)
Survey the CNS to combat infection
Activate and infiltrate injured zones of CNS to scavenge for infection and damage
Some exist within the CNS and others infiltrate from the blood

The Central (CNS) and Peripheral (PNS) Nervous Systems

A. Structural (anatomical) – 2 systems
1) Central Nervous System (CNS)
– brain (cerebrum, cerebellum, brainstem) and spinal cord
2) Peripheral Nervous System (PNS)
– cranial and spinal nerves

Afferent

towards CNS, sensory

Efferent

away from CNS; motor

Extracellular recording

shows the relative frequency and pattern
of action potentials in neurons that form the neural circuits for
the myotatic reflex

Functional Analyses

Functional Brain Imaging
• Positron Emission Tomography (PET scan)
– E.g., radioactive glucose
• Functional Magnetic Resonance Imagine (fMRI)
– Blood Oxygen Level Dependent (BOLD sign

fMRI is a technique which
allows for a good balance
between spatial and temporal
resolution.
Magnetoencephalography
(MEG) is a technique that
allows for mapping brain
activity at high temporal
resolution, but low spatial
resolution.
Positron emission tomography
(PET) acquires signals more
slowly and approaches the
spatial resolution of fMRI.al)

Anterograde tracer

maps connections from source to their termination

Retrograde tracer

maps connections from termination to origin

Tract Tracing

– Pros
• High(est) spatial resolution, i.e, ultrastructural level
(microscale)
• Produces a wealth of quantifiable data
• Generates testable, functional hypotheses
– Cons
• Invasive
• Time intensive

Brain Mapping: Diffusion Tensor Imaging

Diffusion tensor imaging (DTI), is a technique
that allows bundles of axons that carry
information between brain regions to be
visualized.
DTI works by measuring magnetic fields to
calculate the diffusion of water molecules.
The diffusion of water in tissue varies with
direction (anisotropic) which can indicate the
underlying tissue orientation.

Diffusion Tensor Imaging (DTI)

– Pros
• Non-invasive; can be conducted in/ex-vivo
• Fast
• Vast clinical applicability
– Cons
• Low Resolution (Macroscale)
• Indirect Technique