Lecture 4: Motor and Sensory Systems (somatic) Flashcards
Monday 13th January 2025
Is the somatic system voluntary?
yes
In humans, how many pairs of spinal nerves are there?
31
Do spinal nerves also have autonomic fibres?
Yes
Where do motor neurone cell bodies lie?
Motorneuron cell bodies lie in the ventral horn of the spinal cord
What is the corticospinal
pathway?
The corticospinal pathway (or corticospinal tract) is the primary motor pathway responsible for voluntary movement. It connects the brain to the spinal cord, transmitting motor commands from the cerebral cortex to the muscles of the body.
What is a vasicle?
A group of myelinated and unmylinated neurones
Describe a motor neurone
- Has a long axon
- Sits in the ventral horn of the spinal cord
- Lower motor neurones- cause muscles to contract
- Upper motor neurones- come from brain and control the lower motor neurones
- Movement is generated by Motor Cortex (M1)
What does one motor neurone send out its axon to?
Numerous muscle fibres
How is it possible that a single motor neurone can innervate many different muscle fibres at once?
A single motor neuron can innervate many muscle fibers because it has an axon that branches into many terminals, and each terminal forms a connection with one muscle fiber.
What does a motor unit consist of?
A motor unit is the fundamental unit of muscle control in the nervous system. It consists of:
- A single lower motor neuron (from the spinal cord or brainstem).
- All the muscle fibers it innervates (controls).
What is the neuromuscular junction?
The synapse between the axon terminal of the motor neurone and the muscle fibres
What happens when a motor neurone is activated?
When a motor neuron is activated all the muscle fibres that it innervates contract.
Are motor units different sizes?
Yes. (size = number of muscle fibres per motor neuron)
Are motor units intermingled throughout muscles?
Yes
What are smaller motor units in control of ?
Fine control. e.g. extraocular muscles (6-10 muscle fibres per neuron)
What are large motor units in control of?
Coarse control. e.g. gastrocnemius in calf (up to 2000 muscle fibres per neuron)
What are the 2 different types of skeletal muscle that a motor unit can have?
- Type 1 (small)
- Type 2 (large)
Describe type 1 motor units
- slow-twitch motor units that specialize in endurance and sustained activities.
- They are fatigue-resistant and provide low-force, long-duration contractions.
Describe type 2 motor neurones
- Type II motor neurons are fast-twitch motor neurons that control fast, powerful muscle contractions.
- They innervate Type II muscle fibers, which are specialized for quick, high-force movements but fatigue more rapidly than Type I fibers.
How many neuromusclular junctions are there per muscle fibre?
1
What is the neurotransmitter?
Acetylcholine
What is the point of the folds in the muscle fibres?
To increase the surface area for more receptors
What is the neurotransmitter at the neuromusclular junction?
Acetycholine
Describe the action at the neuromusclular junction
- Action potential arrives at the motor neuron’s axon terminal.
- Calcium (Ca²⁺) channels open, allowing calcium to enter the neuron.
- Vesicles release acetylcholine (ACh), a neurotransmitter, into the synaptic cleft.
- ACh binds to receptors on the muscle fiber’s membrane (sarcolemma).
- Sodium (Na⁺) enters, depolarizing the muscle fiber and triggering an action potential.
- Muscle contraction occurs via the sliding filament mechanism.
- Acetylcholinesterase (AChE) breaks down ACh, stopping the signal to prevent continuous contraction.
By which 2 ways can the force of contraction be increased?
- Recruitment
- Temporal summation
What happens during recruitment?
- More motor units are recruited, with the smaller motor units being recruited first
-
Describe how temporal summation increases the force of contraction
- Temporal summation increases the force of muscle contraction by increasing the frequency of action potentials sent to a muscle fiber.
- This leads to stronger and more sustained contractions due to the overlapping of individual muscle twitches.
Is it true that temporal summation is crucial for producing smooth and powerful movements, not just little jerky twitches?
Yes
Muscle fatigue is the result of both local changes in the muscle and signals from the brain, all aimed at protecting the muscle from overuse and damage.
Muscle fatigue is the result of both local changes in the muscle and signals from the brain, all aimed at protecting the muscle from overuse and damage.
What is a twitch?
The response of a muscle to a single action potential
Is fatigue when an action potential is received, but contraction is not carried out?
Yes
Why do muscles show fatigue?
- Generally a protective/defensive mechanism
- Depletion of glycogen
- Accumulation of extracellular K+
- Accumulation of lactate
- Accumulation of ADP + Pi
- Central fatigue from the brain
Do different types of muscle fibres show different resistances to fatigue?
Yes
Do type 1 muscle units show a greater resistance to fatigue?
Yes
Do whole muscles show an intermediatry resistance to fatigue?
Yes
Do type IIb muscles fatigue easily/ show a lesser resistance to fatigue?
Yes
What is muscle tension controlled by?
Muscle tension is controlled by stretch receptors in the golgi tendon organs and the muscle spindles
Muscle spindles
- There is an increase in firing in a stretch receptor when intrafusal muscle is stretched
Muscle spindles…
- Detect changes in muscle length.
- Increase sensory neuron firing when stretched.
- Decrease firing during contraction unless adjusted by gamma motor input.
What’s the difference between the muscle spindle reflex and the golgi tendon reflex?
- The muscle spindle reflex contracts muscles, and prevents overstretching.
- Whilst the golgi tendon reflex relaxes muscles and prevents excessive force.
A receptive field is the specific area of skin (or another sensory surface) that, when stimulated, activates a particular sensory neuron.
A receptive field is the specific area of skin (or another sensory surface) that, when stimulated, activates a particular sensory neuron.
Golgi tendon reflex
- Monitors tension in the muscle
- Protects muscle to prevent damage
- Tendon reflex less sensitive than stretch reflex
but can override it
What are dermatomes?
Spinal nerves that innervate different parts of the body
Describe the process of sensory transduction used by the nervous sytstem
- Stimulus (touch, pressure, hot, cold pain etc)
- receptor
- Change in permeability of nerve ending
- Change in membrane potential of nerve ending (generator potential)
- Generation of action potentials in nerve ending
- Propagation of action potentials to the CNS
- Integration of information by the CNS
“Generator potentials decrement during conduction
Action potentials do not decrement”. Is this true?
Yes
” If a stimulus is very light, the generator potential that is produced won’t reach the threshold, so you won’t feel much”. Is this true ?
Yes
What are the different types of sensory receptors?
Mechanoreceptors mechanical force cutaneous touch
Pain receptors tissue damage cutaneous pain
Thermoreceptors hot and cold cutaneous
Chemoreceptors dissolved chemicals smell taste
Photoreceptors light rods and cones
what is the law of specific nerve energies?
The Law of Specific Nerve Energies, proposed by Johannes Peter Müller in the 19th century, states that the nature of a sensory perception is determined by the specific nerve pathway that carries the signal to the brain, rather than the stimulus itself.
What is adaptation?
When sensory receptors adjust their response to a constant stimulus over time.
What are the 2 types of adaptation?
Tonic (Slowly adapting receptors):
Phasic (Rapidly adapting receptors):
Tonic (Slowly adapting receptors):
- These receptors continue firing as long as the stimulus is present but at a slower rate.
- Example: Merkel’s disks (important for steady pressure and texture detection).
Phasic (Rapidly adapting receptors):
- These receptors fire briefly at the beginning and end of a stimulus, but not during continuous stimulation.
- Example: Pacinian corpuscles (detect vibration and rapid changes in pressure).
What does AM to FM conversion mean?
The amplitude (strength) of the stimulus is converted into a frequency (rate) of nerve impulses.
Why is AM to FM conversion important?
Instead of sending a stronger electrical signal for a stronger stimulus (since action potentials are all-or-none), the nervous system increases the firing rate.
Example:
- A gentle touch produces low-frequency action potentials.
- A firm press produces high-frequency action potentials.
Do Tonic receptors (Merkel’s disks) help detect sustained pressure?
Yes
ventral horn=
motor output
dorsal horn=
sensory input
Do Phasic receptors (Pacinian corpuscles) detect changes in pressure and vibration?
Yes
Does AM (stimulus strength) → FM (nerve firing rate) conversion help the nervous system encode information efficiently?
Yes
What are receptive fields?
A receptive field is the specific area of the body (or sensory space) where a stimulus will activate a particular sensory neuron, resulting in an action potential being produced
What is two point discrimination?
- Two-point discrimination is the ability to distinguish two separate points of contact on the skin.
- It is a measure of tactile acuity and depends on the density and size of receptive fields in a particular area of the body.
What is the flexion reflex?
- Includes both the motor and sensory systems
- Is a protective spinal reflex that causes a limb to rapidly withdraw from a noxious stimulus (i.e pain, extreme temperature).
What is white matter?
the outer part of the spinal cord, full of myelinated axons. Connects the body and the brain
What are spinal nerves?
Nerves that come out from the spinal cord
What information do spinal nerves carry ?
Each spinal nerve carries two types of information:
- Sensory info: from your body to the spinal cord (like touch, pain, temperature).
- Motor info: from your spinal cord to your muscles (to make you move).
What is the corticospinal tract?
The corticospinal tract is a descending motor pathway that starts in the cerebral cortex and travels all the way down to the spinal cord to control voluntary muscle movement
Is it true that acetylcholine activates nicotininc receptors on the muscle fibre, whcih depolarises the muscle fibre?
Yes
