Physiology of the spinal cord Flashcards
What are the physiological functions of the spinal cord?
What kind of nerves allow for this function?
Initial processing of somatosensory input by the CNS
Final processing of motor output by the CNS
Spinal nerves- highways of both somatosensory information traffic to the spinal cord (afferent pathways) and motor information traffic from the spinal cord to the muscles (efferent pathways)
What is the difference between sensation and perception?
Sensation is detected by receptors
Perception is the interpretation by spinal cord and brain circuits.
Receptors are…
Specialised in the transduction of energy generated by external stimuli
How do neurons in sensory systems respond?
What kind of primary sensory receptors are they?
Specific for a narrow range of input by a combination of rate, spatial and temporal codes
Primary sensory recepetors come in two types.
One kind have a cell body that resides within the dorsal root ganglia. And recieves stimulation from the PNS and fires action potentials. Their neurotransmitters are released in the CNS. These are the most common type
The other type is found in sensory organs for taste, equilibrium and hearing. Chemical transmission from the sensory cell to the sensory neuron.
How is different sensory stimulus differentiated?
- Different types of receptors for the same modality (e.g. cutaneous mechanosensory receptors- superficial Meissner’s corpuscle vs Pacinian corpuscle for deep, pressure vibration)
- Spatial distribution of receptors
The higher the density of mechanoreceptors the smaller the distance at which two tactile stimuli can be discriminated. The density of mechanoreceptors is much greater on the hand and face than elsewhere allowing the detection of stimuli at a much greater spatial resolution - Different windows of response intensity
What is the motor system?
What is the simplest motor system?
What is the proof this system exists?
Muscles and the neurons that command them. This system gives rise to behaviour
The spinal cord can generate complex motor patterns independently of the brain. These are motor neurons present in the spinal called -lower motor neurons.
After cord transection, hindlimbs retain function- patterned electrical activity can be recorded from the muscles.
Define and explain the structure and function of the lower motor neurons
The lower motor neurons from the final common path for all signals (neuronal information) from the CNS to skeletal muscles.
Collected in longitudinally organized columns
Each column contains the larger alpha and smaller gamma motor neurons to one muscle (or a few functionally similar muscles)
Each column extends through more than one segment of the cord
Each muscle receives motor fibers through more than one ventral root and spinal nerve
CLINICAL APPLICATION
What would happen if a single ventral root is destroyed?
What is meant by a motor unit?
Destruction of a single ventral root or a single spinal nerve WILL NOT produce paralysis, only weakness (paresis). This is because each muscle receives input from more than one ventral root.
A single motor neuron and the muscle fibres it innervates. Each motor neurons synapses with multiple muscles fibres.
The simplest reflex responses are based on an interaction between a proprioceptive (self-detection) sensory input and a motor unit. Give examples of proprioceptive sensory organs
Muscle spindles- negative feedback regulation of muscle length. Spindles respond to length (due to passive stretch), but in a way modified by the activity of their own contractile elements
Golgi tendon organs- negative feedback regulation of muscle tension. Tendon organs respond to muscle tension (due to contraction)
What is a clasp-knife reflex?
Protects muscles from being tor off by tendon. In event of excessive force, it activates Ib afferent. This will synapse with inhibitory interneuron and turn off motor neuron causing you to remove arm/leg
Consider a stretch reflex circuitry. What happens in a monosynaptic reflex pathway?
A. Muscle spindle acts as the sensory receptor that initiates the reflex.
B. Stretch causes increased Ia afferent activity
Increased a-motor neuron activity leading to contraction of the same muscle
(Ia afferents also excite motor neurons that innervate synergistic muscles, and inhibit motor neurons of the antagonist muscles)
C. Stretch reflex: negative feedback loop to regulate muscle length. ‘Desired’ length is present by descending pathways
Consider the spinal cord circuitry responsible for the flexion reflex. How is this an example os a polysynaptic relflex pathway?
Stimulation of cutaneous pain receptors in the foot lead to activation of spinal cord local circuits that withdraw (flex) the stimulated extremity and extend the other extremity to provide compensatory support
Consider the somatotopic order in the human primary somatic sensory cortex
Electrical activity is recorded following the mechanosensory stimulation of different parts of the body
Somatotopic representation of body parts from medial to lateral
Homunculus constructed on the basis of such mapping. The amount of somatosensory cortex devoted to the hands and face is much larger than the relative amount of body surface in these regions
Consider the effects of lesions to the spinal cord. Describe anterior cord syndrome.
Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons)
Bilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts)
Bilateral loss of pain, temperature and light touch sensations below the level of the lesion (due to loss of anterior and lateral spinothalamic tracts)
Consider the effects of lesions to the spinal cord.
Other than anterior cord syndrome, what other conditions arise?
Brown-Sequard/Cord hemisection syndrome
Complete cord transection syndrome
