Locomotion Flashcards
The body has two motor components, the visceral motor system and the somatic motor system.
What are these responsible for?
Visceral:
- Sympathetic, parasympathetic and enteric divisions
- Autonomic ganglia and nerves
- Signals to smooth + cardiac muscle
Somatic;
- Motor nerves
- Signals to skeletal muscle
Skeletal muscles are innovated by lower motor neurons in a somatotropic fashion.
What does this mean?
- That lower motor neurons that are closer to the centre of the spinal cord are connected to more proximal muscles
- Lower motor neurons closer to the periphery control distal muscles
Outline the neural pathway involved in reciprocal inhibition
- Reciprocal inhibition can happen due to inhibitory interneurons
- A sensory afferent travels from muscle spindle (for example the BICEP) to SC
- It then splits, synapsing with an alpha motor efferent traveling back to the bicep promoting contraction, and an inhibitory interneuron
- The inhibitory interneuron synapses with an alpha motor efferent to the TRICEP, inhibiting contraction
What is the role and structure of the muscle spindle?
- A specialised sensory structure embedded in the muscle involved in proprioception
- Intrafusal muscle fibres arranged IN PARALLEL with extrafusal fibres (like a small muscle within your muscle)
- When alpha motor neurons excite extrafusal fibres causing contraction, gamma motor neurons contract the spindle
- Contraction of the spindle is essential for sensory afferents within spindle
Through what type of motor neuron does the muscle spindle contract?
Gamma motor neurons
What type of sensory afferent axons respond rapidly to stretch and mediate reflex adjustments when the axon is stretched
Type 1 (Ia axon)
What two things does a motor unit consist of?
Alpha motor unit and muscle it innervates
What are the 3 types of input that an alpha motor neuron receives from?
- Spinal interneurons
- Muscle spindles
- Upper motor neurons in the brain
What are the qualities of
a) Slow motor units
b) Fast motor units
a)
- Smaller, consisting of small alpha motor neuron innervating small RED muscle
- Slow contraction
- Small force generated
- Resistant to fatigue (posture e.g.)
b)
- Larger alpha motor neuron innervating larger PALE muscle fibres
- Generate more force
- Easily fatigued as sparse mitochondria
- Brief exertions requiring large forces
What is the size principle in muscle contraction?
- Number of motor units active determines force produced by a muscle
- Gradual increase in tension results from the recruitment of motor units in a fixed order according to their size
Describe the neural mechanism of the Flexion-cross Extension reflex
- In response to pain stimuli (stepping on pin), reflexes allow avoidance
- These are due to polysynaptic connections between sensory neurons and interneurons in the spinal cord
- Sensory afferent carrying pain signal splits into 4
- One excites agonist muscle causing contraction (lifting leg)
- One synapses with inhibitory interneuron causing relaxation of antagonist muscle on same leg
- The other two perform opposite functions on the contralateral leg to shift weight onto other foot
Consider the swing phase and stance phase of limped animals
- Swing = leg flexed, raised off ground
- Stance = planted
What are pattern generators?
neural circuits that generate patterns of neural activity that underlie rhythmic motor behaviours such as walking and swimming
What do CPGs do?
- Autonomous patters that activate antagonistic muscle groups in alteration
- Can occur independent of sensory feedback
- Generates a suitable pattern of output - sensory feedback adjust this output to suit environmental demands
How were Lamprey’s used to study CPGs?
- They have robust sub-program that involves activation of one side of the body with inhibition of the other to produce swimming motion
- This involves glutamate
- Much coordination is required between segments - Achieved through Pacemaking neurons
Outline the molecular basis of CPGs
- Pacemaking neurons in the lamprey generate oscillatory activity
- The basis of this oscillation is the unique properties of NMDA receptors
- This being the voltage-dependent Mg2+ blockade AND K+ channels working in together
Describe descending pathways from the brain that control movement
- Groups of myelinated nerve fibres that carry information from the brain to effector muscles via the sc
- Can be pyramidal (voluntary) or extrapyramidal (involuntary)
Outline the neural systems involved in movement control
Basal Ganglia:
- Involved in movement initiation/suppression
- Signals to motor cortex –> descending pathways
- Involved in voluntary movement
- Somatotropic organisation
Cerebellum:
- Coordination of ongoing movement
- Signals to brainstem centres and to descending system of upper motor neurons
- Involved in rhythmic, stereotyped and postural movement/control
These descending systems relay to SC and brainstem circuits to affect CPgeneration + SENSORY-MOTOR INTEGRATION and motor neuron pools
What is the function of the premotor cortex
Within this is the premotor area required for the planning of movement (NOT movement itself)
What is the function of mirror neurons
These fire when an individual performs an action and when they observe someone else performing the same action
what does the basal ganglia do?
Influences movement by regulating the activity of upper motor neurons
Describe the direct and indirect pathways of the basal ganglia
DIRECT:
- Thalamus sends Glutamatergic signals to the Neocortex
- Neocortex relays to the Striatum (D1 Receptors) via glutamate
- The striatum then inhibits the Internal Pallidum which functions to inhibit the Thalamus (happens via GABA) (see diagram below)
- I.e., the Thalamus is DISINHIBITED by activation of the Striatum
- Direct pathway results in movement initiation! Results in Release
INDIRECT:
- One more inhibitory branch added between the Striatum and the Internal Pallidum
- Working Backwards:
- Thalamus is inhibited by Internal Pallidum
- IP is excited by the Subthalamic Nucleus
- The SN is inhibited by the External Pallidum
- The EP is inhibited by the Striatum
- In the indirect pathway, the striatum communicates with D2 Receptors
Simplified basal ganglia direct / indirect pathways:
Direct:
- Neocortex excited the striatum (D1)
- Striatum disinhibits thalamus
- Release
Indirect:
- Neocortex excites striatum (D2)
- Striatum disinhibits Subthalamic nucleus
- STN excites internal pallidum which inhibits the thalamus
What are 2 conditions that affect the direct/indirect pathway of the BG?
PD:
- Degeneration of DA neurons in the BG
- Decrease direct pathway activation
- Difficulty starting and stopping movements
Huntington’s Disease:
- Direct pathway takes over and releases movements
- Degeneration of medium spiny neurons
The motor cortex broadcasts to 3 areas, what are they?
Basal Ganglia:
- action choice, motivation
Brainstem
- integrates and commands, specialised descending command lines
Spinal cord:
- Execute movement
Why is C. Elegans an ideal organism to study locomotion?
- Simple nervous system is completely known: 302 neurons in 118 anatomically defined classes, 75 motor neurons, 6400 synapses
- Tolerates NS defects
- Basic mechanisms conserved
- Whole brain imaging possible: fluorescent Ca2+ indicator in cell nuclei allows measurement of brain-wide neuronal activity with single cell resolution - possible in live organisms
Sensorimotor integration seen by whole brain imaging in an organism in which all cells are known allows for deep understanding of neural circuits
