Brain, Neurons and Synaptic Transmission
- Dendrites make up 90% of the neurons surface area. Hillock is area of low electrical resistance
dense in voltage-gated Na+ channels. Currents move not by the flow of electrons but by the
shuffling of electrons.
- Chemical synapse is the most common type, high electrical resistance, uses neurotransmitters.
EPSP – 15 ms long, increase in conductance to Na+ (primarily) and K+, is 1 mv in size. IPSP is also
transient and has an increase in conductance to Cl- and K+.
- Neurotransmitters can produce three possible events: ion flow and depolarization of
hyperpolarization, activation of second messengers by calcium influx or activation of GPCRs that
causes long term biochemical and structural changes, neuromodulator that can activate GPCRs to
produce long term changes.
NMDA – class of glutamate-gated cation channels. Play a role in brain development, excitatory
neurotransmission, synaptic plasticity and memory formation. Found in the hippocampus (structure
concerned with memory)
5 classes of neurotranmitters or neuromodulators
(1) – Ach
(2) –Biogenic Amines (dopamine, NE, E, serotonin, histamine)
(3) – Amino Acids (glutamate (excitatory), GABA and glycine (inhibitory))
(4) – Neuropeptides – act as neuromodulators
(5) – Miscellaneous – NO (retrograde transmitter), CO, steroids
- Main excitatory neurotransmitter is glutamate, glycine acts as inhibitory at the spinal cord and
Myasthenia Gravis – increase in width of synapse, decrease in number of receptors
Long-term potentiation – Has been found in the hippocampus and neocortex, maintenance is probably
mediated by both pre- and post-synaptically. Occurs mainly at excitatory synapses which have NMDA
- temporal summation is more effective
Transduction of Environmental Information
- Receptor (generator potentials) are due to increase in permeability to Na+ (and K+), local and
non propagated, graded, can summate. Generator potential has to conduct to the first node of the
receptor neuron to generate an AP (where voltage gated Na+ channels are located). Slowly Adapting – signal amplitude of skin indentation (frequency and population coding)
Rapidly Adapting – signal rate of skin indentation
- Pacinian corpuscles are rapidly adapting because of their inner lamellae which slip back and the
properties of ion channels. Most sensitive mechanoreceptor. Intensity of vibration is signaled by
Somato-sensory system I
- Merkel’s are the only receptor found in the epidermis
Modality Receptor Axon Type Conduction Velocity
Touch: Free C 1 m/s
Merkel, Ruffini Aαβ
Meissners, Pacinian, Hair Aαβ Up to 80 m/s
Vibration: Meissners (5-100Hz) Aαβ
Pacinian (50-1000Hz) Aαβ
Temperature: Warm Free Nerve Ending C
Cool Free Nerve Ending Aδ
Pain: Fast Sharp Free Nerve (Nociceptor) Aδ 20 m/s
Pain: Slow Ache Free Nerve (Nociceptor) C 1 m/s
Itch Free Nerve
Proprioception: Muscle Spindle (primary) Aα (Ia) 90 m/s
Muscle Spindle (sec.) Aβ (II) 50 m/s
Golgi Tendon Organ Aα (Ib)
First system is by sensory and axon physiologists
Aα – alpha motoneurons and skin afferents
Aβ – skin afferents
Aγ – fusimotor neurons
Aδ – pain, temp
C – unmyelinated, pain, temp, crude touch
The second system was used by motor physiologists
Ia – muscle spindle (primary)
Ib – Golgi tendon
II – muscle spindle secondary
III and IV – free nerve endings unmyelinated - Sensitivity is punctuate (occurs at small spots)
Spinothalamic – large receptive fields, convergence of modalities
Dorsal Column, Medial, Lemniscal System – Discrimitive touch, proprioception vibration, only found
in mammals. Small receptive fields, neurons respond to one modality, faithful transmission, fast
conduction. Both systems are contralateral
Somatosensory System II
Somatotopic Organization – distorted homunculus, lateral (outside) is face, lips while medial is legs.
Shown by recording from population of neurons (evoked potential), lesions, cortical stimulation and
epilepsy. Shows plasticity (dormant neural pathways)
One cortical column has same receptive field and same modality.
Posterior Parietal Cortex – located behind the post central gyrus (where information is sent first).
Where body image is assembled. If lesioned, will have a lack of appreciation of spatial aspects of sensory
input from opposite side of external space and side of body.
- Horizontal and Amacrine cells are inhibitory.
Rods – in peripheral part of retina, sensitive to faint light (can detect single photon) have many rods
synapse on one bipolar cell.
Cones – concentrated in fovea, have greater acuity, used in bright light and colour vision, sharper because
- Light leads to hyperpolarization. Retinal ganglion cells are the first cells to emit and action
- Retinal Ganglion cells thalamus visual cortex (simple cells)
Simple cells respond best to bars of light in a specific orientation.
- Information from left visual space goes (falls on right side of retina) and goes to right visual
cortex (fibers decussate).
- Has columnar organization in orientation columns. Series of orientation columns through 180
degrees such that every 50u receptive fields change by 10 degrees. Ocular dominance columns
for left and right eye for one small patch of retina are called a hypercube.
Inferior Temporal Cortex – higher area visual cortex. If lesioned, will cause prospagnosia (inability to
recognize familiar faces)
- Simlar to the somatosensory cortex because both deconstruct then reconstruct, both have ‘maps’,
both have columnar organization and both have specific behavioural disorders associated with
- detects sounds over a frequency range of 20-20,000 Hz and an intensity range of 10-6 with a
spatial resolution of 1 degree arc. Achieved by the cochlea which deconstructs sounds into
specific frequencies and intensities and sends this to the brain. - To transmit a pressure wave from air to fluid, the outer ear is used as a funnel to concentrate
sound, the ossicles in the middle ear act as a lever system, and the area of the tympanic
membrane is greater than that of the oval window.
- Different frequencies produce traveling waves that reach peaks at different regions of the
basement membrane, high frequencies near stapes, low frequencies near apex of cochloea, this is
how complex sounds are deconstructed.
o Intensity is coded by the number of receptors activated and the frequency of firing of the
- The basilar membrane is narrow and stiff near the oval window and wide and more flexible near
apex. The length and stiffness of the hair cells is different along the membrane.
Choclea nucleus medial and lateral superior olive inferior colliculus thalamus auditory cortex
- Sound localization first occurs in the medial and lateral superior olive. Uses sound intensity and
time of sound arrival to localize.
- Contralateral and ipsilateral cochlea nerves project to dendrites of the same neuron in the medial
and lateral superior olive; one produces excitation the other inhibition.
- Tonotopic organization and columnar organization in the auditory cortex. A lesion to the auditory
cortex will create a deficit in locating sounds and in recognizing sound patterns.
Vestibular System and Eye Movement
Vestibular system – detects positions and motion of the head in space, damage can lead to nausea,
Semicircular canal – has ampulla (with cupula) detects angular acceleration of the head in any three
Otolith organs – utrical and saccule, have otoliths, detect linear acceleration and gravity.
Benign position vertigo – vertigo and nausea, usually caused by trauma, otolith crystals in semicircular
- When hair cells are bent toward the kinocilia, causes depolarization. Work in pairs (one side will
depolarize, other will hyperpolarize)
- Function: sense of eq