The nervous system

1. PHYSIOLOGY OF THE
NERVOUS SYSTEM

2. Outline of the Nervous System
Cellular Components of the Nervous System
Nervous Tissue of the Nervous System
Organization of the Nervous System
Overview of the Divisions of the Nervous System
CNS and its functions
Divisions under PNS
PNS functions
Reflex Action
Action Potential
Homeostasis Mechanism

5. Nerve cell or
Neuron
Nerve tissue

7. Organization of the Nervous System
The nervous system orchestrates body functions to maintain
homeostasis. it is made up of neurons and glia cells
Functions
Controls and coordinates the body functions and generate an
appropriate motor response to adjust activity of muscles and
glands
Keeps the previous stimuli as the experiences or memory which
guide the animal in the future
Coordinates the visceral functions to maintain a homeostasis in
the body

8. Nervous System

9. Organization of the
Nervous System
 2 big initial divisions:
1. Central nervous system
 the brain + the spinal cord
 the center of integration and control
2. Peripheral nervous system
 the nervous system outside of the brain and spinal
cord
 consists of:
 31 spinal nerves
 carry info to and from the spinal cord
 12 cranial nerves
 carry info to and from the brain

10. CNS
Consists of the brain and spinal
cord
Is the control centre of nervous
system interpreting,
integrating, and issuing
commands to the other
branches of the nervous
system.
Encased in bone and protected
by CSF and the meninges

11. Peripheral Nervous System
Responsible for communication between the cns and the rest of the body.
can be divided into:
Sensory/ afferent division
conducts impulses from receptors to the cns
informs the cns of the state of the body interior and exterior
sensory nerve fibers can be somatic (from skin, skeletal muscles or
joints) or visceral (from organs within the ventral body cavity)
Motor/ efferent division
conducts impulses from cns to effectors (muscles/glands)
motor nerve fibers

12. Motor/Efferent Division
Can be divided further:
Somatic nervous system
voluntary
somatic nerve fibers that conduct impulses from the cns to
skeletal muscles
Autonomic nervous system
involuntary
conducts impulses from the cns to smooth muscle, cardiac muscle,
and glands.

13. Autonomic Nervous System
Consists of nerves that carry information to visceral organs and glands
their activities are not of conscious control
Has 2 major branches and 1 sub-branch
Sympathetic nervous system
Parasympathetic nervous system and
Enteric nervous system

14. Autonomic Nervous System
 can be divided into:
sympathetic nervous system
“fight or flight”
parasympathetic
nervous system
“rest and digest”
These 2 systems are antagonistic.
Typically, we balance these 2 to keep ourselves in a state of dynamic
balance.
We will go further into the difference between these 2 later!

16. The Human Brain
Inside the developing embryo, the brain starts off as a neural tube which then
differentiate into the spinal cord, three primary vesicles (prosencephalon
(forebrain), mesencephalon (midbrain), rhobencephalon (hindbrain) which
essentially becomes the structure of the brain.
Prosencephalon (Forebrain) divides into
1. Telencephalon
2. Diencephalon
Mesencephalon (Midbrain) divides into
3.Myelencephalon
4. Metencephalon
These divisions or regions may or may not interact with each other to perform a
particular activity

18. The Human Brain
It is the site of major coordination in the nervous system
It contains around 100 billion neurons linked to up to 10000
synaptic connections
Weighs about 1.36kg in adults
Four main regions
1. Cerebrum
2. Diencephalon
3. Brainstem
4. Cerebellum

19. Cerebrum: involved in coordinating intellectual activites such as
reasoning, memory attention, thought, language and consciousness;
movements. The largest division of the brain
Consists of two sides: right and left cerebral hemispheres
The hemisphere are covered by a thin layer of gray matter known as
the cerebral cortex
Each hemisphere of the cerebral cortex is divided into four lobes
Occipital lobe
Temporal lobe
Parietal lobe
Frontal lobe

20. Frontal lobe: reasoning, planning,
speech, movement, emotions,
problem solving
Parietal lobe: movement,
orientation, recognition, perception
Occipital lobe: coordinates sight or
visual processes
Temporal lobe: auditory
perception, memory and speech

21. Brainstem
Controls the most vital life functions e.g. Breathing rate, heart rate,
sleep and consciousness
It is the base of the brain
It adjoins the spinal cord
Consists of the: medulla oblongata, pons and mid brain
Medulla oblongata
Controls:
Heart rate/ action
Vasoconstriction/ blood vessel diameter
Breathing, peristalsis, reflexes such as: swallowing, coughing,
sneezing, vomiting, hiccupping

22. Pons
Controls breathing, reflexes such as pupillary reflexes (constricts the
pupil to limit light entering the eye)
Midbrain
Controls reflexes such as eye movement

23. Cerebellum: The “little brain” structure sits below the cerebrum has an
outer cortex of gray matter has two hemispheres
Receives/ relays information via the brainstem
Controls
Coordinates motions, movement, Posture and balance of the limbs
during movements, Fine motor control, Eye movement

24. The cerebellum can be
permanently damaged by trauma
or stroke or temporarily affected
by drugs such as alcohol.
These alterations can produce
ataxia – a disturbance in balance.

25. Thalamus: It receives many sensory inputs with the exception of smell sensory
inputs and relays the information to the parietal lobe of the cerebrum.
It also stimulates feeling of wakefulness or alertness
It also allows the cerebrum to focus on an important task
Hypothalamus: control body temperature,
body rythym (biological clock-processes that
occurs in a day), regulate food intake and
coordinate emotional responses such
as anger, anxiety etc.
Mainly, the hypothalamus links
the nervous system to the endocrine
system (production of hormones)

26. The Spinal Cord
A large, nearly circular mass of nerve tissue which runs along the dorsal side of the
body
• Links the brain to the rest of the body
• Encased in the vertebral column/spine
• The gray matter of the spinal cord consists mostly of cell bodies and dendrites
• The surrounding white matter is made up of bundles of interneuronal axons
(tracts)
• has two functions:
• Provides the two-way routes to/ from(afferent/ efferent) the brain.
• Serves as the reflex centre for all spinal reflexes

29. Reflex Action
A rapid automatic response to a stimulus
• requires no conscious thought
• the neural pathway that mediates a reflex action is the reflex arc
The reflex arc is made up of five basic elements:
1. the receptor
2. the sensory neuron
3. integrating centre (ie. an associated neuron in the central nervous system;
this may be absent in some reflex arcs
4. the motor neuron
5. effector

32. Communication Between Neurons

33. It is a means by which impulses are transmitted from one neuron to
another.
• Communication is by the liberation of signal
molecules like hormones, neurotransmitters
• Signal molecules initiate their action
by binding to receptors and
generating a response
• communication may be from
 nerve cell to nerve cell
 from nerve cell to muscle
 from nerve cell to gland
How do neurons talk to each other?

34. • The site of transmission of information is the synapse
A typical synapse consists of
 presynaptic cell
 postsynaptic cell
synapses may be
 electrical found at gap junctions
 chemical found at synaptic clefts

40. • Action potential at the presynaptic neuron opens voltage-
gated Ca2+ channels concentrated in the nerve terminal
• Ca2+ flow into the cell
• Ca2+ cause the synaptic vesicles to fuse with the cell
membrane, releasing their contents (the neurotransmitter
chemicals) by exocytosis
• The neurotransmitters diffuse across the synaptic cleft

41. •The neurotransmitter binds to the receptors in the post-synaptic
membrane
• The transmitter-receptor complex causes a transient change in
the conductance of post-synaptic membrane to one or more ions
causing a transient change in membrane potential
• A transient depolarization causes an excitatory postsynaptic
potential (EPSP)
• A transient hyperpolarization causes an inhibitory postsynaptic
potential (IPSP)
• Action Potentials are not produced at the synapse

42. Neurotransmitters
A chemical substance which is released at the end of a neuron
by the arrival of an impulse and, by diffusing across the synapse
or junction, effects the transfer of the impulse to another neuron,
a muscle fibre, or some other structure.

43.  Neurotransmitters
They may be
 neuroexciters
 neuroinhibitors
o Neuroexciters
They cause depolarization of the postsynaptic membrane
resulting in EPSPs
egs norepinephrine (NE), Epinephrine (E), dopamine (DA),
aspartate, glutamate

44. o Neuroinhibitors
They cause hyperpolarization of the postsynaptic
membrane resulting in IPSPs
egs include GABA, glycine, and ACh acting
on heart muscle

45. Neurotransmitter Removal

47. • Integration (summation) can be
 spatial
 temporal

49. RESTING MEMBRANE POTENTIAL (RMP)
basic definition of RMP: the voltage/charge difference across
the cell membrane when the cell is at rest
RMP exits in all cells
in neurons, RMP is -70mv

50. A resting cell is said to be polarized

51. • Increasing the external K+ concentration
increases the resting membrane potential.(---)
Why?
• Decreasing the external Na+ concentration
has little effect on the resting membrane
potential
WHY?

53. Two types of responses can be elicited if a neuron is stimulated.
The type of response depends on the intensity of stimulation.
These are:
• Graded potential
• Action potential

54. Graded Potentials
Any stimulus that opens a gated channel produces a graded
potential.
Graded potentials are changes in the membrane potential that
cannot spread far from the site of stimulation elicited by a
stimulus of subthreshold strength
Threshold: The voltage needed to open voltage gated Na+
channels in the axons (-55mV)

55. WHAT DOES GRADED POTENTIAL DO?
1. Moves RMP closer to threshold causing slight depolarization-Excite
the cell
Depolarization- making the cell less negative after hitting the threshold
strength
2. Moves it further away from threshold causing hyperpolarization-
Hyperpolarization- making the cell more negative

56. • Graded Potential arise mainly in dendrites and cell bodies
• May be hyperpolarizing (inhibitory to generation of action
potential) or depolarizing (excitatory to generation of action
potential)

57. • A graded potential that is capable of reaching the threshold
(-55mV) depolarizes or excites the axon of the neuron
Excitatory can be done in two ways
1. Temporal Summation: The combination of two or more graded
potentials from one presynaptic neuron to reach a threshold in
Postsynaptic neuron
2. Spatial Summation: The combination of two or more graded potentials
from many presynaptic neurons to reach a threshold in
Postsynaptic neuron

58. Action Potential
A self-propagating electrical potential difference produced across
the plasma membrane of neuron or muscle cells when they are
stimulated
• An Action Potential is initiated by a stimulus above a certain intensity
or threshold
In a neuron;
• Arise at trigger zones and propagate along axon
• Propagated, thus permit communication over long distances
• All or None
• Typically of shorter duration, ranging from 0.5–2 msec

59.  Phases of the Action Potential
• Latent period
• Depolarization-making the cell positive or less negative
• Repolarization-returning the cell back to negative (RMP)
• Hyperpolarization- making the cell more negative
 Latent Period
 Time taken by stimulus to travel from point of stimulation to the
recording electrode
 Depolarization
A depolarization of adequate rate and threshold causes the voltage-gated
Na+ channels to open (Activation gate opens)

60. • Having reached the action threshold around that axon, more
voltage-gated Na+ channels open by a positive feedback
mechanism
• Then Na+ influx drives the interior of the cell membrane
from the threshold (-55mV) up to about +30mV
• Depolarization wave moves across the axon terminal
causing voltage Ca+ ion channels to open and releasing
neurotransmitters
• Now purpose for AP achieved, axon voltage needs to return
to negative again -Repolarization

61. Repolarization
Voltage-gated K+ channels open. Since the K+ channels are much slower
to open, the depolarization has time to be completed.
o The Na+ channels show rapid inactivation for some time,
resulting in a significant decrease in Na+ conductance before
closing
o Repulsion of Na+ by the positive interior limits
influx of Na+
o prolonged opening of the voltage-gated K+
channels increases K+ conductance and returns
the potential back to the resting level

62. Hyperpolarization
With both the voltage-gated and non-gated K+ channels open, the
membrane potential becomes more negative than the resting
membrane potential
The RMP is regained by
 closure of the voltage-gated K+ channels
 efflux of K+ through non-gating K+ channels
 restoration of resting ion concentrations By the Na+-K+ pump
Decreasing the external Na+ concentrations decreases the size of the
AP

66.  Refractory Period
The period during an action potential when the
ability of the membrane to respond to a second
stimulus is markedly altered
 it limits the number of APs that can be
produced by an excitable membrane in a given
period
 it is a key in determining the direction of AP
propagation
It is made up of two phases. These are:
• absolute refractory period
• relative refractory period

67.  Absolute Refractory Period
It is the period during an action potential, when a
second stimulus will not produce a second action
potential no matter the strength of the stimulus
o it corresponds to the period when the
voltage-gated Na+ channels are open or in the
inactivation state
o coincides with the entire duration of the action
potential

68.  Relative Refractory Period
It is the period during an action potential when
another action potential can be produced, but
only if the stimulus strength is greater than the
threshold stimulus, then gradually by stimuli of
progressively lesser magnitude

70. Homeostasis and Control Systems

71. 71
Homeostasis
• It is the maintenance of nearly constant internal environment despite
changes that may be occurring in or outside the body.
• This implies that a given physiological variable is NOT rigidly constant
with respect to time but that it fluctuate within a predictable and often
a narrow range.
• The internal environment of the body (ECF) is said to be in dynamic
state of equilibrium.
HOMEO- SAME
STASIS- STANDING STILL

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• Homeostasis maintains a normal range around a set point
Fasting Blood glucose level 3.9mmol/L-5.6mmol/L
Body Temperature 36.1°C-37.2 °C
Body PH 7.35 -7.45
Blood Pressure levels
Systolic BP 90-120 mmHg
Diastolic BP 60-80 mmHg

73. 73
• The essential variables of the internal environment that are
maintained within limit are;
 concentration of O2 and CO2
 concentration of nutrients and waste products
 volume and pressure of the extracellular fluid
 temperature
 pH
 concentration of salt and other electrolyte
• Homeostasis is continuously being disturbed by
o external stimuli
– heat, cold, lack of O2 , pathogens, toxins

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o internal stimuli
– body temperature, blood pressure, concentration of water,
glucose, salts, O2, physical and psychological distress
• Disturbances in homeostasis leads to homeostatic imbalance.
– mild and moderate imbalance leads to sickness
– severe imbalance can lead to death
• Different organ systems operate in harmony to provide
homeostasis
• Most often, the nervous and the endocrine system working together or
independently, provide the needed corrective measures.

75. 75
Homeostatic Control Systems
• They are made up of body components that generate compensatory
regulatory responses to maintain relatively stable condition of the
internal environment.
• They are made up of three basic components:
 Receptors/sensors
 Integrating/control center
 Effectors

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Homeostatic control system

77. 77
• Receptors
 detect change in a monitored variable (stimulus) eg. change in
temperature
 receives and responds to the changes by sending information
to a control center.
• Control center/Integrating center
 determines the set point at which the variable is maintained
 analyses the input from the receptor
 sends instructions (output) to the effector

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• Effectors
 receive directions from the control center
 carry out instructions
 produces a response that changes the controlled condition
and the value of the variable.
• Two basic mechanisms are used by the body to maintain
homeostasis
– Feedback
– Feedforward
 Feedback control; impacts or ‘feeds back’ or respond to influence
the input or stimulus (anything that changes the environment).

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• Two types of feedback loop;
 Negative feedback
 Positive feedback
 Negative feedback
• Occurs when a change in controlled variable triggers a response that
opposes the change.
• This form of feedback helps to maintain equilibrium.
• Most feedback systems in the body are negative.
• Examples of negative feedback controls
- temperature regulation, blood sugar regulation, blood pressure
control

81. Negative feedback control of body temperature

82. Negative feedback control of blood glucose

83. Negative feedback control of blood pressure

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 Positive feedback
• Occurs when a change in controlled variable initiate a response that
intensifies the change.
• Here the response reinforce the stimulus rather than decreasing or
removing it.
• Because this type of feedback escalate the response, it requires some
interventions or event outside the loop to stop it.
• Leads to instability and can cause death in some cases
• Sometimes useful in blood clotting, childbirth, lactation

86. Positive feedback loop; childbirth

87. Positive feedback loop; lactation

88. Positive feedback loop; blood clotting

89. 89
 Feedforward
• This refers to the type of process in which changes in a regulated
variables are anticipated and prepared for before they actually occur.
• Here, a change in variable is anticipated and the body prepares for it
before it occurs.
It functions to;
 improve speed of the body’s homeostatic response.
 minimize fluctuations in the levels of variables being regulated.

90. 90
• Example;
– preparation of the GIT for the arrival of food in response to sight,
smell or thought of food
– Erection of sexual organs in anticipation of sexual activities, as
a response to visual or tactile stimulation or thoughts