Human System

1. NERVOUS SYSTEM
Dr Wairimu Mwaura

2. OBJECTIVES
• Organization of the nervous system
• Divisions of the nervous system
• Functions of the nervous system
• Brain and the blood brain barrier
• Auditory system
• Visual system

3. ORGANIZATION OF THE NERVOUS SYSTEM
• The nervous system can be classified according to:
i. Structural classification- according to anatomical
structures found within the nervous system.
ii. Functional classification- according to the activities that
take place within the nervous system.

4. STRUCTURAL ORGANIZATION
• The structural classification has two subdivisions:
i. Central nervous system
ii. Peripheral nervous system

6. STRUCTURAL ORGANIZATION
i. The central nervous system:
- Consists of the brain and spinal cord.
- Due to their importance they are encased in bone for their
protection.
- The brain is in the cranial vault and the spinal cord is in the
vertebral canal.
- Although they are considered as separate organs, the brain
and spinal cord are continuous at the foramen magnum.
- Function :act as the integrating and command centers of
the nervous system.

7. THE BRAIN
• The human brain is the center of the nervous system.
• It contains 100 billion neurons & 1 trillion glial cells.
• Each neuron has 10,000 connections with other neurons.
• It commands and controls our bodies.
• Is the part of the CNS contained in the cranial cavity.
• The four divisions of the brain are:
i. Brainstem
ii. Cerebellum
iii. Cerebrum
iv. Diencephalon

9. BRAIN STEM
• Connects the spinal cord to the base of the brain.
• Consists of the medulla oblongata, pons and midbrain.
• Also contains the reticular formation.
• The brain stem is responsible for many essential functions-
heart rate, breathing, swallowing.
• Damage to the brain stem often causes death.

11. CEREBELLUM
• Little brain.
• Is attached to the brainstem posterior to the pons.
• Consists of 3 parts: flocculonodular lobe, vermis & 2 lateral
hemispheres.
• Flocculonodular lobe: controls balance & eye movement.
• Vermis: control posture, locomotion & fine motor
coordination.
• Lateral hemispheres: complex movements.

12. CEREBELLUM

13. DIENCEPHALON
• Part of the brain between the brainstem & the cerebrum.
• Its main components are: hypothalamus, thalamus,
epithalamus & subthalamus.
• Thalamus: sensory relay center of the brain.
• Hypothalamus: controller of the endocrine system.
• Subthalamus: controls motor functions.
• Epithalamus: control of behaviour& the sleep wake cycle.

14. CEREBRUM
• Accounts for the largest portion of the brain-1200 -1400
grams.
• The cerebrum is divided into the right and left hemispheres
by the longitudinal fissure.
• It consists of an outer cerebral cortex and an inner region of
grey and white matter.
• Gyri: numerous folds on the surface of the brain.
• The grooves between the gyri are called sulci.

17. CEREBRUM
• Each cerebral hemisphere is divided into 5 lobes.
i. Frontal lobe: voluntary motor function, personality & decision
making.
ii. Parietal lobe: receives and evaluates sensory information.
iii. Occipital lobe: receives & integrates visual input.
iv. Temporal lobe: receives input for hearing, smell and plays a role in
memory.
v. Insula: receives and evaluates taste information.

18. STRUCTURAL ORGANIZATION
ii. The peripheral nervous system:
- Part of the nervous system outside of the CNS
- Consists of nerves and ganglia.
- Nerves are bundles of nerve fibers.
- Cranial and spinal nerves extend from the CNS to peripheral
organs e.g. muscles and glands.
- Ganglia are collections of nerve cell bodies outside the CNS.

19. FUNCTIONAL CLASSIFICATION
• The functional classification is concerned only with the PNS.
• The PNS is subdivided into two:
i. Afferent (sensory) division:
ii. Efferent (motor) division

20. AFFERENT (SENSORY) DIVISION
• Transmits impulses from sensory receptors located in
various parts of the body to the CNS.
• Two types:
i. Somatic sensory fibers: transmit impulses from the skin,
skeletal muscles, and joints.
ii. Visceral sensory fibers: transmit impulses from the
visceral organs e.g. stomach, liver, bladder.

21. EFFERENT(MOTOR) DIVISION
• Transmits impulses from the CNS to effector organs e.g.
muscles or glands.
• Is divided into:
i. Somatic nervous system
ii. Autonomic nervous system
• Somatic NS: is involved in voluntary control.
- Conducts impulses from the CNS to skeletal muscle.

22. EFFERENT(MOTOR) DIVISION
ii. Autonomic nervous system:
- Conducts impulses to cardiac muscle, smooth muscle & glands.
- It regulates involuntary functions.
- It is divided into the parasympathetic, sympathetic and enteric
nervous system.
- Sympathetic NS: mobilizes body systems during activity, flight or
fight response.
- Parasympathetic NS: conserves energy, maintains body functions at
rest.
- Enteric nervous system: controls the functions of the GIT.

24. FUNCTIONS OF THE NERVOUS SYSTEM
i. Monitoring change: the nervous system uses its sensory
receptors to monitor changes occurring both inside and
outside of the body.
- These changes are called stimuli, and the gathered
information is called sensory input.
ii. Interpretation of sensory input: it processes and
interprets the sensory input and decides what should be
done at each moment, a process called integration.

25. FUNCTIONS OF THE NERVOUS SYSTEM
iii. Effects responses: it then effects a response by activating muscles
or glands (effectors) via motor output.
iv. Mental activity: the brain is the center of mental activity,
including consciousness, thinking, and memory.
iv. Homeostasis: is the maintenance of a constant internal
environment despite changes in the external environment.
- This function depends on the ability of the nervous system to
detect, interpret, and respond to changes in external conditions.
- It can help stimulate or inhibit the activities of other systems to help
maintain a constant internal environment.

27. SENSORY PHYSIOLOGY:
TASTE
Dr Wairimu Mwaura

28. PRIMARY TASTE SENSATIONS
• There are 5 primary taste
sensations that stimulate and
are perceived by our tastes
buds.
• They include:
i. Sweet
ii. Salty
iii. Sour
iv. Bitter
v. Umami

29. PRIMARY TASTE SENSATIONS
i. Sour taste:
• Sourness is a taste that detects acidity
• Taste buds detect hydrogen ions released from acidic
substances
• The more acidic the food, the stronger the sour sensation
becomes
ii. Salty taste:
- Is elicited by ionised salts, mainly by Na+
- Sodium cations are mainly responsible for the salty taste
- Sodium anions contribute to a lesser extent

30. PRIMARY TASTE SENSATIONS
iii. Sweet taste:
• The sweet taste is the sensitivity of taste cells to the
presence of glucose dissolved in saliva
• Molecules that are similar in structure to glucose will have
similar effect on the sensation of sweetness
- Fructose , sucralose, aspartame, saccharine
• The affinity for each of these molecules varies, and some
will taste “sweeter” than glucose because they bind to the
sweet receptor differently.

31. PRIMARY TASTE SENSATIONS
iv. Bitter taste:
• Stimulated by a large number of molecules known as alkaloids
• Alkaloids are long chain organic substances that contain nitrogen
- Hops (in beer), tannins (in wine), tea, quinine, caffeine, nicotine
• Threshold for stimulation of the bitter taste is the lowest
• When enough alkaloids are contained in a substance it can stimulate the
gag reflex.
• This is a protective mechanism because alkaloids are often produced by
plants as a toxin.
• Humans tend to avoid eating bitter foods.
• When we do eat bitter foods, they are often combined with a sweet
component to make them more palatable (milk and sugar in coffee, for
example).

32. PRIMARY TASTE SENSATIONS
v. Umami:
- Japanese word meaning delicious
- Pleasant taste sensation that is different from sweet, sour,
salty& bitter
- Savoury taste
- Umami is the dominant taste of foods containing glutamine
- It is considered to be the taste of proteins and is associated
with meat containing dishes

33. TASTE BUDS
• The sense of taste is mediated by taste receptor cells which
are bundled in clusters called taste buds.
• Adults have 3000-10000 taste buds.
• Beyond 45 years taste buds degenerate causing the taste
sensation to progressively decrease with old age.
• Each taste bud responds mostly to one of the five primary
taste sensations when the taste substance is in low
concentration.
• At high concentration most taste buds are excited by two or
more of the primary taste stimuli.

34. TASTE BUDS
• Taste buds consist of three cell types:
i. Supporting cells: are found among taste receptor cells
- Do not respond to taste stimuli
ii. Basal cells: undifferentiated stem cells that serve as precursors to taste
receptor cells
- Undergo continuous replacement, every 10 days they differentiate into
new taste receptor cells to replace those sloughed off from the tongue
iii. Taste receptor cells: line the taste buds and extend microvilli into the
taste pores
- Microvilli provide a large surface area for detection of chemical stimuli
- The taste receptor transduces the chemical stimuli into electrical signals
that are transmitted to the CNS via afferent nerve fibres

35. TASTE BUDS

36. LOCATION OF TASTE BUDS
• Taste buds on the tongue are organised in papillae
• Four types of papillae: filiform, fungiform, foliate and
circumvallate
• Filiform papillae do not contain taste buds
• Circumvallate: largest papillae, contain half the total number of
taste buds
- Form a V on the posterior surface of the tongue
• Foliate papillae: located on the lateral borders of the tongue
• Fungiform papillae: located on the anterior surface of the
tongue
• Additional taste buds are located on the palate, tonsillar pillars,
proximal esophagus and epiglottis

37. PAPILLAE

38. TASTE PATHWAY
• Taste begins with transduction of chemical signals in the
taste receptor cells
• Chemical signal is converted into an electrical signal that is
transmitted to the CNS
• Different regions of the tongue are innervated by the 3
cranial nerves:
i. Anterior 2/3 of the tongue (sweet, salty and umami
sensations are most sensitive) – facial nerve
ii. Posterior 1/3 of the tongue (bitter & sour)-
glossopharyngeal nerve
iii. Back of the throat & epiglottis – vagus nerve

39. TASTE PATHWAY
• The 3 cranial nerves enter the brain stem and ascend in the
solitary tract
↓
Terminate on neurons in the solitary nucleus of the medulla
↓
Posteromedial nucleus of the thalamus
↓
Neurons leave the thalamus and terminate in the taste
cortex

40. TASTE PATHWAY

41. FACTORS INFLUENCING TASTE SENSATION
i. Age: taste discrimination decreases with increasing age
due to atrophy of taste buds
ii. Temperature: maximum sensitivity to taste is at or
slightly below body temperature
iii. Olfaction – as it affects flavour of food
iv. Disease : patients with cancer and anorexia have reduced
taste sensitivity due to their compromised physical
condition
- Patients suffering from a cold will complain of loss of their
sense of taste

42. FACTORS INFLUENCING TASTE SENSATION
v. Sex : women are more sensitive to sweet and salt and less
sensitive to sour than men
vi. Taste medium: taste buds can only detect flavours that are
dissolved in a liquid.
- You cannot taste a dry substance with a dry tongue
- Increased viscosity reduces tastes sensitivity.
- It is easiest to detect tastes in liquid state, harder in foams
and more difficult in gels.
vii. Smoking : places the taste buds in contact with chemical
compounds that decrease the ability to register the primary taste
sensations
viii.Adaptation: taste buds adapt quickly to a particular taste

43. ABNORMALITIES OF TASTE
i. Ageusia: inability to detect any taste
- Can be congenital or due a lesion of the facial or
glossopharyngeal nerve
ii. Hypogeusia: reduced ability to taste
iii. Dysgeusia : altered taste sensation
- Salty / foul/metallic taste sensation persists in the mouth
iv. Familial dysautonomia: rare condition where a high
concentration of glucose fails to produce sweet taste
sensation
v. Selective taste blindness: high increase in threshold to bitter
taste
- All other taste sensations remain normal

44. THANK YOU