SOMATOSENSORY SYSTEM
The Somatosensory system transmits sensations of the body (from the skin and the
musculoskeletal system) to the brain.
It has three components/parts
Sensory receptor
Sensory Pathway
Sensory Cortex (Primary, Secondary & Association Cortex)
Types of Sensation
Skin: Pain, Temperature, Touch, Pressure & Vibration
Musculoskeletal System: Pain & Proprioception
SENSORY PHYSIOLOGY
Sensory Transduction (conversion): Receptors transform an external signal (Pain, Touch,
Temp) into a membrane potential
All sensory input ascends via spinothallamic pathway and ends in thalamus.
Specific thalamic areas send the ascending sensory information to the respective primary
sensory area
From the primary sensory areas, the information converges in the association cortices &
perceived as sensation.
SENSORY CODING
The kind of receptor activated determined the signal recognition by the brain.
It must convey the intensity of the stimulus  the stronger the signals, the more frequent
will be the APs
It must send information about the location and receptive field, characteristic of the
receptor.
SENSORY PATHWAYS
The sensory pathways convey the type (because of the type of receptor activated) and
location (because the brain has a map of the location of each receptor)of the sensory
stimulus
These pathways involve a chain of neurons:
o
First-order neuron – to the CNS
o
Second-order neuron – an interneuron located in either the spinal cord or the brain
stem
o
Third-order neuron – carries information from the thalamus to the cerebral cortex
o
Third order neuron ends at the primary sensory area in the parietal lobe
o
The axon of the first-order or second-order neuron crosses over (decussation)
o
In the posterior column and spinothalamic pathways axons of the third-order ascend
within the internal capsule to synapse on neurons of the primary sensory cortex of
the cerebrum
Sensory Homunculus: Functional map of the primary sensory cortex
SENSORY RECEPTOR
o Classification based on type of stimulus it carries
o Mechanoreceptors (touch, pressure vibration): Skin: Meissners corpuscle,
Merkles corpuscle / discs, Pancinian corpuscle
o Thermoreceptors (free nerve endings activated by a rising temperature OR a
falling temperature)
o Nociceptors (free nerve endings activated by mechanical deformation
chemicals and temperature)
o Proprioceptors (muscle spindles in muscle; tendon organs in tendon) are
responsible for the awareness of muscle length and body position in space.
o Classification Based on function
o Tonic receptors -- slow acting, -- no adaptation: continue to for impulses as
long as the stimulus is there (ex: proprioceptors)
o Phasic receptors -- quick acting, adapt: stop firing when stimuli are constant
(ex: smell)
Receptive field is defined as an area of skin supplied by a single sensory nerve fiber.
SNSORY PATHWAYS
Dorsal Column Medial Lemniscus System/Posterior or Dorsal Column Tract
o Carries Discriminative/fine touch & Conscious Proprioception
o Have three order of neurons
o Runs ipsilateraly in spinal cord
Anterior Spinothalamic Tract
o Carries Crude touch & Pressure
o Have three order of neurons
o Runs contralateraly in spinal cord
Lateral Spinothalamic Tract
o Carries Pain & Temperature
o Have three order of neurons
o Runs contralateraly in spinal cord
Spinocerebelar Tract
o Carries subconscious proprioceptive information which ends in Cerebellum.
Diseases affecting the sensory system.
Gullian Bare Syndrome
Multiple Sclerosis
Brown Sequard Syndrome
MOTOR SYSTEM
Definition
Motor system consists of parts of CNS and PNS, which is specialized for control of limb,
trunk, and eye movements. Holds us together and is responsible for simple reflexes (knee
jerk) to voluntary movements.
Major components of motor system
1. Parietal cortex: visual and proprioceptive processing gives positional info of an object
relative to body (i.e. the hand) and sends this info to the motor cortex.
2. Motor cortex: It is somatotopicaly organized (motor HOMUNCULUS) and compute
forces needed to cause desired motor action, which results in commands that are sent
to the brainstem and spinal cord.
3. Brainstem: tells spinal cord how to maintain balance during movement
4. Spinal cord: Motor neurons send the commands received form the motor cortex and
the brainstem to the muscles.
5. Cerebellum: coordination of multi-joint movements, postural stability, and motor
learning
6. Basal ganglia: learning and stability of movements, initiation of movements,
emotional and motivational aspects of movement
III. Properties of muscle fibers and muscle movement
1. Extrafusal fibers attach to tendons, which in turn attach to the skeleton. They produce
the force that acts on bones and other structures.
2. Intrafusal fibers attach to the extrafusal fibers, produce a negligible force, and instead
play a sensory role. They contain muscle spindles which, innervated by muscle
spindle afferents, provide the CNS with info about muscle length.
3. There are two major types of motor neurons:  motor neurons innervate large,
extrafusal muscle fibers that generate force;  motoneurons innervate intrafusal fibers
that house the muscle’s sensory system.
4. A motor unit refers to an alpha motor neuron and all the muscles fibers it controls.
5. Motor Neuron pool: Motor neurons innervating a single muscle are situated close to
each other in the ventral horn, and are called a motor neuron pool. In the ventral
horn, there are two groups of motor neuron pools, medial & lateral motor neuron
pool.
Organization of Movements
•
Reflexes: Stimulus-evoked involuntary muscle contraction controlled by spinal cord
circuits
•
Postural adjustments: Maintenance of upright posture during rest & movement,
controlled by spinal cord & brain stem
•
Voluntary movements : Organized around purposeful acts controlled by spinal cord,
Brain stem, & cortex
Descending Control of Movement
The brain influences activity of the spinal cord
Voluntary movements
Hierarchy of controls
Highest level: Strategy- Cortical
Middle level: Tactics- Sub cortical (Basal Gangalia)
Lowest level: Execution-Spinal (Anterior Horn Cell)
Sensorimotor system
Sensory information: Used by motor system
Axons from primary motor cortex descend to the spinal cord via two groups
•
•
Lateral group: controls independent limb movements
•
Corticospinal tract: hand/finger movements
•
Corticobulbar tract: movements of face, neck, tongue, eye
•
Rubrospinal tract: fore- and hind-limb muscles
Ventromedial group control gross limb movements
•
Vestibulospinal tract: control of posture
•
Tectospinal tract: coordinate eye and head/trunk movements
•
Reticulospinal tract: walking, sneezing, muscle tone
•
Ventral corticospinal tract: muscles of upper leg/trunk
Diseases Affecting the Motor System
Amyotrophic Lateral Sclerosis (ALS): Motor neuron disease
Parkinson’s disease (PD) involves muscle rigidity, resting tremor, slow movements
Huntington’s disease (HD) involves uncontrollable, jerky movements of the limbs
Duchenne Muscular Dystrophy: Due to dystrophin deficit
Myasthenia Gravis: Autoimmune ACh receptors are affected