Nerve Regeneration Overview I • Mature neurons do not divide to replace

injured cells. • If injury occurs close to the cell body or the

damage is severe, the entire axon, and therefore cell, may die

• If the cell body is intact damaged axons of peripheral nerves can regenerate.

• Distall portion will disintegrate in a process called Wallerian degeneration, but the Schwann cells will survive and begin to divide due to mitosis stimulatin chemicals releases by macrophages removing cellular debris.

• These Schwann cells release growth factors to encourage growth and form a regeneration tube to encourage axonal growth.

Nerve Regeneration Overview II • Axons regenerate at a rate of ~1.5mm/day, the

greater the distance between severed nerve endings, the smaller the change of regeneration

• CNS nerves do not regenerate, making nerve or spinal cord damage irreversible.

• This is believed to be due to the neuroglial cells of the CNS and not the CNS nerves directly.

• Microglial cells and astrocytes remove cellular debris, but slower than macrophages in the CNS and the oligodendrocytes die, failing to guide fiber regeneration. CNS cells are also surrounded by growth inhibitory proteins surround CNS cells

Guidance Channels

Neurotrophic Growth Factors

Stem Cells

Gene Therapy

Neural Integration• The nervous system is organized in a

hierarchal, or ladder-like, fashion.• Neurons function in groups, rather than

individually or in pairs as we have been looking at them.

• Each group then contributes to broader neural function.

• This fusing of the individual components of the nervous system into a smoothly operating unified system is called neural integration.

Neuronal Pools• Neuronal pools, the first level of neural

integration, are functional groups of neurons that integrate incoming information and forward it to other parts of the nervous system.

• Neurons in the discharge zone are closely associated with the incoming fibers and are most likely to generate impulses

• The faciliated zone, at the peripherary of the pool, are not usually excited by the EPSPs but they are facilitated and can easily be stimulated by another source.

Neural Circuits• Neural circuits, or the patterns of synaptic connections in

neuronal pools, determine the functional capabilities of the neuronal pool.

• There are four potential circuit patterns:– Diverging: one incoming fiber triggers a response in an continually

increasing numbers of neurons farther along the circuit; common in sensory & motor systems

– Converging: opposite of diverging, pool receives input from several pre-synaptic neurons causing a concentrating effect; also common in sensory & motor systems

– Reverberating (Oscillating): positive feedback loop between neurons in the pathway causing continuous output until one neuron fails; involved in rhythmic activities such as breathing

– Parallel after-discharge: incoming fiber stimulates several neurons that eventually stimulate a common output cell, but at different times causing a burst of impulses after initial input has ended; involved in complex, exacting mental processing (problem-solving).

Patterns of Neural Processing• The input processing in these circuits is both serial

and parallel.• Serial processing: the whole system works in a

predictable all or none manner; spinal reflexes are the primary example

• Parallel processing: inputs are segregated into many different pathways; delivered information is dealt with simultaneously by different parts of the neural circuitry; ex) smell triggers multiple thoughts/memories simultaneously