Cortical Map Plasticity. Gerald Finnerty Dept Basic and Clinical Neuroscience

Transcription

1 Cortical Map Plasticity Gerald Finnerty Dept Basic and Clinical Neuroscience

2 Learning Objectives Be able to: 1. Describe the characteristics of a cortical map 2. Appreciate that the term plasticity is used in many ways. 3. Understand the basic mechanisms that underpin cortical map plasticity. 4. Give examples of neuroplasticity associated with sensory experience or learning. 5. Describe instances where plasticity might promote recovery from disease or, conversely, be part of a disease process

3 What is (neuro)plasticity? William James (Principles of Psychology, 1890) the possession of a structure weak enough to yield an influence, but strong enough not to yield all at once Konorski (Conditioned Reflexes and Neuron Organization, 1948) differentiated between the immediate reaction of neurons which he refers to as excitability and the long-lasting transformation of a system which he called plasticity. Persistent change in a neuron, a neural circuit or set of neural circuits that occurs in response to its history

4 LEVEL OF PLASTICITY Neuron Ion channels Synapse Receptive field (single-unit firing) EM Cortical Circuits Microcircuits in a cortical column Cortical maps, local and cross-modal Higher cortical functions, memory, emotions fmri

5 Where are the cortical maps? From Norman Geschwind

6 What is a cortical map?

7 Cortical space is a transformation of the sensory periphery

8 HISTORY Cortical Map Plasticity Hubel and Wiesel (1960s) - Ocular dominance map of kittens (monocular deprivation) - plasticity restricted to a critical period Merzenich (J Neurophysiol 1983) - Median nerve section - Reorganization of the map of the hand in contralateral SI 1990s e.g. Diamond et al Science 1993 non-traumatic sensory experience induces adult cortical plasticity Nudo (Science 1996) - Motor map reorganizes after stroke

9 Plasticity toolbox changing activity in necortical circuits 1. Modify synaptic strength synapse-specific e.g. LTP, LTD global, homeostatic plasticity 2. Alter neuronal excitability spike number firing pattern 3. Rewire neural circuits

10 Imaging whisker representations with fmri A Alonso et al Cereb Cortex 2008

11 Whisker map plasticity imaged with BOLD fmri Control 3 day trim Group maps, 5 Hz whisker deflection Albieri et al Cereb Cortex 2014

12 Whisker representations expand initially and then retract Albieri et al Cereb Cortex 2014

13 What are the cellular mechanisms underlying cortical map plasticity?

14 Synaptically-connected pyramidal neurons in SI 20 m APV + CNQX Cheetham et al J Neurosci 2007

15 Mechanisms of cortical map plasticity depend on whether sensory input is retained or lost Part 1 - Cortex that has lost its sensory input (deprived)

16 Extensive rewiring of excitatory cortical microcircuits after 3 days Albieri et al Cereb Cortex 2014

17 Rewiring of excitatory cortical microcircuits in deprived cortex Albieri et al Cereb Cortex 2014

18 Mechanisms of cortical map plasticity depend on whether sensory input is retained or lost Part 2 - Cortex with retained sensory input (spared)

19 Local excitatory connections strengthen slowly in spared cortex Cheetham et al J Neurosci 2007

20 Synaptic contact zone area is linearly related to synapse size Cheetham et al Cereb Cortex 2014

21 Strengthened synapses exhibit structural plasticity - pansynaptic growth Cheetham et al Cereb Cortex 2014

22 Inhibition and Cortical Map Plasticity 1. Different types of Interneurons Calbindin Parvalbumin (PV+) 2. Different types of inhibitory circuit Feedforward Feedback

23 Excitation of Fast-spiking Interneurons Albieri et al Cereb Cortex 2014

24 Inhibition of Pyramidal Neurons Albieri et al Cereb Cortex 2014

25 EXPERIENCE-DEPENDENT PLASTICITY, LEARNING AND THE NORMAL BRAIN Highly-trained musicians have a larger cortical representation of piano tones with a concomitant increase in the size of auditory cortex. String players develop an enlarged representation of fingering digits in sensori-motor cortex. Pascual-Leone et al studied patients who had been blind for many years. MEG maps of index finger used for reading Braille. Size of map varies with holidays and even over a weekend off work. Cognitive enhancers

26 PLASTICITY AND NEUROLOGICAL DISEASE Cortical plasticity contributes to recovery from brain damage (Nudo, 1996) Cortical plasticity and therapeutics. Problem: excessive or aberrant reorganisation has been proposed to cause perceptual illusions and disease e.g. phantom limb pain (Ramachandran) and tinnitus.

27 Plasticity and disease Cheetham & Finnerty ACNR 2007