Somatic (motor) Nervous System / Flashcards
What are the components of the NS?
Sensory components: sensory ganglia and nerves, sensory receptors at surface and within the body >
Cerebral hemispheres, cerebellum, brainstem, spinal cord = CNS - analysis and integration of sensory and motor information
Motor components:
-visceral motor system - sympathetic, parasympathetic and enteric divisions, autonomic ganglia and nerves > effectors - smooth muscles, cardiac muscles and glands.
- Somatic motor system - motor nerves > effectors - skeletal muscles
What is the spinal organisation of neurons?
Sensory neuron cell bodies in the dorsal root ganglia, axon terminals in the dorsal horn. Project into skin and muscle, detect muscle tension. Synapse in monosynaptic reflex arcs with alpha motor neurons or with inhibitory interneurons
Interneurons carry sensory information and regulate motor activity
Alpha motor neurons in the ventral horn
What are somatic afferent/efferent and visceral afferent/efferent examples?
Somatic afferent: somatosensory afferents within nerves - pain, touch, position, temperature, itch e.g. localised pain
Somatic efferent: motor axons within nerves e.g. skeletal muscle activation
Visceral afferent: viscerosensory afferents within nerves of the viscera and blood vessels e.g. GI pain
Visceral efferent: smooth and cardiac motor axons within nerves e.g GI muscle activation e.g. vomiting
What is the pyramidal system?
Descending tracts that which originate in the cerebral cortex and carry motor fibres to the spinal cord and brain stem and responsible for voluntary control of the musculature of the body and face
Corticospinal and corticobulbar tracts
What is the cortex divided up into?
Brodmann areas defined by different thicknesses of the 6 layers
There are 52
Areas will fit into regions such as premotor cortex, primary motor cortex, somatosensory cortex, visual cortex etc.
Primary motor cortex
Topographic map of the body exists in the primary motor cortex (M1) where motor commands for each area originates
Density of the cortical neurons that generate information is proportional to the degree of control in the body
Body represented in the medial part of M1 near longitudinal fissure (corticospinal tract)
Face represented in the lateral part of M1 (corticobulbar tract)
Where do the pyramidal tracts travel through in the brain?
Corticospinal tract originates from medial primary motor cortex and travels through the posterior limb of the internal capsule
Corticobulbar tract originates from the lateral primary motor cortex and travels through the genu of the internal capsule (white matter)
Label the three parts of the internal capsule
Where does the corticospinal tract travel?
Project down from primary motor cortex, through
internal capsule,
cerebral peduncles of midbrain,
pyramids of the MO,
Decussate at the pyramids, travel contralaterally through lateral funiculus in the corticospinal tract of the spinal cord
Synapse with motor neurons in the ventral horn of the spinal cord
Most axons will decussate, some will remain ipsilateral
Where does the corticobulbar tract travel?
Originate from lateral aspect off primary motor cortex
Fibres converge and travel through the genu of the internal capsule
Terminate on the motor nuclei of cranial nerves, synapse with LMN - carry motor signals to the facial muscles
Most UMN innervate motor neurons bilaterally except;
UMN for facial and hypoglossal nerve provide contralateral innervation
What will damage to the UMN innervating the facial nerve cause? When can this damage happen?
Palsy of inferior facial muscles in one half of the face - if unilateral damage - below the eyebrow due to bilateral innervation of the the forehead muscles, and UMN from the other side of the brain that are still intact can compensate.
The forehead is spared
Can be due to stroke, MS, subdural haematoma
What will damage to the LMN - facial nerve cause? What can cause this?
Palsy of superior and inferior aspects of one half of the facial muscles - includes forehead
Can be caused by Bell’s palsy
What are lower motor neurons?
Motor neurons that directly innervate the muscle
Cell body lies in ventral horn of spinal cord or in brainstem motor nuclei of the cranial nerves
a-motor neurons and y-motor neurons
What are upper motor neurons?
Control and synapse onto the LMN
Do not leave the CNS
Originate from primary motor cortex
Give rise to the descending motor tracts
How is proprioception given?
Proprioceptive afferents sense muscle tension
Intrafusal muscle fibres are innervated by type 1a and type II afferents to sense tension
Intrafusal muscle fibres do not contract the muscle, they only allow the NS to detect level of contraction
Tendon tension is sensed y type 1b proprioceptors innervating golgi tendon organs - part of the tendon at the end, as it attaches to the muscle
What is the difference between proprioceptive afferent activity of intrafusal fibres and GTOs in passive and active stretching?
Intrafusal fibres and GTOs have same response during passive muscle stretching - both under tension, and afferent activity is sent
If you contract the muscle, intrafusal muscle fibres will have less tension as muscle is getting shorter, but pull on tendons, so GTOs will still be stretched
Therefore the difference between active and passive muscle stretching can be felt
What motor neurons provide proprioception?
y-motor neurons
Innervate the intrafusal fibres and stimulate them to contract when muscles contract - maintains sensitivity of proprioception - still get some signalling activity
What is gain?
The sensitivity of proprioception controlled by the modulation of levels of y-motor neuron activation relative to a-motor activation - coactivation maintains ability to detect tension in them
High gain e.g. when walking on tightrope - concentrated on movement, fine sense of proprioception > accurate movements
Controlled by lower subconscious circuits and upper conscious circuits - concentration
Are alpha and gamma motor neurons controlled together?
When corticospinal tract activates a-motor neurons, it also activates y-motor neurons
Why does UMN lesion cause clonus?
Series of contractions that occur in a muscle when it is suddenly stretched and held in that position. Likley arises via a reduced descending inhibition, which leads to an exaggerated stretch reflex.
Control of flexors and extensors in reflex system and normal spinal function
Depend upon being stimulated but also inhibited by GABAergic interneurons
When activate e.g. flexor, also activate inhibitory interneurons that inhibit MN controlling extensors
Why does hyperreflexia and tremors occur?
UMN lesions
UMN can no longer activate inhibitory interneurons for the opposite muscle, so MN that control the opposing muscle
This activates both muscles at the same time = tremors
Reduced inhibition - exaggerated reflex
What are extrapyramidal tracts/what do they do?
Originate in brain stem, carrying motor fibres to the spinal cord.
Responsible for involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion
Reticulospinal tract function
Reticular system - collection of nuclei in brainstem
Responsible for much subconscious motor control e.g. posture, breathing
What are vestibular reflexes?
Vestibular sensation is sense of balance and head position
Vestibulospinal reflex: stops you falling over by innervating spinal cord LMNs
Vestibulocollic reflex: keeps head up by innervating cervical LMNs
Vestibulo-ocular reflex: keeps gaze fixed by innervating cranial nuclei of III, IV and VI
What is the tectospinal tract?
Tectum:
Superior colliculus - responsible for visual reflex and movements
Inferior colliculus - auditory reflex movements
Where does the vestibulospinal tracts go?
Medial and lateral pathways
Arise from vestibular nuclei, that receive input from organs of balance, at level of pons and medulla
Terminate in ventral horn and synapse with motor neurons in spinal cord
Where does the reticulospinal tracts go?
Medial reticulospinal arises from pons - facilitates voluntary movements and increases muscle tone
Lateral reticulospinal arises from medulla - inhibits voluntary movements and reduces muscle tone
Where does the tectospinal tract go?
Originates at superior colliculus of midbrain that receives input from optic nerves
Neurons decussate and enter spinal cord
Terminate at cervical levels of the spinal cord
