contrast general senses and special senses
GENERAL SENSORS:
- nociceptors (pain receptors)
- thermoreceptors (sensitive to warm or cold; or sensitive to change in temperature)
- mechanoreceptors (tactile receptors and proprioceptors)
- sensory receptors distributed throughout the body, mostly in skin
- information is MAINLY transmitted via spinal nerves to spinal cord
SPECIAL SENSES:
- olfaction (smell; chemoreceptors)
- gustation (taste; chemoreceptors)
- vision (sight; photoreceptors)
- hearing + equilibrium (balance); mechanoreceptors)
- sensory receptors found in the head
- information is ONLY transmitted via cranial nerves to brain
structure of spinal nerves
DORSAL = SENSORY VENTRAL = MOTOR
spinal nerve —> dorsal/ventral roots —> dorsal root ganglia —> dorsal/ventral rootlets
- dorsal rootlets enter the spinal cord at the dorsolateral sulcus
- ventral rootlets exit the spinal cord at the ventrolateral sulcus
- dorsal root ganglia contain cell bodies of sensory neurons
- dorsal & ventral roots merge to form the spinal nerve
- dorsal medial sulcus down centre of spinal cord
relationship between receptor type, fibre diameter and conduction velocity
EXPLORATIVE SENSES:
- touch + proprioception
- mechanoreceptors
- have larger axon diameter = faster conduction
- part of dorsal column medial system
- larger cells
PROTECTIVE SENSES:
- pain, temperature, itch
- have smaller axon diameter = slower conduction
- part of anterolateral system
- smaller cells
anatomy of the spinal cord (individual vertebrae)
dorsal side:
- dorsal median sulcus
- posterior intermediate sulcus (thoracic and cervical)
- posterolateral sulcus
- gracile fascicles (medial)
- cuneate fascicles: larger than gracile –> more nerve fibres for innervation of fine motor control in upper limbs
anterior side:
- anterior median fissure
- anterolateral sulcus
white matter surrounds grey matter
white matter divided into 3 funiculi:
- –> dorsal funiculus (column)
- ** dorsolateral sulcus ***
- –> lateral funiculus
- ** anterolateral sulcus ***
- –> anterior funiculus
grey matter divided into 3 horns:
- –> dorsal grey horn
- –> lateral grey horn
- –> anterior grey horn
ratio of white/grey matter changes throughout spinal cord:
- —> largest amount of white ma_er is found in the cervical cord as all axons from/to the brain pass through it
- —> grey matter of cervical/lumbar/sacral are expanded to include more sensory input from limbs and more cell bodies for motor control of limbs
- anterior horn carries motor commands from brain to body (α-Motoneurons)
- dorsal horn carries sensory (e.g. temperature, pain) feedback from body to brain
- lateral horn carries autonomous nervous system for visceral organs (present only at T1-L2, S1-S3)
- white mater (3 funiculi) contain ascending and descending axons (nerve fibres)
gross anatomy of spinal cord
- spinal cord is 45cm long
- contains 31 pairs of spinal nerves
- cervical enlargement:
- —> spinal nerves that supply shoulder and upper limbs
- lumbar enlargement:
- —> spinal nerves that supply pelvis and lower limbs
- at the caudal end, tapers into conus medullairs (a cone shaped structure)
- thread of fibrous tissue attaches the conus medullaris to the 1st coccygeal vertebra = filum terminale
- cauda equina = bundle of spinal nerve roots below conus medullaris
- —> the spinal cord stops growing in childhood, while the vertebral column continues growing
- —> spinal cord ends at L1/2 in adults
where is sensory information processed before reaching the cortex?
sensory information passes through the thalamus before reaching the cortex
the thalamus consist of discrete nuclei and each sensory modality has a different relay nucleus
each of these nuclei projects to a particular cortical area, called the primary sensory cortical area for that sensation
olfaction bypasses thalamus and goes straight to cortex
after the thalamus, what structure does sensory information pass through before reaching the cortex?
sensory information reaches the cortex via the posterior limb of the internal capsule
dorsal column-medial lemniscus’ pathway
how explorative senses (proprioception + touch) reach the cortex
—> receptive organ
—> dorsal root ganglion (1st order)
—> dorsal root
—> dorsal column:
~~~~~~lower body fibres enter the gracile fascicle (T6 and down —> )
~~~~~~upper body fibres enter the cuneate fascicle (above T6)
—> dorsal column nuclei in medulla (2nd order)
—> 2nd order neurones travel through medial lemniscus pathway and decussate via internal arcuate fibres to contralateral VPL thalamus (3rd order)
—> primary somatosensory cortex via posterior limb of internal capsule
anatomy of the dorsal column
composed of fascicles:
- —> gracile fascicles either side of dorsal median sulcus
- —> cuneate fascicles lateral side of dorsal intermediate sulcus
contains gracile tubercle (nucleus) and cuneate tubercle (nucleus)
anterolateral pathway
how protective senses (temp + pain + itch) reach the cortex
—> receptive organ
—> dorsal root ganglion (1st order)
—> dorsal rootlets
—> ducussate at ventral commisure
—> ascend via spinothalamic tract
—> travel to contralateral VP thalamus (3rd order)
—> primary somatosensory cortex via posterior limb of internal capsule
pathway for ‘unconscious’ proprioception
—> dorsal root ganglion (1st order)
—> Clarke’s nucleus (2nd order = collectively called spinocerebellar tracts)
—> all spinocerebellar tracts end in ipsilateral cerebellar part
which cranial nerve is the principle pathway for sensation from the head and face?
where does it originate?
what is the thalamic relay for this nerve?
which branches supply why areas?
trigeminal nerve (CN V)
originates at the pons
thalamic relay for sensory trigeminal nerve is ventral posterior medial nucleus (VPM)
the ophthalmic branch (V1) carries info from orbit, cornea, bridge of nose, scalp and forehead
the maxillary branch (V2) carries info from corners of eyes and mouth
the mandibular branch (V3) carries info from lower part of face. also carries motor fibres for muscles of mastification
describe the different nuclei contained within the sensory trigeminal nerve (CN V)
MAIN TRIGEMINAL NUCLEUS
- located at pons
- relays touch and mechanical sensation
- joins anterolateral system
SPINAL TRIGEMINAL NUCLEUS
- located in lower pons and medulla
- 2º neurons
- relays pain, temperature and itch
*** main and spinal trigeminal nuclei project with fibres of medial lemniscuss + anterolateral pathway (respectively) to VPM
MESENCEPHALIC TRIGEMINAL NUCLEUS
- primary sensory neurons with cell body located in brain
- receives input from jaw proprioceptors
- projects to main + spinal trigeminal nuclei
- fibres decussate to join medial lemniscuss
how is visual sensory information transmitted from eye to brain?
- light hits fovea
- light travels to back of eye, detected by photoreceptors in neural retina
- converted to electrical signals
- travels to interneurons
- retinal ganglia relay to brain via the optic nerve (CN II)
- a) terminates in dorsal layers of superior colliculus = unconscious reflexive eye movement
- b) lateral geniculate nucleus in thalamus —> primary visual cortex = conscious
describe the different types of cells in the eye
cone cells:
- function in bright lgiht
- colour vision
- high visual acuity as they only connect to few bipolar cells
rod cells:
- function in low light
- night vision
- low acuity as they connect to many bipolar cells
horizontal cells:
- sharpen contrast
amacrine cells:
- regulate sensitivity
retinal ganglion cells:
- carry visual info to brain via optic nerve
retinal ganglion cell axons from which hemiretinae decussate at the optic chiasm?
what does this achieve?
RCG axons from the nasal hemiretina decussate at the optic chiasm
temporal hemiretina axons stay ipsilateral
allows left and right visual fields to be separated and processed in the contralateral hemisphere
describe the organisation of the primary visual cortex
primary visual cortex = V1
superior half of visual field = inferior bank of calcarine sulcus
inferior half of visual field = superior bank of calcarine sulcus
macula (centre of visual field) = occipital pole
peripheral visual field = rostral V1
describe the organisation of the lateral geniculate nucleus (LGN)
consists of 6 layers which each receive exclusive input from ipsilateral or contralateral eye:
(ventral) C, I, I, C, I, C (dorsal)
parvocellular layers:
- dorsal 4 layers
- receive projections from P-type RGCs
- specialised for shape and colour
magnocellular layers:
- ventral 2 layers
- receive projections from M-type RGCs
- specialised for movement detection
in between the 6 layers, there are INTRALAMINAR NEURONS which relay colour info to V1
what are ocular dominance columns
are there any other specialised columns?
nuclei are segregated into columns
keeps left and right visual info separated at thalamus and V1
info only integrated at higher cortical areas
removing one eye shows some ocular dominance columns becoming inactive
ORENTATION COLUMNS:
- segregated by intracortical connections as opposed to the eye
COLOUR BLOBS:
- located in cortical layers 2 and 3 of ocular dominance columns
where does visual info project after reaching V1?
projection depends on what type of RCG the signal originated from
M-type RGCs:
- –> magnocellular neusons
- –> travels upwards to posterior parietal lobe
- —> V5 = “where:, movement, depth
P-type RGCs:
- –> parvocellular neurons
- –> travels to temporal lobe
- –> V4 = “what”, object recognition
V1 and V2 are involved in both streams
describe the structure of the superior colliculi
laminated structures = layered
visual info processed in dorsal layer
anterolateral and auditory info processed in ventral layer
tectospinal pathway originates in ventral layer
auditory pathway
- soundwaves vibrate the tympanic membrane
- auditory ossicles vibrate to amplify the pressure
- pressure waves created by stapes push on oval window
- scala vestibula (fluid in cochlea) vibrates
- sounds in hearing range vibrate the basilar membrane
- inner hair hells and stimulate vestibular-cochlea nerve
- travels to cochlea nuclei:
- –> dorsal portion processes sound from vertical axis and connects directly to inferior colliculus
- –> anteroventral portion receives sound from horizontal plane (superior olivary complexes
- –> posteroventral portion - travels to medial geniculate nucleus via inferior brachium
- primary auditory complex (posterior and superior portion of superior tempotal gyrus)
- –> high tones are medial
- –> low tones are lateral - travels to secondary auditory cortex for interpretation of sound
- (“where” pathway) projections to posterior parietal cortex for interpretation of where sound came from
- connections to prefrontal and motor regions for planning movement in response to sound
“what” pathway is less well defined
what is the role of the superior olivary complex?
spatial localisation of sounds from horizontal plane
low frequency: compares how long it takes to travel from left to right ear
high frequency: compares intensity between left and right ears
olfactory pathway
** sensory neurons = bipolar neurons **
- olfactory nerve fibres dangle through olfactory epithelium
- project through cribiform plate to synapse on olfactory bulb (olfactory bulbs connected via anterior commissure [medial stria])
- 2º neurons in olfactory bulb connect via olfactory tract to uncus (laterl stria)
- uncus connects to olfactory cortex
NOTE: no thalamic relay
