INTS 9: Motor and Sensory Pathways Flashcards
Where do the main descending motor pathways originate from?
- they originate from the cerebral cortex and the brainstem
Where do the principal descending motor pathways act on?
- they act via lower motor neurons on skeletal muscles
Which motor pathways originate from the cerebral cortex?
Via where?
- corticospinal tracts: via spinal cord
- corticobulbar tracts: via brainstem
Which motor pathways originate from the brainstem?
- vestibulospinal tracts
- reticulospinal tracts
- rubrospinal tracts
- tectospinal tracts
Give a brief description of what somatosensory pathways do
- they carry information from the periphery (skin, joints) into the CNS
- relaying information relating to
- fine touch
- pressure
- vibration
- crude touch
- pain
- temperature
Which two motor pathways originate from the primary motor cortex?
- corticospinal pathway
- corticobulbar pathway
What is the neuron from the primary motor cortex to the spinal cord (or brainstem) known as?
- upper motor neuron
What is the neuron from the spinal cord (or brainstem) to the muscle known as?
- lower motor neuron
What does it mean that the body is represented somatotopically in the primary motor cortex?
- a specific region of the body can be mapped to a specific region in the nervous system
What does this diagram show?
- they are projections of the corticospinal (a) and corticobulbar (b) tracts from the primary motor cortex through the internal capsule
Describe how the corticospinal fibres are distributed throughout the brain
How do they form the lateral corticospinal and anterior corticospinal tract?
- see image below
- 85-90% of corticospinal fibres cross in the medulla (the decussation of the pyramids) to form the lateral corticospinal tract
- the remaining 10-15% form the anterior corticospinal tract
What movements do the lateral corticospinal tract and anterior corticospinal tract control?
- lateral corticospinal tract:
- skilled voluntary movements
- anterior corticospinal tract:
- postural adjustments following limb movements
Describe the pathway and function of the vestibulospinal tract
- the vestibulospinal tract conveys signals from the inner ear (vestibular apparatus) to the musculature
- it controls movements of the neck muscles and postural adjustments by facilitating extensor muscles and inhibiting flexor muscles
- image shows the posterior view showing origin of vestibulospinal tracts from pons/medulla
Describe the pathway and function of the rubrospinal tract
- fibres originate from the red nucleus in the midbrain
- they travel through the brainstem and synapse on lower motor neurons innervating the flexor muscles of the upper limbs
Describe the pathway and function of the vestibulospinal tract
- fibres originate from the reticular formation of the pons and medulla
- they control the orientation of the body towards or away from stimuli
Describe the pathway and function of the tectospinal tract
- fibres originate from the superior colliculus of the midbrain
- controls neck musculature in response to visual stimuli
- orientates the head during eye movements
Familiarise yourself with the transverse (horizontal) section through half a spinal cord
Recall functions of the major motor pathways shown
Which neurons are somatosensory pathways generally composed of?
- generally composed of three neurons:
- primary
- secondary
- tertiary
Describe the pathways of the primary, secondary and tertiary neurons of the touch and proprioceptive (the dorsal column) somatosensory pathways
- the primary neurons of the touch and proprioceptive pathway (the dorsal column pathway) enter the spinal cord
- they ascend on the same side (ipsilaterally) to the medulla where they synapse onto secondary neurons
- the cell bodies of the secondary neurons are located in the gracile and cuneate nuclei
- the ascending tracts from these nuclei cross to the opposite (contralateral) side and ascend as the medial lemniscal pathway to the thalamus
- there, they synapse onto tertiary neurons
- these project to the relevant area of the primary somatosensory cortex
Describe the pathways of the primary, secondary and tertiary neurons of the pain and temperature (the spinothalamic tract) somatosensory pathways
- the primary neurons enter the spinal cord and synapse onto secondary neurons
- they cross the midline and ascend contralaterally to the thalamus, where they synapse onto tertiary neurons
- these project to the relevant area of the primary somatosensory cortex
Describe the dorsal column pathway
- one of the principal pathway systems
- the receptors for touch and proprioception are mechanoreceptors
- they transduce (convert) mechanical stimuli into electrical signsals
- they are the modified terminals of the peripheral axons of primary sensory neurons
- cell bodies are in the peripheral nervous system
- dorsal root ganglia for the body
- trigeminal ganglia for the head
- the axons are fast conducting and innervate receptors located in the muscles (spindles) and tendons (Golgi tendon organs)
- the intensity of the stimulus is coded by the frequency of firing of neuron
- the amplitude of action potential does not change
- firing frequency is related to log: 10 fold stimulus increase, then firing frequency doubles
- somatotopic organisation is maintained through the entire pathway from dermatomes to cortex
- decussation of the pathway for information coming from the body and head occurs in the brainstem
- the difference between adjacent inputs is enhanced by lateral inhibition
Describe the spinothalamic pathway
- the receptors for pain and temperature are polymodal (triggered by mechanical, thermal or chemical stimulus) free nerve endings
- they are slowly adapting and have high thresholds.
- cell bodies are in the peripheral nervous system
- dorsal root ganglia for the body
- or trigeminal ganglia for the head
- receptors are innervated by 2 axon types:
- larger myelinated fibres: which are faster conducting and produces sharp pain
- smaller unmyelinated fibres: which are slower conducting and produce dull aching pain.
- the receptive fields (see below) are usually large and the intensity of the stimulus is coded by frequency of firing.
- somatotopic organisation is maintained through entire pathway from dermatomes to cortex
- decussation (crossing) of the pathway for information coming from the body occurs in the spinal cord.
- decussation of the pathway for information coming from the head occurs in the brainstem.
- cortical representation of pain is not well understood.
- localisation and intensity of stimulus only may register there
What is the receptive field?
- a region of skin innervated by a single sensory neuron
- the firing rate of the neuron is altered by a stimulus in the receptive field
- there is a varying degree of overlap in receptive fields of sensory neurons
- the size of the receptive fields coupled with the degree of overlap influences the degree to which the CNS localises stimuli
What is lateral inhibition?
- precision of localisation of a stimulus may be enhanced with lateral inhibition
- the central portion of a receptive field has an excitatory region surrounded by an inhibitory regions
- this creates contrast in sensation allowing sensory precision
