motor systems Flashcards
human reflexes
- Rene Descartes considered animals as ‘reflexive’ machines where behaviour was automatic consequences of sensory stimulus.
- Flame touches the skin and animal withdraws limb from source of obnoxious
stimulus. - ‘….The fire produces inner vibrations of some sort in the skin of the foot which
then propagate up the nerve tube to the brain, where they open a pore in one
of the brain ventricles, releasing “animal spirits”. These animal spirits travel back down the nerve tube in order to pull the foot away from the fire….’ - He called this a reflex from latin reflectere (to bend back on itself)
complexeties of a simple reflex
- The stretch reflex – stretching a muscle in the body causes
the same muscle to contract due to an increase in tension - The classic example is the knee-jerk (patellar) reflex
- A clearer example come from thinking about forces on a
simpler joint
monosynaptic stretch reflex
Stretching muscle activates stretch receptors that via a sensory-motor synapse in
the spinal cord causes the same muscle to contract.
This involves a simple monosynaptic excitatory synaptic connection between the sensory and motor neuron
secondary reflex
Reflex circuitry has to involve inhibitory pathways, stretching a muscle not only excites the muscle that is stretched but also inhibits the antagonistic muscle.
Inhibition of antagonistic muscles involves a special type of inhibitory interneuron
in the spinal cord
fixed action patterns: complex behavioural sequences
- Many behaviours are much more complex than a simple reflex response
- A basis for complex motor output was proposed by Lorenz and Tinbergen, who had observed that animals show repeated stereotyped sequences of behaviour:
- Fixed Action Patterns
simple systems approach
- The Tritonia brain is made up of balls of neurons called ganglia
- Each neuron has a large cell body and can be recorded with micro-electrodes
- Each neuron has specific intrinsic firing properties and are arranged into circuits with precise connectivity
- These circuits are the same from animal to animal
escape behaviours in insects
Dedicated neuron responds to
looming visual stimuli
Visual information bypasses
brain and targets motor areas
escape reflexes cane be flexible
- Drosophila responds to visual looming stimuli just
like locusts - Take-off direction depends on stimulus direction
- Before jumping they adjust their posture to move
their centre of mass relative to middle legs
escape responses in vertebrates
- Free swimming fish can be placed in a closed loop immersive VR
- At the same time individual neural activity can be imaged by calcium imaging
- Possible because the fish are transparent
- Many vertebrates show similar responses to looming
- Larval zebrafish are a model system in neuroscience, very tractable and with structural similarities to mammals
parallel circuits for different visual behaviours
- Visual reflexes can drive attraction as well as avoidance.
- Attraction to small objects can drive hunting of paramecium in larval zebrafish.
- With genetic tools and imaging, the Retinal Ganglion Cell to optic tectum circuits can be traced for distinct behaviours
central pattern generators
- Last time we looked briefly at the rhythmic circuit of Tritonia swimming
- The circuit responsible for simple patterned behaviour is referred to as a central pattern
generator (CPG) - A network of neurons capable of generating patterned (rhythmic) activity in the absence of sensory input to drive the timing of output.
- Drives behaviours such as walking swimming, flying, breathing, chewing
- Stereotyped but complex and allows voluntary control
intrinsic/endogenous properties:
Cellular properties of neurons which give them particular characteristics
intrinsic/endogenous properties of neurons:
- Firing threshold
- Spike frequency adaptation
- Post-inhibitory rebound
- Plateau potentials
- Endogenous bursting
synaptic properties:
- Sign (inhibitory/excitatory)
- Strength
- Time course
- Mode of transmission (chemical/electrotonic)
- Facilitation/depression etc
patterns of connections:
- Reciprocal inhibition
- Parallel excitation and inhibition
- Mutual excitation
a simple oscillating circuit:
Could use:
* Reciprocal inhibition
* Endogenous bursting
* Plateau potentials
* Post-inhibitory rebound
Could use:
* Recurrent inhibition
* External excitation
making a cpg multifunctional
- CPGs need to be flexible
– Within a behaviour: eg. changing stepping pattern of legs with increasing
locomotion.
– For different behaviours: eg. dog hind-limb may be involved in rhythmic
locomotion with other legs OR active alone in rhythmic scratching
reconfiguring the pyloric rhythm
Dopamine changes:
- the strength of the connections between neurons
- the endogenous properties of individual cells
