brain plasticity and integrated brain

Brain plasticity

Synaptic plasticity

The brain is always active (even in sleep)

The pattern of activation of neurons can itself impact future activation of neurons

The adaptability/changeability of neural connections is referred to as synaptic plasticity

Existing synapses can be strengthen or weakened

New synapses can be generated existing synapses can shrink or be removed

Long term potentiation (LTP)

Synaptic connections becomes stronger when one synapse is repeatedly accompanied by an action potential at the post synaptic neuron

Post synaptic neuron will become more “sensitive” to neurotransmitter release from presynaptic neuron and more likely to reach the threshold for activation

Increasing the likelihood of a post-synaptic potential allows the neural signal to be transmitted more quickly (remember the “rate law”)

Important in learning (and motor speed that comes with practiced movements)

Long-term potentiation (LTP) referred to as either the Herb Rule or Hebbian Learning in recognition of Donald Hebb’s 1949 proposal that “neurons that fire together wire together”.

Long Term Depression (LTD)

Long-term depression (LTD) can be broadly conceptualised as the opposite of LTP with weakening of synaptic connections.

The post-synaptic neuron becomes less sensitive to neurotransmitter release by the presynaptic neurons and will be less likely to fire in response

Memories can fade and skills can be lost if the underlying neural networks are not stimulated sufficiently - “use it or lose it”

It is important complement to the strengthening of synaptic connections in LTP and weakens synaptic connections that are infrequently stimulated.

LTP & LTD together enable the brain to adapt to current needs

Neurogenesis

Neurons generated through ‘neurogenesis’

This process is largely restricted to the Hippocampus (involved in the consolidation of long-term memories) and the olfactory bulb (involved in detection of odour/smell)

Brain rehabilitation

Bad news

Neurogenesis does not occur in all brain regions.

People typically experience some degree of sustained impairment after severe stroke or traumatic brain damage.

The neurons that are destroyed are typically not replaced in the brain and function will remain impaired.

Good news

Thanks to LTP and LTD, there is tremendous capacity for surviving neurons to adapt, increasing connections between healthy brain regions > weakening or removing connections with damaged areas.

This leads to improved function and rehabilitation through “retraining” their brain to compensate for some lost functioning.

Section summary of brain plasticity

The brain adapts and changes based on “experience” (past activation of neurons impacts future activation of neurons)

Synaptic plasticity

• Long term potentiation (LTP) strengthens connections

• Long term depression (LTD) weakens connections

Neurogenesis

• new neurons can be generated in hippocampus & olfactory bulb

Brain rehabilitation

• recovery of function is possible due to LTP and LTD

Motor Control

Consider the case of hitting a cricket ball

Eyes and ears provide important visual and auditory cues

Some sensory signals may be important for successful execution of the task other signals might be distracting (such heckling from the opponent)

The brain must select the relevant sensory cues depending on the current task or goals which are maintained in working memory and supported by the frontal cortex.

Emotion

The concept of emotion has several different meanings

• positive or negative feelings we experience (happiness, fear, etc)

• Emotion can also be used to refer to the physiological or behavioural changes (increase in heart rate, rapid breathing, eye gaze etc.

Emotions consist of patterns of physiological response and species-typical behaviours

In humans these physiological responses are accompanied by feelings

Feelings are powerful motivators

Emotions are likely to have evolutionary signficance

Emotional responses

Three components of emotional responses include:

• behavioural

• Autonomic

• Hormonal

Muscular changes that are appropriate to the situation that elicits them

Eg. A dog may defend its territory by adopting an aggressive posture (growling, ears forward, back and tail up, teeth bared)

If this behavioural causes fear the intruding dog may adopt a submissive posture (ears back, back and tail down). Otherwise, the defending dog may run toward the intruder and attack

Emotional responses - autonomic

Physiological changes induced by the autonomic nervous system facilitate the behavioural responses.

Eg. In dog defending territory, activating of the sympathetic branch of the autonomic nervous system increases whereas activity of the parasympathetic branch decreases (heart rate increases, and blood is diverted from the digestive system to the muscles).

Parasympathetic - rest and digestive

Sympathetic - flight or fight

Emotional responses - hormonal

Within the body hormones reinforce the autonomic changes

The adrenal gland secretes adrenaline

Adrenaline acts to further increase blood flow to the muscles and cause nutrients stored in the muscles to be converted into glucose

Emotional responses - integration

Three components of the emotional responses (behavioural, autonomic and hormonal) involve separate aspects of the nervous systems

The amygdala plays an important role in coordinating the emotional response and sending the appropriate signals to the autonomic and hormonal responses

The “lie detector” (polygraph)

Emotional reactions to questions are used to determine “truthfulness” of answers

Measures physiological responses associated with activity of autonomic nervous system

Still used routinely by law enforcement authorities in many countries

Consciousness

Clear that different regions of the brain do different things

Clear that different brain regions are highly integrated through a highly complex network of neural connections

Understanding this complex connectivity is a major current goal of neuroscience

Understanding how brain activity supports subjective/conscious experience of the mind remains a mystery

Section summary of integrated brain (functional connectivity)

As our understanding of brain function increases the focus is turning to how different areas of the brain work together to support complex function

Motor control and emotion provide a great example of the integration required to support basic behaviours and mental function

Exactly how the brain generates a mind is not yet known