Biopotential, Motor & Sensory Function Flashcards
What is the soma?
Cell body of a neuron
What are dendrities
End of neurone to receive information
How many neurons in the brain?
10^11
What is the function of the axons?
Transmit electrical signal output
How does information travel from 1 neuron to another?
Synapses
How do nerve cells survive?
Oxidative metabolism only
Die in anaerobic conditions
Can nerve axons regenerate?
In PNS, not CNS
How many synapses in an adult?
100-500 trillion
Different parts of a neuron
Cell Body (soma) Cell Membrane Nucleus Dendrites Terminal Branches Axon Myelin Sheath Node of Ranvier
What are the types of neurons?
Sensory (afferent)
Motor (efferent)
Interneurons
What do sensory neurons do?
Send info from sensory receptors (in skin/nose/eyes/tongue/ears) TOWARDS the CNS
What do motor neurons do?
Send info AWAY from CNS to muscles or glands
What do interneurons do?
Send info between sensory and motor neurons. Most interneurons are located in CNS
What is the concentration gradient on ions across the nerve cell? How does it work?
- high concentration of K+ on the inside
- high conc. of Na+ on outside
- in resting = K+ diffuse through membrane creating a potential difference which limits perfusion
- typically -70 to -80mV
- active pump which pumps K+ in and Na+ out of the cell balances diffusion
What is the role of ion selective gates?
- can be controlled (on or off)
- electrically controlled according to membrane potential or chemically
What is the role of the K+ gate?
- allows K+ ions to diffuse out of the cell creating a potential across the membrane
- acts like a resistor capacitor circuit for small electrical disturbances
- makes cell more negative = hyperpolarisation
- makes cells more positive = depolarisation
How is the action potential created?
- depolarisation beyond the threshold (-45mV) occurs
- this means sodium ion gates open causing further depol -> peaks to +55mV
- sodium ion gates close
- potassium ion gates open = hyperpolarisation
- switch in potential is called an action potential
Positive Feedback Process
- after threshold is reached
- the stimulus causes the sodium ions to open
- as more sodium ions on the outside, inside is negative so sodium ions rush into the neuron
- neuron becomes positive = depolarised
- takes longer for K+ channels to open
- when they do, K+ rush out = hyperpolarisation
- sodium channels close at this point
- action potential goes back to -70 (repolarisation) but actually goes past -70 (hyperpolarisation) as K+ channels stay open for a bit too long
- gradually concentrations return to resting levels
What is the action potential followed by?
- absolute refractory period
- then a relative refractory period
How does the action potential get transferred?
- propagation
- action potential at one site of the membrane causes depolarisation in neighbouring regions
Nodes of Ranvier
- breaks in myelin sheath allowing action potential to propagate
- otherwise speed of propagation is limited by membrane capacitance
What is the effect of increasing axon diameter?
- increases conductance by r^2
- increases capacitance by only r
- hence increases speed
How can capacitance be reduced?
- use of an insulating material called the myelin sheath
- Schwann cells wrapped around the axon
How do synapses work?
- electrical or chemical
- most are chemical
- action potential arrives
- calcium channels open
- calcium diffuses into pre-synaptic terminal
- causes release of neurotransmitter from vesicles
- neurotransmitter diffuses across synaptic cleft
- detected at post-synaptic receptors (chemical controlled ion gates)
What are the different types of post synaptic potentials?
- excitatory = open sodium channels and depolarise membrane
- inhibitory = increase membrane polarisation
What happens to neurotransmitters which do not bind with receptors?
Metabolised by enzymes and pulled back into the pre-synaptic terminal through reuptake
Neuromuscular Junction
- functions the same way as a inter-neuronal synapse does
- action potential from motor neuron invokes an action potential in innervated muscle fibre
- action potential propagates bi-directionally from NMJ in same way as action potential does along a nerve fibre
- induces a shortening of the muscle fibre
Parts of NMJ
- motor nerve fibre
- myelin
- axonal terminal
- Schwann Cell
- active zone
- sarcolemma
- synaptic vesicles
- basement membrane
- synaptic cleft
- region of sarcolemma with Ach receptors
- junctional folds
- nucleus of muscle fibre
What are the senses?
- touch
- taste
- smell
- sight
- sound
- balance
- temperature
- position
- force
Simple motor control test
- how is motor control regulated with and without sight
Reflex test
- what happens when tendon is tapped
What does the motor unit consist of?
- motor neuron
- NMJ
- muscle fibres it innervates
- smallest functional block for control of movement
How is force controlled?
By changing the frequency of firing of the motor neuron and by recruitment of more motor units, beginning with the smallest
Large vs. small motor neurons
- largeer = require more excitation = thicker axons = innervate more muscle fibres
Feedback through afferents
- short feedback loops allow for fast involuntary reflexes
- may be mediated by higher control
- receptor -> sensory neuron -> integrating centre -> motor neuron -> effector
Muscle spindle
- a type of displacement transducer
- regulate muscle length by feeding back directly to alpha motor neurons
Gamma motor neuron
- innervate muscle spindles
Knee jerk reflex
- monosynaptic
- simplest possible reflex
Golgi tendon organ
- type of force transducer
- feeds back negatively to motor neuron to regulate muscle tension
How do sensors respond to passive stretch?
- muscle spindles lengthen
- output a train of action potentials
- when muscle is activated, spindle is shortened and becomes ‘silent’
How do sensors respond to active stretch?
- force is applied to the Golgi tendon organ
- outputs a train of action potentials
- activation of the muscle produces the same effect
