Download Nervous System

Nervous System
1.Functions of the nervous system:
a. The sensory function: is to sense changes in the
internal & external environment through sensory
receptors and then pass the information through
sensory pathways to the CNS (Brain).
b. The integrative function: This is to analyze the
sensory information, store some aspects and
make decision regarding appropriate behavior.
c. The motor functions: This is to respond to stimuli
by initiating actions, such as movements,
secretion. Motor neurons serve these functions—
muscle contraction and gland secretion.
2.Organization of the nervous system:
i.Central Nervous System (CNS): Brain & spinal cord
i. The Peripheral Nervous System (PNS):
Consists of: cranial nerves, Spinal nerves,
sensory and motor component, ganglia & sensory
receptors.
PNS is divided into somatic, autonomic & enteric
nervous system
a. The sensory system: consists of variety of
receptors & sensory neurons.
b. The motor system conducts nerve impulses
from the CNS to muscles and glands.
2a. The somatic nervous system: (SNS):
Consists of
 sensory neurons that conducts impulses from
cutaneous & special sense receptors to the
CNS
 Motor neurons that conduct impulses from the
CNS to the skeletal muscles.
2b.The autonomic nervous system (ANS)
Consists of
i. sensory neuron from visceral organs
ii. Motor neurons that convey impulses from the CNS
to the smooth muscles, cardiac muscle & glands.
 Motor neuron is again consists of
---Sympathetic division
--- Parasympathetic division.
These two divisions have opposing actions on
the same organ.
2c. The enteric nervous system: consists of neurons in
the wall of the gastro-intestinal tract (GIT).
--They work independently of the ANS and CNS.
3. Histology of the nervous system:
A. Neurons: They are the structural & functional unit
of the nervous system. They consist of nerve cell
(cell body & soma) and many dendrites and usually
a single axon.
a. Structural and functional variation in neurons:
 Structurally they are classified as:
--Multipolar
--Bipolar
--Unipolar
e.Interneurons: They are connecting neurons
between two neurons. Example: Purkinji cell or
Renshaw cell , Pyramidal cell
B. Neuroglia:
--They are specialized tissue cells that support
neurons, attach neurons to blood vessels, and produce
myelin sheath around axon.
 they are of 4 types: astrocytes
Oligodendrocytes
Schwann cell
ependymal cell
C. Myelination:
 a multi-layered lipid & protein covering called
Myelin sheath and it is produced by schwann cell &
oligodendrocytes around axon.
 This sheath is electrically a insulator & protects
axon & increases the speed of nerve impulse
conduction.
 Schwann cell produces Myelin sheath in PNS.
 Outer nucleated cytoplasmic layer of schwann cell
is called neurolemma. The neurolemma aids in
regeneration in an injured axon by forming
regenerative tube that stimulates regrowth of the
axon
D. Gray and white matter:
i.
ii.
White matter composed of aggregation of
myelinated processes, but gray matter contains
nerve cell bodies, dendrites, axon terminals or
bundles of unmyelinated axons & neuroglia
In the spinal cord, gray matter form H shaped
inner core surrounded by white matter. In the
brain, a thin outer shell of gray matter covers the
cerebral hemisphere.
4. Electrical signals in Neurons:
A. Action potential & Graded potential
Excitable cells communicate with each other by action
potential (AP) for long distance and by graded
potential for short distance.
*Production of both types of potentials depend upon
the existence of a resting membrane potential (RMP)
and the presence of certain types of ion channel.
*The RMP is an electrical voltage across the
membrane at rest. When Na+ enters from the outside
to inside the cell, it causes depolarization.
B. Ion channels:
 There are two types of ion channels:
--Leakage (non-gated): are always open.
Examp.: K-channel on the cell membrane.
 Gated channels open and close in response to some
stimulus, for example:
--In response to voltage changes.
--In response to chemical (ligands) or mechanical
pressure.
 Voltage gated channels respond to a direct change
in the membrane potential (fig. 12.8a)
 Ligand gated channels respond to specific chemical
stimulus.
 Mechanically, gated ion channels respond to
mechanical pressure and vibration.
C. Resting membrane potential (RMP):
--When any membrane is at rest, it is positive outside
and negative inside due to distribution of different ions
across the membrane and relative permeability of the
membrane towards Na+ & K+
--A typical value of RMP is –70mv & the membrane
is said to be polarized.
C. Resting membrane potential:
At rest cell membrane are polarized. Such seperation of
+ve from –ve is form of potential energy. This is measurEd in mv
It varies from cell to cell (-40 to –90 mv). This resting
Membrane potential
C1. It is maintained by two factors.
i. Unequal distribution of ions across plasma membrane.
ii. Relative permeability of plasma membrane to Na+ &
K+. In a resting cell, K permeability is 50-100 times
greater than that of Na+
D. Graded potential:
When chemical or mechanical stimulus causes Ligend
gated channels to open or close, cell produce Graded
Potential
This can be Hyperpolarizing Graded potential or
Depolarizing graded potential
Occurs mostly in the dendrites & cell body. Have short
distance communication
E. Action Potential (AP):
 It is an impulse which is a sequence of rapidly
occurring events that decreases & eventually
reverses the membrane potential called
depolarization (make it less negative & even
positive) and
 Then restore it to the resting stage
(repolarization). So, AP has both depolarization
and repolarization.
*During AP, two types of voltage-gated channels
open and close in sequence.
Channels are present in plasma membrane.
 Rapid opening of 1st. Voltage-gated channels
cause Na+ to enter & cause depolarization.
When depolarization reaches a threshold, the
membrane potential reverses & AP is generated.
 The second voltage gated K+ channel open
allowing K+ to flow out. The slower opening of
voltage-gated K+ channels & closing of previously
open NA+ channel—leads to repolarization.
* Local anesthetics prevent opening of voltagegated Na+ channel. So, nerve impulse cannot pass
through the obstructed region.
5. Synaptic Transmission (signal transmission in
synapse):
A. synapse is a functional junction between one
neuron and another or between a neuron and an
effector organ such as muscle or gland.
B.Chemical Synapse: At a chemical synapse, there is
only one-way information transfer from a presynaptic
neuron to a post-synaptic neuron
i. Excitatory post-synaptic potential (EPSP) is a
depolarizing postsynaptic potential.
 A single EPSP normally does not originate a
nerve impulse but it causes the postsynaptic
neuron to become more excitable so that next
EPSP can make the postsynaptic neuron to reach
threshold excitability to produce a nerve
impulse.
ii. Inhibitory postsynaptic potential (IPSP):
An inhibitory neurotransmitter hyperpolarizes
the membrane of the post-synaptic neuron,
making the inside more negative & generation
of nerve impulse more difficult. So a
hyperpolarizing PSP is inhibitory and is
termed IPSP.
c. Summation: if several presynaptic neurons
release their neurotransmitter at about the same
time , the combined effect may generate a nerve
impulse due to summation.
Summation may be temporal and spatial
6. Regeneration and repair of Nervous Tissue:
A.Through out life, the nervous system exhibits
plasticy, the capability to change with learning.
i.
Despite plasticity, neurons have a limited
capacity to repair or replicate themselves.
ii. In PNS, damage to dendrites and myelinated
axons may be repaired if the cell body remains
intact or if Schawann cells are active (fig. 12.19b)
iii. In the CNS, there is little or no repair of damage
to neurons.
The Nervous System
1. Functions of the nervous system.
2. Organization of the nervous system.
Somatic nervous system
Autonomic nervous system.
Enteric nervous system
3. Histology of the nervous system
Neurons.
Myelination.
Gray and white matter.
4. Electrical signals in neuron.
Ion channels
Resting membrane potential (RMP)
Graded potential.
Action potential.
All or None principle
5.Synaptic transmission:
Chemical synapse.
Neuro-transmitters.
EPSP & IPSP
Summation
6.Regeneration and repair of nervous tissue
7. Disorders: Multiple sclerosis (MS) & Epilepsy