KIN 313 Lecture 6 - Motor units, sensory receptors, afferent nerve fibres and graded potentials

23 September 2025

direction of afferent info

from periphery to CNS

path of travel for afferent info

stimulation of a receptor in the periphery

mylenated axon carries signal towards CNS

afferent nerve cell body (soma) is in dorsal root ganglion

mylenated axon projects into spinal cord

enters white matter and splits

• either projects up through a pathway to the brain

• or into the grey matter to give sensory information directly to the motor neuron pool or by first synapsing to an interneuron and then to the motor neuron pool

definition of a ganglion

collection of cell bodies in the periphery

path of sensory info coming to CNS and then out to muscles

stimulation of a receptor in the periphery

mylenated axon carries signal towards CNS

afferent nerve cell body (soma) is in dorsal root ganglion

mylenated axon projects into spinal cord

enters white matter and splits

• either projects up through a pathway to the brain

• or into the grey matter to give info directly to muscles

  • through a synapse directly onto a motor neuron

  • or by first synapsing to an interneuron and then to the motor neuron pool

then the signals from the motor neurons are sent out the anterior horn via the ventral route and sent out to muscles

sensory innervations to the muscle spindles

1. primary afferent a (Ia)

2. secondary afferents (II)

sensory innervation to GTOs

Primary afferent b (Ib)

how many afferent nerve types are there, what are they labeled as and how are they ranked

4 afferent nerve types

labelled based on cross sectional diameter

I, II, III, IV

I = largest/fastest

IV = smallest/slowest

how are sensory nerves / afferent fibres categorized?

by diameter (velocity)

where they go (spindle or GTO)

overlap of Ia and Ib on a finer diameter vs number of fibres graph

at the places of overlap, a finer of that size could be Ia or Ib, only way to know is by looking at where they are going (spindle vs GTO)

are all afferent filers myelinated?

no

synaptic potentials

cause membrane shift on postsynaptic membrane. if the shift is big enough shift, and AP is generated in the axon hillock of the postsynaptic membrane

where are APs generated and why

in the axon hillock because there are lots of channels here waiting for the membrane to hit threshold. as soon as it does, they open and flood the hillock causing a rapid shift in membrane potential → AP

AP vs EPSP

Action Potentials

APs are “all or none” events

they happen in the axon hillock, and once they start, they can’t be stopped. 

they can travel down the axon, with or without myelin

has a refractory period where the potential is so negative that there is no way to fire another

Excitatory Post-Synaptic Potentials

graded → size can vary, magnitude is proportional to the amount of neurotransmitter released

if the magnitude isn’t enough to trigger an AP, then it doesn’t do anything 

local → not propagated → potential dissipates as you move down the cell → brief and transient

no refractory period → can send another right after the last one → they can summate together

results of interplay of post-synaptic potentials (EPSPs and IPSPs)

• lots of EPSPs = shifts membrane enough to fire AP

• equal EPSP and IPSP = nothing happens to the membrane

• lots of IPSPs = very negative membrane → definitely no AP fired

how is size of EPSP or IPSP determined

magnitude is proportional to amount of neurotransmitters released

what determines if a post-synaptic potential is excitatory or inhibitory? (EPSP vs IPSP)

the type of neurotransmitter fired

divergence

when one neuron synapses/affects multiple neurons

seen in muscle spindle

convergence

multiple neurons influence fewer neurons

either directly or indirectly through other neurons first

seen in motor neurons and muscle spindle

convergence results in what kind of summation?

spatial summation

all influencing together by hitting the same motor neuron

if they all hit at the same time, they summate

graded EPSPs or IPSPs arrive at the hillock and add up/summate to hopefully trigger an AP

spatial summation

motor neurons converge onto the same motor neuron

if they all fire at the same time, they summate

graded EPSPs or IPSPs arrive at the hillock and add up/summate to hopefully trigger an AP

effect of an IPSP on EPSPs

IPSP diminishes the effects of the EPSPs

if you have enough EPSP to trigger an AP but you throw in a few IPSPs, they will no longer be able to trigger an AP

temporal summation

one thing stimulating lots of times quickly so that the EPSP doesn’t have time to diminish 

summates to hopefully reach an AP

true or false: temporal summation and spatial summation are independent of each other

false. they can work together

can specially and temporally summate together

how much EPSP to trigger an AP in a small motor neuron vs a big one

small MN has a low threshold and high resistance so it doesn’t need as many EPSPs

big MN has a high threshold and low resistance so it needs more EPSPs

the only thing that has a monosynaptic connection to the motor neuron pool

muscle spindle

things that determine if an AP happens

• how frequently do EPSPs come

  • temporal summation

• how many neurons converge onto the same neuron

  • spatial summation

• are there any IPSPs

  • diminish EPSPs

• how big are EPSPs/IPSPs

  • how many neurotransmitters are released