digestive system

1. ZLY 106
NERVOUS SYSTEM
DR. O. A. IYIOLA

2. OUTLINE
• Introduction
• Nervous system in lower organisms.
• Components of Nervous System (neurons).
• Types & structure of neurons(Sensory/ Afferent,
Motor/Efferent &Relay/Association).
• Organization of the nervous system into CNS & PNS.
• The components, functions of brain & spinal cord
• Mechanism of Transmission of Nerve Impulses
• Action & Resting Potentials
• Neurochemicals
• Reflex actions(examples), Reflex arc, Synapse,

3. INTRODUCTION
• The nervous system is the principal regulatory system in
animals, an animal’s life style is closely related to the
organization & complexity of its nervous system
• Nervous & endocrine system work in close partnership to
bring about coordination of activities & regulation of many
behaviours & physiological processes in the body.
• In nervous system, messages are transmitted as electrical
impulses via the nerves in a very rapid, very precise & well
coordinated fashion.
• The effectors(muscles & glands) receive the messages &
respond quickly.
• Responses & the effects may be felt immediately, the effects
are usually brief & very localized.

4. NERVOUS SYSTEM IN INVERTEBRATES
• Hydra & other Cnidarians have nerve nets made up of
interconnected neurons with no central control organ.
• In planarian flatworms the bilateral nervous system includes
cerebral ganglia &, usually, two solid ventral nerve cords
connected by transverse nerves.
• Annelids & arthropods typically have a ventral nerve cord &
numerous ganglia.
• The cerebral ganglia of arthropods have specialized regions.
• Octopods & other cephalopod mollusks have highly
developed nervous systems with neurons concentrated in a
central region.

5. THE NERVOUS SYSTEM
• The vertebrate nervous system consists of the
central nervous system (CNS) & peripheral
nervous system (PNS).
• The CNS consists of the brain & dorsal,
tubular spinal cord.
• The PNS consists of sensory receptors &
nerves.

6. NERVOUS SYSTEM IN HIGHER VERTEBRATES
• The PNS consists of sensory receptors & nerves, including the
cranial nerves & spinal nerves & their branches.
• The somatic division of the PNS responds to changes in the
external environment. The autonomic division regulates the
internal activities of the body.
• The sympathetic system permits the body to respond to
stressful situations.
• The parasympathetic system influences organs to conserve &
restore energy.
• Many organs are innervated by sympathetic parasympathetic
nerves, which function in opposite ways.
• For example, the sympathetic system increases heart rate,
whereas the parasympathetic system decreases heart rate.

7. REFLEX ACTION
• An automatic, involuntary response to a given
stimulus that generally functions to restore
homeostasis.
• The circuit of a reflex action begins with a
sensory neurone.
• A reflex protects the body by triggering a rapid,
involuntary response to a particular stimulus.
• For example, if you put your hand on a hot
burner, a reflex begins to pull your hand back well
before the sensation of pain has been processed in
your brain.

8. REFLEX ARC

9. REFLEX ARC

10. EXAMPLES OF REFLEX ACTION
• Withdrawing hands from a hot pan
• Blinking of eyes
• Contraction of pupils
• Knee jerking

11. ACTION & RESTING POTENTIALS
• An action potential is generated by an influx of Na+
& an efflux of K+
• When a stimulus is strong enough, a rapid, large
change in membrane potential occurs, depolarizing
the membrane to a critical point known as the
threshold level.
• At that point, the neuron fires a nerve impulse, or
action potential, an electrical signal that travels
rapidly down the axon into the synaptic terminals.
• All cells can generate graded potentials, but only
neurons, muscle cells, & a few other cell types
(certain cells of the endocrine & immune systems)
can generate action potentials.

12. WHAT IS ACTION POTENTIALS?
• An action potential is a short-lasting event in which the
electrical membrane potential of a cell rapidly rises and falls, following a
consistent trajectory.
• It occurs in several types of animal cells, called excitable cells, which
include neurons, muscle cells, and endocrine cells, as well as in some plant
cells.
• In neurons, they play a central role in cell-to-cell communication.
• In other types of cells, their main function is to activate intracellular
processes. In muscle cells, for example, an action potential is the first step
in the chain of events leading to contraction. In beta cells of the pancreas,
they provoke release of insulin.
• Action potentials in neurons are also known as "nerve impulses" or
"spikes", and the temporal sequence of action potentials generated by a
neuron is called its "spike train". A neuron that emits an action potential is
often said to "fire".
• Action potentials are generated by special types of voltage-gated ion
channels embedded in a cell's plasma membrane.

13. ACTION & RESTING POTENTIALS
• Neurons are excitable cells. They can respond to stimuli &
convert stimuli into nerve impulses.
• An electrical, chemical, or mechanical stimulus may alter the
resting potential by increasing the membrane’s permeability to
sodium ions.
• When a stimulus causes the membrane potential to become
less negative (closer to zero) than the resting potential, that
region of the membrane is depolarized.
• Because depolarization brings a neuron closer to transmitting a
neural impulse, it is described as excitatory.
• In contrast, when the membrane potential becomes more
negative than the resting potential, the membrane is
hyperpolarized.

14. Membrane, Resting & Action potentials
• Action potentials are the signals conducted by axons
• K+ & Na+ play critical roles in the formation of
resting potential
• Neurons have gated ion channels that open or close
in response to stimuli, leading to changes in
membrane potential.
• A change in the membrane potential toward a more
negative value is a hyperpolarization; a change
toward a more positive value is a depolarization.
• Changes in membrane potential that vary with the
strength of a stimulus are known as graded potentials.
• After action potentials, the Na+ are actively pumped
out by a process called sodium pump.

15. Production & Conduction of Action potentials
• An action potential is generated by an inflow of Na+ &
an outflow of K+ ions across the membrane at one
location
• An action potential functions as a long-distance signal
by regenerating itself as it travels from the cell body to
the synaptic terminals, much like a flame traveling
along a lit fuse.
• At the site where an action potential is initiated (usually
the axon hillock (neck), Na+ inflow during the rising
phase creates an electrical current that depolarizes the
neighboring region of the axon membrane.
• Note that the membrane is much more permeable to
K than to Na,

16. The Resting potentials
• Resting potentials occurs when the nerve cells are not
conducting any signals.
• Although the resting potential is primarily established
by K, & less so by Na, Cl also contribute slightly
because the plasma membrane is permeable to
negatively charged Cl- ions.
• Because the membrane is much more permeable to
K+ than to Na+, the resting potential of the neuron is
closer to the K+ equilibrium potential than to the Na+
equilibrium potential.

17. MECHANISM OF TRANSMISSION OF IMPULSES
• This involves 3 phases
• Resting potential- Here the cell surface is +vely charged with
respect to the inside which is –vely charged. This is due to
accumulation of Na+ outside & K+ inside due to permeability
of membrane to K+
• Action potential- This is the passage of impulse & this
involves entry Na+ into the nerve by diffusion & exit of K+.
This is due to membrane now becoming permeable to Na+.
• Repolarization- This is the re-establishment of the original
resting potential of the membrane. Cell surface now once
again become +vely charged while the inside is –vely charged.

18. MECHANISM OF TRANSMISSION OF IMPULSES
• Ion pumping maintains the gradients that determine the resting
potential.
• The neuron plasma membrane has very efficient sodium–
potassium pumps that actively transport Na out of the cell & K
into the cell.
• Both Na+ & K+ are pumped against their concentration &
electrical gradients, & these pumps require ATP.
• For every three Na pumped out of the cell, two K+ are
pumped in. Thus, more positive ions are pumped out than
in.
• The sodium–potassium pumps maintain a higher concentration
of K inside the cell than outside, & a higher concentration of
Na outside than inside.

19. TRANSMISSION OF IMPULSES
• Transmission is the process of sending messages along a
neuron, from one neuron to another or from a neuron to a
muscle or glands.
• An action potential travels from the axon hillock (axon closest
to cell body) to the synaptic terminals by regenerating itself
along the axon.
• The speed of conduction of an action potential increases with
the diameter of the axon &, in many vertebrate axons, with
myelination.
• The depolarization of the action potential spreads to the
neighbouring region of the membrane, reinitiating the action
potential there.
• Action potentials in myelinated axons jump between the
Nodes of Ranvier, a process called Saltatory conduction.

20. INTRODUCTION TO NEURONS
• Neurons is the basic functional unit of the
nervous system
• Nervous tissue consists of neurons & glial cells.
• Neurons are specialized for receiving &
transmitting signals.
• Neurons are nourished & supported by Glial
cells.
• A typical neuron has a cell body containing the
nucleus as well as two types of cytoplasmic
extensions.

21. INTRODUCTION TO NEURONES
• Certain neurons receive signals from the
external or internal environment &transmit
them to the brain &spinal cord.
• Other neurons relay, process, or store
information. Still others transmit signals from
the brain & spinal cord to the muscles
&glands.
• Neurons communicate at junctions called
synapses.
• A nerve consists of a great many neurons
bound together by connective tissue.

22. PICTURE OF A TYPICAL NEURON

23. STRUCTURE & FUNCTION OF A TYPICAL NEURONE
• Neurons are specialized to receive stimuli & transmit
electrical & chemical signals.
• A typical neuron consists of a cell body, dendrites &
an axon
• 1. Cell body: contains the nucleus & contain most of
a neuron's organelles including its nucleus.
• 2. Dendrites: usually numerous- Dendrites are highly
branched, cytoplasmic extensions specialized for
receiving signals & transmitting them to the cell
body.
• 3. Axon: It is single, long & extends from the cell
body.

24. STRUCTURE OF A TYPICAL NEURON
• Each branched end of an axon transmits
information to another cell at a junction called
a synapse
• The single axon transmits signals, called nerve
impulses, away from the cell body.
• Axons range in length from 1 or 2 mm to more
than a meter.
• The expanded portion of the neuron is called
soma

25. STRUCTURE & FUNCTION OF A TYPICAL NEURONE
• Many axons are surrounded by an insulating Myelin
sheath.
• In the PNS myelin sheath is formed by Schwann cells
but in the CNS the sheath is formed by other glial
cells.
• Nodes of Ranvier are gaps in the sheath between
successive Schwann cells.
• A nerve consists of several hundred axons wrapped in
connective tissue; a ganglion is a mass of neuron cell
bodies in the PNS.

26. SYNAPSE, PRE AND POST-SYNAPTIC NEURONS
• Myelinated neurons transmit impulses rapidly
• In vertebrates, another strategy has evolved that speeds
transmission—myelinated neurons.
• Myelin acts as an effective electrical insulator around the axon
except at the nodes of Ranvier, which are not myelinated.
• A synapse is a junction between two neurons or between a
neuron & an effector, such as between a neuron & a muscle
cell.
• A neuron that terminates at a specific synapse is called a
presynaptic neuron, whereas a neuron that begins at that
synapse is a postsynaptic neuron. Note that these terms are
relative to a specific synapse. A neuron that is postsynaptic
with respect to one synapse may be presynaptic to the next
synapse in the sequence.

27. TYPES OF NEURONS
• Sensory/Afferent- transmit information to the CNS.
Afferent neurons generally transmit information to
interneurons, or association neurons, in the CNS.
• Motor /Efferent– send signals to the skeletal muscles
& glands. The motor neurons signal muscles in the
abdomen to contract.
• Interneurons- Connect both motor & sensory
neurons.
• The vast majority of neurons in the brain are
interneurons & infact 99%, are interneurons. Their
function is to integrate input & output

28. SUMMARY OF INFORMATION FLOW THROUGH
NERVOUS SYSTEM
• Integration involves sorting & interpreting incoming sensory information
&determining the appropriate response.
• Neural messages are transmitted from the CNS by efferent (meaning “to carry
away”) neurons to effectors—muscles & glands.
• Sensory receptors, afferent & efferent neurons are part of the peripheral nervous
system (PNS).
• In summary, information flows through the nervous system in the following
sequence:
• Reception by sensory receptor transmission by afferent neuron
integration by interneurons in CNS transmission
• by efferent neuron action by effectors

29. NERVOUS SYSTEM IN HIGHER
VERTEBRATES
• The CNS consists of the brain and the spinal cord
• The PNS consists of sensory receptors & nerves, including the cranial
nerves & spinal nerves & their branches.
• The somatic division of the PNS responds to changes in the external
environment. The autonomic division regulates the internal activities of the
body.
• The autonomic system is divided into sympathetic & parasympathetic
nervous system.
• The sympathetic system permits the body to respond to stressful situations.
• The parasympathetic system influences organs to conserve & restore
energy.
• Many organs are innervated by sympathetic & parasympathetic nerves,
which function in opposite ways.
• Eg the sympathetic system increases heart rate, whereas the
parasympathetic system decreases heart rate.

30. Picture showing the Human Brain

31. THE 3
BRAIN
REGIONS
EMBRYONIC
BRAIN REGION
BRAIN STRUCTURES
PRESENT IN ADULT
FOREBRAIN TELENCEPHALON Cerebrum (including cerebral
cortex, white mater, basal nuclei
DIENCEPHALON Thalamus, Hypothalamus and
Epithalamus
MIDBRAIN MESENCEPHALON Midbrain (part of the brain stem),
Optic lobes
HINDBRAIN METENCEPHALON Cerebellum, Pons (part of the
brain stem.
MYELENCEPHALON Medulla oblongata (part of brain
stem)
BRAIN DEVELOPMENT DURING HUMAN EMBRYONIC AND ADULT STAGE

32. THE HUMAN BRAIN
• The largest part of the human brain is the cerebrum, which is
divided into two hemispheres.
• Underneath lies the brainstem, & behind that sits the
cerebellum.
• The outermost layer of the cerebrum is the cerebral cortex,
with 4 lobes: the frontal, parietal, temporal & occipital
lobes.
• Like all vertebrate brains, human brain develops from 3
sections known as the forebrain, midbrain and hindbrain.
• Each of these contains fluid-filled cavities called ventricles.
• The forebrain develops into the cerebrum and underlying
structures; the midbrain becomes part of the brainstem; and the
hindbrain gives rise to regions of the brainstem and the
cerebellum.

33. The cerebrum
• The human cerebral cortex consists of gray matter, which forms
folds or convolutions.
• Deep furrows between the folds are called fissures.
• Ventricles deep in the brain's interior contain cerebrospinal fluid,
• Most of the gray matter is on the surface of the brain, surrounding
the white mater
• The cerebrum is functionally divided into sensory areas that receive
incoming sensory information; motor areas that control voluntary
movement; & association areas that link sensory & motor areas.
• The white matter of the cerebrum lies beneath the cerebral cortex.
The corpus callosum, a large box of white matter, connects right &
left hemispheres.

34. FUNCTIONS OF SOME PARTS OF THE BRAIN
• Cerebrum - centre for learning, reasoning, language, thought, & judgment.
• Midbrain contains centers for receiving & integrating several types of
sensory information. It also sends coded sensory information along neurons
to specific regions of the forebrain.
• In non mammalian vertebrates, portions of the midbrain form prominent
optic lobes that in some cases are the animal's only visual centers.
• In mammals, vision is integrated in the cerebrum, not the midbrain.
• The midbrain instead coordinates visual reflexes, such as the peripheral
vision reflex.
• Medulla oblongata contains centers that control several automatic,
homeostatic functions, including breathing, heart, blood vessel activity,
swallowing, vomiting & digestion.
• Pons contains centers that help regulate respiration & nuclei that relay
impulses from the cerebrum to the cerebellum.
• The medulla and pons also help coordinate large-scale body movements,
such as running & climbing.

35. FUNCTIONS OF SOME PARTS OF THE BRAIN
• Cerebellum – receives sensory information about the
position of the joints and the length of the muscles, as
well as input from the auditory (hearing) and visual
systems.
• It also monitors motor commands issued by the
cerebrum. Information from the cerebrum passes first
to the pons and from there to the cerebellum.
• The cerebellum integrates this information as it
carries out coordination & error checking during
motor and perceptual functions.
• Reflex center for muscular coordination &refinement
of movements.

36. HYPOTHALAMUS CONTROLS BIOLOGICAL CLOCK
• Hypothalamus is one of the most important brain regions for the
control of homeostasis & biological clock regulation.
• The hypothalamus contains the body's thermostat, as well as
centers for regulating hunger, thirst & many other basic survival
mechanisms.
• The hypothalamus is the source of posterior pituitary hormones
& of releasing hormones that act on the anterior pituitary.
• Hypothalamic centers play a role in sexual & mating behaviours,
the fight-or-flight response & pleasure.
• In mammals, circadian rhythms are coordinated by a group of
neurons in the hypothalamus called the suprachiasmatic
nucleus.
• The thalamus is a relay center for motor & sensory information.

37. The structure & functions functions
of the human spinal cord
• The human brain & spinal cord are protected by bone & three
meninges—the dura mater, arachnoid & pia mater.
• The PNS transmits information to and from the CNS and plays
a large role in regulating an animal's movement and internal
environment
• Like the brain, Spinal cord is cushioned by cerebrospinal fluid
(CSF).
• The spinal cord transmits impulses to & from the brain &
controls many reflex actions.
• The spinal cord consists of ascending tracts, which transmit
information to the brain & descending tracts, which transmit
information from the brain.
• Its gray matter contains nuclei that serve as reflex centers.

38. Neurotransmitters
• It is the chemical messengers used by neurons to
signal other neurons.
• They conduct the neural signal across the synapse &
bind to chemically activated ion channels in the
membrane of the postsynaptic neuron.
• This binding triggers specific gated ion channels to
open (or close), resulting in changes in permeability
of the postsynaptic membrane.
• When a postsynaptic neuron reaches threshold
depolarization, it transmits an action potential.
• Examples are acetylcholine, dopamine,
Norepinephrine, serotonin, glycine, glutamate,
carbonmonoxide, Nitric oxide

39. Neurotransmitters
• Acetylcholine is a low-molecular-weight neurotransmitter
released from the brain, autonomic nervous system, motor
neurons & triggers muscle contraction.
• Cells that release acetylcholine are called cholinergic
neurons.
• Neurons that release norepinephrine are called adrenergic
neurons.
• Serotonin belong to a class of compounds called biogenic
amines.
• Biogenic amines affect mood & their imbalance has been
linked to several disorders, including major depression,
attention deficit disorder (ADD) & schizophrenia.
• Several amino acids function as neurotransmitters.
• Glutamate is the major excitatory neurotransmitter in the brain.

40. Approximate plot of a typical action potential shows its various phases as the action potential
passes a point on a cell membrane. The membrane potential starts out at -70 mV at time zero. A
stimulus is applied at time = 1 ms, which raises the membrane potential above -55 mV (the
threshold potential). After the stimulus is applied, the membrane potential rapidly rises to a
peak potential of +40 mV at time = 2 ms. Just as quickly, the potential then drops and
overshoots to -90 mV at time = 3 ms, and finally the resting potential of -70 mV is
reestablished at time = 5 ms.

41. Glial Cells: Types & Functions
• Glial cells provide support, nourish the neurons & are
important in neural communication.
• Types of Glial cells are
• (1) Astrocytes: support & provide nutrients to neurons, help
regulate extracellular fluid components in the CNS by taking
up excess K+ ions, important in memory & learning, guide
neurons during embryonic development, communicate with
one another & with neurons & stabilize synapses.
• (2) Oligodendrocytes: form myelin sheaths around axons in
the CNS; Schwann cells form sheaths around axons in the
PNS.
• (3). Microglia: This secretes growth factors & are phagocytic
cells.
• 4. Ependymal cells line cavities in the CNS & contribute to
formation of cerebrospinal fluid. They also serve as neural
stem cells.