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:

• 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.

Receptors

Telereceptors(distance receptors)e.g. rods & cons for light

Exteroceptors(external environment)e.g. touch & Temperature receptors

Interoceptors(internal environment)e.g. chemoreceptors

Propriocepters(position of body)Nociceptors

Pain receptors

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:

• Spatial recognition:

• Localization of the site of the stimulus and two-point discrimination.

• Recognition of relative intensities of different stimuli.

• 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.