Neural Development 2: Regressive

Apoptosis

Normally Occurring Cell Death. Extrinsic signal triggers a lethal gene cascade “programmed cell death.” Cell commits suicide.

What does cell death create?

Nervous Systems; massive death of neurons during development. Coincident with the establishment of inputs and outputs.

Targets

Supply neurotrophic factors, some suppress expression of lethal gene cascades, some induce expression of protective molecules that block lethal gene products. Production is activity dependent.

Synapse Elimination

Retraction of axon or dendritic branches. NOT death of neuron - just loss of some synaptic connections (error correction, refinement.) Activity Dependent

Polyneuronal Innervation

The phenomenon where multiple motor neurons innervate a single muscle fiber during development. As maturation occurs, redundant connections are eliminated, leading to a more efficient neuromuscular junction.

Polyneuronal Innervation

• Neonatal muscle fibers get input from multiple motoneurons

- corrected by an activity dependent synapse elimination

- Muscle fibers “compare” activation

- active synapses are retained preferentially

• At maturity

- motor units (“motoneuron and all of the muscle fibers it innervates”)

- each muscle fiber driven by only one motoneuron.

Ocular Dominance Columns

• Segregate to define clear

ocular dominance columns

• Activity dependent

• Thalamic axons carrying information from left

and right eyes initially overlap

(active synapses are retained preferentially)

• Interaction between information from both eyes occurs in layer IV of

primary visual cortex

• Most neurons in primary visual cortex are binocular- respond to

input from either eye

• But have a preferential

response to one eye:

ocular dominance