Transcript nervous system

Nervous System
Nervous System
1) Central Nervous System.
2) Peripheral Nervous System.
A) Afferent Division.
B) Efferent Division
(Somatic N.S. and Autonomic N.S.)
3) Enteric Nervous System.
Central N.S
• central N.S. consists of:
1) Brain.
2) spinal cord.
• gets information about:
1) external environment:(sensory information).
2) internal information:(visceral information).
Peripheral N.S.
• Consists of:
A) Afferent Neurons.
B) Efferent Neurons.
Afferent Neurons
• transmit sensory and visceral information
from organs to (CNS).
• sensory information:
a) somatic senses: 1- skin, 2- muscles, 3- joints
b) special senses: 1- vision, 2- hearing, 3equilibrium, 4- smell, 5- taste.
• visceral information (visceral senses):
a) fullness of stomach.
b) blood pressure.
c) blood PH.
Efferent Neurons
• transmit information from C.N.S. to organs in
the periphery (called effector organs), like
muscles and glands.
• Efferent Neurons constitute:
1) Somatic N.S.
2) Autonomic N.S.
Somatic N.S
• Somatic N.S. for motor neurons to skeletal
muscle (under voluntary control).
Autonomic N.S.
• Autonomic N.S. for internal organs, cardiac
muscles, smooth muscles, sweat glands, blood
vessels,… (not under voluntary control)
• Autonomic N.S. consists of:
a) sympathetic N.S.
b) parasympathetic N.S.
(they have opposite effects on organs).
Enteric N.S.
• consists of intricate network of neurons in the
gastrointestinal tract,
• can function independently of the rest of N.S.
• (but communicates with autonomic N.S.)
Cells of N.S
1) Neurons: the functional unit.
• excitable (produce action potentials).
2) Glial cells: constitute 90% of cells in N.S.
• provide structural and metabolic support to
neurons.
Neurons
• Consist of:
1) cell body.
2) dendrites.
3) axon.
• Mature neurons lose the ability to undergo cell
division.
• Recent studies suggest that in the few areas of
the adult human brain, new neurons can develop
from undifferentiated cells.
• dendrites: receive information.
• axon: sends information(branches of axon is
called collaterals).
• axon hillock: the site where the axon
originates from cell body, is specialized for the
initiation of action potentials.
• axon terminal: is specialized to release neural
transmitter on arrival of action potential.
Structural classification of neurons
1) Bipolar :two projections – sense of smell
and sense of vision.
2) Multipolar (most common).
3) Pseudo-unipolar (most sensory neurons):
dendrite is modified to be peripheral axon
(because it originates in the periphery and
functions like an axon).
Functional classification of neurons
1) Afferent neurons.
2) Efferent neurons.
3) Interneurons.
1) Afferent Neurons
• Transmit sensory and visceral information
from periphery to C.N.S.
• The cell body is outside the C.N.S. and located
in a ganglion.
( A ganglion is a cluster of neural bodies
outside C.N.S.)
2) Efferent Neurons
• Transmit information from central nervous
system to effector organs ( muscles, glands).
• cell body and dendrites are located in the
C.N.S. but axons travel to effector organs they
innervate (to be part of peripheral N.S.)
3) Interneurons
• Account for 99% of all neurons in the body.
• They locate entirely in C.N.S., carry out
complex functions of the brain such as
thought, memory and emotions.
Glial Cells
• 5 types of glial cells:
1) Astrocytes.
2) Ependymal cells.
3) Microglia.
4) Oligodendrocytes.
5) Schwan cells.
Oligodendrocytes & Schwan cells
• They form wrap of myelin around axons for
insulation, thus axon can transmit action
potential efficiently and rapidly.
• Nodes of Ranvier are gaps in the myelin.
• One oligodendrocyte can form myelin for
many axons and that is in the C.N.S.
• One Schwan cell forms myelin only for one
axon and that is in peripheral N.S.
Resting membrane potential
• A cell at rest has a potential difference across
its membrane.
• (The inside is negatively charged relative to
the outside).
• Resting Vm = -70 mV
(All cells have resting Vm ranging (-5 mV) ~(-100mV)
• Resting Vm depends on:
1) Concentration gradients of ions (Na+, K+).
2) Presence of ion channels in the plasma
membrane.
Localization of Ion Channels
in Neurons
A) Leak channels (nongated channels):
They are located throughout the neuron.
B) ligand – gated channels:
• They open or close in response to the binding of
a chemical messenger to a specific receptor in the
plasma membrane.
• They are located densely in the dendrites and
cell body.
C) Voltage-gated channels
They open or close in response to the charges in
membrane potential.
Voltage-gated K+ channels and Voltage-gated
Na+ channels are located densely in axon
(especially hillock).
Voltage-gated Ca+ channel is located densely in
the axon terminal.
Understanding Resting Vm
1) Membrane is permeable only to K+.
2) Membrane is permeable only to Na+.
3) Membrane is permeable to K+ and Na+.
Only permeable to K+
• K+ ions are inside the cell.
• The chemical driving force is from the inside
to the outside.
• Electrical force increases from the outside to
the inside.
• K+ equilibrium potential is -94 mV.
Only permeable to Na+
• Na+ ions are outside.
• The chemical driving force is from the outside
to the inside .
• Electrical force increase from the inside to the
outside.
• Na+ equilibrium potential is +60 mV.
Permeability to both K+ & Na+
• Membrane is more permeable to K+ because
of number of open K+ channels.
• The outflow of K+ slows down.
• The inflow of Na+ speeds up.
• The final Resting Vm is -70 mV and much
closer to the K+ equilibrium potential.
• We say the cell is not at equilibrium but in a
steady state.
Na+/K+ Pumps
• They transport Na+ outside the cell and K+
inside the cell.( 3 Na+ ions outside and 2 K+
ions inside need one ATP)
• They establish the concentration gradients.
• They also maintain them.