Transcript Central Nervous System

Central Nervous System
•Introduction
•The Sensory System
Introduction: Cells of the nervous System
• Neurons:
• The functional unit of the nervous system.
• They vary in size and shape, but
they all basically consist of:
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Cell body (soma).
One or more dendrites.
Axon.
Synaptic terminals.
Introduction
• The CNS
compromises the
brain lying within the
skull and the spinal
cord lying within the
vertebral column.
Brain
Spinal Cord
31 segments
•Cerebrum
•Cerebellum
•Diencephalon
•Brain stem
•8 cervical
•12 thoracic
•5 lumbar
•5 sacral
•3 coccygeal
The Brain
The Brain-Brain Stem
• Consist of medulla, Pons and midbrain.
• The medulla:
• Contains motor and sensory nuclei of the throat, mouth
and neck.
• Nuclei for respiratory and cardiovascular control centers.
• Nuclei for cranial nerves.
• The pons:
• Contains control centers for the respiratory and
cardiovascular systems.
• Nuclei of some cranial nerves.
• The midbrain:
• Contains nuclei of cranial nerves III and IV which
mediate the pupillary reflexes and eye movement.
The Brain-Diencephalon
• Composed of :
• Two thalami.
• Hypothalamus.
• The Brain- Cerebllum
• Control of rate, range and
direction of movement.
• Control of muscle tone.
• Control of equilibrium and
posture.
The Brain- cerebrum
• Consist of right and left
cerebral hemispheres.
• The superficial layer is
composed of grey matter- the
cerebral cortex.
The Spinal Cord
• Consists of segments, each of which has a pair of nerve
roots, on each side.
• Dorsal roots-carry impulses from periphery into the
spinal cord.
• Ventral roots-carry impulses to the periphery (i.e.
muscles)
• Spinal cord functions include:
• Transmission of sensory (afferent) impulses coming from
peripheral receptors to the brain and of motor (efferent)
impulses from the brain to motor neurons, which supply
effector organs (i.e. muscles and glands).
• Serving as a center for some reflexes, some of which are
the basis of movement and posture, e.g. stretch reflex.
The Sensory
System
Introduction
• The sensory system provides us with information about
our environment – both internal and external
environment.
• All information come to us through our sense organs,
which contain structures called Receptors.
• Receptors are detectors and transducers that convert
various forms of energy (light, sound, chemical,
mechanical..etc) into action potentials.
• When these action potentials reach the brain, we
experience a sensation, e.g. vision, hearing, pain, touch,
cold, atc.
Telereceptors
(distance
receptors)
e.g. rods &
cons for light
Interoceptors
(internal
environment)
e.g.
chemoreceptors
Receptors
Exteroceptors
(external
environment)
e.g. touch &
Temperature
receptors
Nociceptors
Pain
receptors
Propriocepters
(position of body)
Properties of receptors
• Specificity:
• Each type of receptor is most sensitive to one specific
form of energy which is called its adequate stimulus.
• Excitability:
• Different types of receptors are excited in different ways.
• Some stimulated by: - mechanical deformation
- or chemicals
- or temperatures
• Adaptation:
• When a stimulus of constant strength is contineously
applied to a receptor, the frequency of action potentials
in the afferent nerve fiber from the receptor gradually
declines.
• Slowly adapting receptors (tonic receptors):
• Muscle spindle, for information about posture.
• Joint receptors, for information about position of
different parts of the body.
• Rapidly adapting receptors (phasic receptors):
• Touch receptors.
• NOTE: Pain receptors do NOT adapt at all.
Somatic Sensation
• Classification:
• Mechanoreceptive senses: stimulated by mechanical
displacement of body tissue (touch, pressure, vibration
and tickle)
• Thermoceptive sense: stimulated by heat and cold.
• Pain senses: tissue damage.
Cutaneous Receptors
•Merkel’s and Ruffini
endings:
-Slow adapting touch
receptors.
•Meissner’s corpuscles:
-For fine touch.
•Pacinian corpuscles:
-For vibration.
•Pacinian corpuscles and
Ruffini endings:
-For proprioception.
•Free nerve endings:
- For pain and temperature.
Thermoceptive sensation:
• Cold receptors:
• More densely distributed than warmth receptors (three to
ten times more).
• They respond to skin temperatures from 10º to 40º C.
• Warmth receptors:
• Respond from 30º to 45º C.
At temperature below 10º and above 45º C, tissue damage begin to
occur and the sensation experienced is pain.
Temperature ranging from 20º to 40ºC are subject to rapid adaptation .
Pathways for general sensations
• Sensory information that reaches our consciousness is
carried by 2 main ascending tracts:
1. Dorsal column-medial lemniscal system (DC-MLS):
- Fine touch
- Proprioception
- Vibration
2. Anterolateral spinothalamic system:
- Pain
- Temperature
- Crude touch
Somatosensory Tracts:
Figure 10-9: Sensory pathways cross the body’s midline
Postcentral
gyrus
It is the location of
primary
somatosensory
cortex.
it tells how much
brain power is
dedicated for
sensing different
body parts.
Sensory homunculus
Role of thalamus and sensory cortex in
appreciation of sensation
• All sensory tracts, except the olfactory pathway, synapse
in the thalamus on their way to cerebral cortex.
• When impulse reach the thalamus, we become crudely
aware of the sensation but we can’t perceive all of its
fine details.
• Interpretation of the quality and localization of these
sensations occurs at the level of the cerebral cortex.
The sensory cortex is concerned with three
discriminative faculties:
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Spatial recognition:
Localization of the site of the stimulus and twopoint discrimination.
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Recognition of relative intensities of
different stimuli.
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Stereognosis:
Ability to identify objects by touch without the
aid of vision.
•Two point discrimination
• The distance at which two simultaneous stimuli can be
perceived as separate, is called Two-point
discrimination.
• The spatial thresholds are roughly:
• 1mm on the fingers, lips and tip of the tungue.
• 4mm on the palm of the hand.
• 15mm on the arm.
• Over 60mm on the back.