Somatic Nervous System Flashcards
What are the classifications of the somatic nervous system?
-
somatic afferent- somatosensory afferents within nerves – pain, touch, position, temperature, itch
eg. localised pain (somatosensory fibres/nerves – pain, touch, -
somatic efferent- motor axons within nerves
eg. skeletal muscle activation -
visceral afferent- viscerosensory afferents within nerves of the viscera and blood vessels
eg. GI pain -
visceral efferent- smooth and cardiac motor (ie. gland- and gut wall-innervating) axons within nerves
eg. GI muscle activation (vomiting)
How do the CB, CS and pyramidal tracts project?
Lateral M1 neurons controlling the face project down the corticobulbar tract and medial M1 neurons controlling the body project down the corticospinal tract. These pyramidal tracts project through the genu (CBT) and posterior limb (CST) of the internal capsule
What is the corticospinal tract?
The CS tract(s) convey motor commands to lower motor neurons in the ventral horn of the spinal cord via the internal capsule, cerebral peduncles, medullary pyramids, and lateral funiculus
What is the corticotubular tract?
The CB tract is the functional equivalent of the CS tract, but it synapses with the cranial motor nuclei – the brainstem analogue of the ventral horn in the spinal cord. Clinically, the pattern of innovation is particularly important for the facial motor nucleus (VII), since it controls facial motor tone, and that is very diagnosable.
What is Bell’s palsy and how can it be distinguished from UMN lesions?
Bell’s palsy is a common condition that results from damage to the facial nerve. Bell’s palsy, a lower motor neuron lesion (C, right), can be distinguished from upper motor neuron lesions (A, B, right) based upon whether the whole face or just the lower face loses its muscle tone. The ‘upper’ motor neurons innervate the superior face ‘lower’ MNs bilaterally, but the inferior face LMNs predominantly unilaterally, so if there is an UMN lesion, only lower face tension is lost
Lower vs upper motor neurons
Lower motor neurons- are motor neurons that directly innervate muscle eg. the ⍺ motor neurons of the ventral horn or cranial motor neurons
Upper motor neurons- control lower motor neurons. An example are the layer 5 Betz neurons of the primary motor cortex, whose axons form the corticospinal and corticobulbar tracts.
What happens in proprioception?
Proprioceptive afferents sense muscle tension. While extrafusal muscle fibres cause muscle tension, intrafusal muscle fibres are innervated by proprioceptors (type Ia and type II afferents) to sense tension. Tendon tension is sensed by type Ib proprioceptors innervating golgi tendon organs
Intramural fibres vs GTOs
What is the action of alpha and gamma motor neurons?
𝞪 motor neurons innervate extrafusal fibres, which generate contractile force of the muscle;
𝞬 motor neurons innervate intrafusal fibres, and stimulate them to contract, maintaining the sensitivity of proprioception.
- Modulation of the levels of γ motor contraction relative to α motor contraction controls the sensitivity of proprioception – this sensitivity is called gain. Gain is controlled by lower (spinal cord) subconscious circuits and upper (brain) conscious circuits
- By tensing the intrafusal fibres so that as the muscle gets shorter, they contract, maintaining the proprioceptors’ ability to detect tension in them.
Why does an UMN lesion cause clonus?
Loss of inhibition of inter neurons. The action of inhibitory neurons ensures that muscle contraction does not occur simultaneously (= prevents spasticity).
What is the reticular system?
is an enormously complex collection of nuclei in the brainstem. It is responsible for lots of subconscious motor control (eg. posture, breathing).
What is the vestibular system and the reflexes?
Vestibular sensation is the sense of balance/head position.
Vestibulospinal reflex: stops you falling over by innervating spinal cord LMNs.
Vestibulocollic reflex: keeps your head up by innervating cervical LMNs.
Vestibulo-ocular reflex: keeps your gaze fixed by innervating cranial nuclei of III, IV, and VI (extra-ocular nerves)
What is the tectospinal tract?
It’s the ‘roof’ over the cerebral aqueduct between the 3rd and 4th ventricles. Tectum = superior colliculus (responsible for visual reflex movements) + inferior colliculus (auditory reflex movements)
sends motor signals to coordinate with eye and head movements (neck and shoulder LMNs + nuclei of III, IV, and VI).
What do somatosensory pathways consist of?
Consist of 3 Order Ascending Neurons:
- Sensory neurons
- Spinal cord/brain stem
- Thalamus
What are the somatosensory receptors and their function?
What are the somatosensory nerve fibres and what sensation are they responsible for?
What is the physiology of pain?
What is transduction?
Transforming noxious stimuli into electrical activity. Nociceptors are polymodal. Capable of creating an AP from different stimuli:
- Temperature extremes
- Trauma
- Hypoxia
- Chemicals
- Mechanically gated Na & Ca ion channels
- Heat sensitive ions channels
- Mast Cells: 5HT/Histamine
- Damaged Cells:
→ Bradykinin on G-protein coupled receptors
→ ATP on ATP-sensitive K channels
→ Potassium
→ Prostagladins
What is transmission?
conduction of electrical impulse through the nervous system
1stOrder Neurone
- A-beta/A-delta & C
- Dorsal Horn: Laminae I-V
2ndOrder Neurone
- Decussate
- Ascend in thelatspinothalamic tract
3rdOrder Neurone
- Thalamus
- Sensory Cortex
What is modulation?
process of alterations in the pain signals along the transmission pathway of pain
What are the excitatory and inhibitory neurotransmitters and neuropeptides?
Where do analgesics work?
What are the domains of chronic pain?
Nociceptive vs neuropathic pain
What is the WHO pain ladder?
What is the mechanistic approach to pain therapy?
