Senses and perceptions 2 Flashcards
What are the components of a sensory pathway?
receptor endings, peripheral nerve, ascending sensory pathway, cortical representation
What axons are found in a peripheral nerve?
axons from sensory and motor neurons
What are sensory neurons known as?
afferents
What are motor neurons known as?
efferent
Where are the axon terminals of sensory neurones located?
in the CNS
Where are the cell bodies of motor neurones located?
in the CNS
What is a fascicle?
a bundle of neurones
What is the endoneurium?
loose vascular supporting tissue that surrounds each nerve fibre in the fascicle
What is the perineurium?
condensed collagenous tissue layer that surrounds each fascicle
What is the epineurium?
loose collagen tissue that binds fascicles together and condenses peripherally to form a sheath
Function of the epineurium
provides structural integrity and mechanical protection to the nerve
Why is the presence of blood vessels crucial within nerves?
AP conduction is metabolically demanding. Vasculature supplies oxygen and glucose and removes waste products
What are myelinated axons encased in?
a series of Schwann cells line the length of the axon, each having a wrapped coating of myelin insulating the axon
What are unmyelinated axons encased in?
Schwann cell cytoplasm but no wrapped coating of myelin
What are the 4 types of afferent axons?
Aa (A alpha), AB (A beta), Ad (A delta), C
Which axon types are myelinated?
Aa, AB, Ad
What axon type is unmyelinated?
C fibres
Which axon type has the largest diameter and thickest myelin sheath?
Aa (A alpha) - diameter 13-20um
Which axon type has the fastest conduction speed?
Aa - speed 80-120m/sec
Which axon type has a relatively large diameter and myelin sheath thickness?
AB (A beta) - diameter 6-12um
Which axon type has relatively fast conduction?
AB 35-75m/sec (not as fast as Aa)
Which type of myelinated axon has the smallest diameter and myelin sheath thickness?
Ad (A delta) 1-5um
Which myelinated axon has the slowest conduction speed?
Ad 5-30 m/sec
Which axon type has the smallest diameter?
C fibres with a diameter 0.2-1.5 um
Which axon type has the slowest conduction speed?
C fibres 0.5-2 m/sec
Which sensory receptors are associated with Aa fibres?
proprioceptors of skeletal muscle
Which sensory receptors are associated with AB fibres?
mechanoreceptors of skin (innocuous tactile sensation e.g. pressure, brushing, touch)
Which sensory receptors are associated with Ad fibres?
nociceptors, thermoreceptors (pain and temp)
Which sensory receptors are associated with C fibres?
thermoreceptors, nociceptors, itch
What is the arrangement of white and grey matter in the spinal cord?
white matter surrounds the grey matter (central core)
What are the 3 sections the grey matter of the spinal cord is divided into?
dorsal horn (posterior), intermediate horn, ventral horn (anterior)
Where are the cell bodies of sensory neurones located?
dorsal root ganglia
How many dorsal root ganglia are there?
31 pairs (either side of vertebral column)
Describe the course of an action potential from a pin prick into the spinal cord
free nerve ending (nociceptors) of Ad and C fibres are activated. AP sent along sensory afferent, past dorsal root ganglia, into the dorsal horn
Describe the course of an AP from the spinal cord to skeletal muscle
motor efferent transmits the impulse from the cell body in the ventral root to the effector
What is a dermatome?
an area of skin supplied by a single spinal nerve
What does the dermatome map demonstrate?
demonstrates which segment of the spinal cord sensory input from distinct body regions project into
What is a myotome?
A group of muscles innervated by a single spinal nerve
What does the myotome map demonstrate?
the motor output controlled by segments of the spinal cord
What are the branches of the trigeminal nerve (CN V)?
ophthalmic (CN Va), maxillary (CN Vb) and mandibular (CN Vc) branch
Function of ophthalmic branch of the trigeminal nerve (CN Va)
sensory innervation of the scalp, forehead, dorsum of nose, upper eyelid, ethmoid, sphenoid and frontal sinuses
Function of the maxillary branch of the trigeminal nerve (CN Vb)
sensory innervation to lower eyelid, cheeks, maxillary sinus, lateral nose, upper lip, gingiva and teeth and superior palate
Function of the mandibular branch of the trigeminal nerve (CN Vc)
mixed sensory and motor. Sensory innervation to the chin, lower lip, teeth and gingiva, anterior 2/3 of tongue. Motor axons to the muscles of mastication
What are the muscles of mastication?
temporalis, masseter, medial and lateral pterygoids
How are the neural pathways in the CNS divided?
mechanoreception (touch) and nociception (pain) have separate pathways with the trigeminal nerve (face innervation) and spinal nerves (rest of body) considered separately.
How many neurones are typically involved in a sensory pathway?
3
Outline the roles of the 3 interconnected neurones in a sensory pathway
Primary sensory neuron transmits impulse from the periphery to the secondary sensory neuron in the brainstem/spinal cord. The secondary sensory neuron crosses the midline in the brain and synapses with the tertiary sensory neuron in the thalamus which transmits the signal to the cerebral cortex.
What does cortical representation refer to?
the representation of every sensory neuron (receptive field) in the cortex
Why is the body image on the sensory homunculus distorted?
Body regions with high sensory innervation (high density of receptor endings and small, overlapping receptive fields) have greater cortical representation
How do smaller receptive fields impact 2 point discrimination values?
the smaller and more overlapping the receptive fields are, the smaller the 2 point discrimination values
What is somatic mechanosensation?
the perception of innocuous tactile stimuli from the periphery/different parts of the body
What is the name of the sensory pathway for somatic mechanosensation from the trunk/limbs?
Dorsal column-medial lemniscal pathway
Describe the dorsal column-medial lemniscal pathway
Primary sensory neuron transmits impulse from receptor endings into the dorsal horn and up the dorsal column to the medulla oblongata where it synapses with the secondary sensory neuron. Secondary sensory neuron crosses the midline and synapses in the thalamus with the tertiary sensory neuron which terminates in the appropriate region of sensory cortex.
What is the name of the sensory pathway for somatic mechanosensation of the face?
posterior (dorsal) trigeminothalamic pathway
Which nerve conducts the posterior (dorsal) trigeminothalamic pathway?
trigeminal nerve (CN V)
Where are the cell bodies of the trigeminal afferents collected?
trigeminal ganglion
Describe the posterior (dorsal) trigeminothalamic pathway
The primary sensory neuron transmits information from the face to the brainstem where it synapses with the secondary sensory neuron. This crosses the midline and synapses with the tertiary sensory neuron in the thalamus. Neuron 3 terminates in the opposite region of the sensory cortex
What is the name of the somatic nociception sensory pathway from the trunk/limbs?
spinothalamic pathway
Describe the spinothalamic pathway
the primary sensory neuron transmits information from the free nerve endings to the dorsal horn where it synapses with the second sensory neuron. The second sensory neuron crosses the midline in the spinal cord and projects the impulse through the brainstem and to the thalamus where it synapses with the tertiary sensory neuron. Neuron 3 terminates in the somatosensory cortex.
What is the name of the somatic nociception pathway for the majority of the face?
anterior (ventral) trigeminothalamic pathway
Describe the anterior (ventral) trigeminothalamic pathway
trigeminal afferent transmits impulse from the face to the spinal nucleus where it synapses with the secondary sensory neuron. Neuron 2 crosses the midline in the spinal cord and transmits the impulse to the thalamus where it synapses with the tertiary sensory neuron which transmits the impulse to the somatosensory cortex.
Where do the primary and secondary sensory neurons synapse in the the somatic mechanosensation pathways?
in the brainstem nucleus
Where do the primary and secondary sensory neurons synapse in the the somatic nociception pathways?
spinal nucleus
Where do the secondary and tertiary neurons synapse?
in the thalamus
How was Wilder G Penfield first able to describe the sensory homunculus?
discovered electrical stimulation of sensory cortex lead to perceptions of touch despite no peripheral stimulus
Sensation definition
process of sensing our environment (touch, taste, smell, sight) by activation of sensory receptors and associated sensory pathways
Perception definition
the interpretation of sensory input (interpretation of sensation) - requires higher degree of cortical processing
2 examples of higher degree cortical processing
stereognosis and proprioception
Stereognosis definition
the mental perception of objects referenced by touch
What aspects of higher degree cortical processing is required for stereognosis?
- integrating information on several aspects of touch e.g. texture, weight, size
- retrieve previous experience
- generate 3D mental image
What components of the brain are required for stereognosis?
sensory cortex (bilateral - info from both hands), hippocampus, basal ganglia and motor cortex
What is the function of the sensory cortex in stereognosis?
processing sensory information from both hands
What is the function of the hippocampus in stereognosis?
retrieving information
What is the function of the basal ganglia and motor cortex in stereognosis?
movement to manipulate the object
Proprioception definition
sense of self-movement and body position
What is an alternative term for proprioception?
kinaesthesia
Why does proprioception require higher degree cortical processing?
requires the integration of mechanosensory input
What structures provide mechanosensory input for proprioception?
balance organs in inner ear, muscle/joint receptors, mechanoreceptors, periodontal receptors
What are the names of the balance organs in the semi-circular canals?
utricle and saccule
What do muscle receptors detect?
muscle length, tension - information about which muscle groups are contracted/relaxed
What do joint receptors detect?
joint position
What do mechanoreceptors detect for proprioception?
skin stretching, tension from clothes
What are the names of the 2 pain pathways (somatic nociception)?
spinothalamic pathway and anterior (ventral) trigeminothalamic pathway
What are the names of the 2 tactile pathways (somatic mechanosensation)?
dorsal (posterior) column-medial lemniscal pathway and posterior (dorsal) trigeminothalamic pathway
