Neuroscience of Sensory Processing and Motor Control

Sensory receptor cells

respond to changes in energy in the outside world

Transduction

physical energy into activity of the neurons

Neural pathways

carry sensory information to other brain regions

Primary cortical area

receives sensory information

Higher cortical areas

give rise to sensory perception

Cornea

bends light towards pupil

Iris

regulates amount of light

Lens

focuses light on retina

Retina

contains photoreceptors

Fovea

receives light from center of visual field

Optic disc

area where optic nerve exits the eye - results in a blind spot

Photoreceptors

Light sensitive neuron at the back of the retina.

Cones

Located in the fovea

High acuity

Can distinguish wavelengths (chromatic)

Rods

Located in periphery

High sensitivity

Achromatic

Hocenter-surround receptive fieldrizontal cells

lateral inhibition

Amacrine cells

center-surround receptive field

optic chiasm

crossing point

Optic tract

carries information from the retina to three key brain regions.

Lateral geniculate nucleus (LGN)

relays signals to v1

superior colliculus

Visual reflexes

Superchiasmatic nucleus (SCN)

Receptive field

The region of the retina that when stimulated produces a change in the firing rate of a neuron.

Ganglian cell receptive fields

When light falls on the central part of the receptive field, the GC rate of firing increases

“On-center”

When light falls on the outer part of the receptive field, the GC rate of firing decreases.

“Off-surround”

The LGN has six layers of cells

Parvocellular layers- Color and detail

Magnocellular- Motion

Simple cells

Respond to lines of a particular orientation in a particular location.

Complex cells

Orientation and movement specific

Less location specific

Hypercomplex cells

Orientation, movement, and length specific

Somatotopic organization

Different areas of cortex control different regions of the body.

Greater cortical representation equals greater motor control.

From the Cortex to the Head

Corticobulbar tract

Upper motor neurons originate in the primary motor cortex.

Descend to multiple regions in the brainstem.

Alpha motor neurons travel via cranial nerves.

From the Cortex to the Body

Corticospinal tract

Upper motor neurons originate in later V of primary motor cortex.

Contralateral fibers

Ipsilateral fibers

Lower motor neurons travel via spinal nerves to skeletal muscle.

Spinal Cord

Comprised of four major regions:

Cervical

Thoracic

Lumbar

Sacral

Injury to the spinal cord results in

paralysis

Alpha motor neurons originate in the ventral horn and exit the spinal cord through the

ventral roots

Alpha motor neurons synapse upon muscle fibers

Fewer synapses, finer control.

Each muscle is made up of many muscle fibers.

When an action potential arrives

the alpha motor neuron releases acetylcholine into the synapse

Premotor Areas

Provides input to the primary motor cortex to organize and initiate movements.

Dorsal premotor area

Becomes active when we prepare to make a movement

Ventral premotor area

Incorporates visual information to facilitate grasping movements.

Finger placement and separation

Supplementary motor area (SMA)

Coincides with the desire to want to move.

Tends to be a specific movement of a specific body part.

Inferior Parietal Lobe when stimulated...

gives rise to the desire to move.

Damage to inferior parietal lobe

Alien hand syndrome

Mirror Neurons

Found within the premotor cortex (and other areas)

Prefrontal Cortex

Prioritizing and maintaining goals.

damage to prefrontal cortex

Damage can lead to utilization behavior

Basal Ganglia

Choosing which behavior is most appropriate given your current state.

Incorporates sensory information about the environment with current body position.

Through repetition of successful behaviors, basal ganglia automatize these behaviors (habits).

Striatum

Caudate nucleus

Putamen

Circuit of the Basal Ganglia

Cortex provides input to striatum

Sensory information

Motor plans

Striatum influences activity of globus pallidus.

Globus pallidus sends output to the cortex via thalamus.

Looped circuit

Basal Ganglia: Direct Pathway

Sensory and premotor cortices and substantia nigra excite the striatum

Striatum inhibits the globus pallidus

Globus pallidus inhibits thalamus an appropriate amount

Thalamus excites the primary motor cortex

Basal Ganglia (Parkinson's)

Progressive motor disorder that arises from the death of dopamine producing neurons in the substantia nigra.

Nigrostriatal pathway

Striatum inhibits the globus pallidus

too little

Globus pallidus inhibits the thalamus

too much

Levodopa (L-DOPA)

Dopamine agonist

Deep brain stimulation (DBS)

Exciting the subthalamic nuclei

Cerebellum

Receives a copy of efferent commands from the motor cortex.

Combines info with sensory information

Visual, auditory, tactual, proprioceptive.

Cerebellum facilitates the execution of

accurate, coordinated movements.