• In lecture today:• Structure and function of neurons and other cells in the nervous

system.

• Blood-Brain Barrier.

• Properties of Action Potentials

• Conduction of nerve impulses

• Synaptic transmission & neurotransmitters

CHAPTER 7

Nervous System

• The nervous system is divided into:

1- Central Nervous System (CNS)– Brain and Spinal cord

2- Peripheral Nervous System (PNS)– Cranial and Spinal nerves, ganglia

Neurons and Glial Cells

• There are two types of cells in the nervous system:

1- Neurons: are the functional units of the nervous system.

2- Supporting cells: help functioning of the neurons.

Neuron

• In general the structure of neuron is divided into:

1- cell body - contains nucleus and is the nutritional center of neuron.groups of cell bodies in the CNS = neucleigroups of cell bodies in the PNS = ganglia

2- dendrites- transmit electrical signals to the cell body.3- axon- longer than dendrite and transmits signals away from cell

body.

Specialized Transports in Neurons

Substances synthesized in the cell body are transported through the axon by twomechanisms:

1- axoplasmic flow- wave like contraction that pushes cytoplasm away from the cell body.

2- axonal transport- more rapid and can occur in both directions.

Neurons

Classification of Neurons

Neurons maybe classified according to their function:

1- Sensory or afferent neurons

2- Motor or efferent neurons

3- Interneurons or association neurons

• OR according to their structure:

1- Pseudounipolar-

2- Bipolar-

3- Multipolar-

There are 6 types of supporting cells in the nervous system:

1- Schwann cells- form myelin sheath in PNS

2- Oligodendrocytes- form myelin sheath in CNS

3- Microglia- phagocytic cells in CNS

4- Astrocytes- regulate ECF of CNS

5- Ependymal cells- separate CNS from CSF

6- Satellite cells- support cell bodies in PNS

Supporting Cells

Cells in Nervous System

• Some axons in the PNS are surrounded by a myelin sheath formed by Schwann cells, this type of axons are called myelinated.

• The insulation in the myelinated axons is by successive wrapping of the cell membrane of Schwann cells around the axon.

• Unmyelinated is the type of axons that do not have myelin insulation.

• Each Schwann cell wraps only a mm of length of axon leaving gaps (node of Ranviour) of exposed axon between adjacent Schwann cells.

Schwann Cells

Myelination

Myelination in the CNS• The process of forming a myelin sheath around axons in the CNS is carried

out by oligodendrocytesoligodendrocytes. These cells form myelin sheath around several axons in the CNS.

Astrocytes• The most abundant type of supporting cells in the CNS are astrocytes.

• Astrocytes support the function of neurons in by:

1- Regulating extracellular K+ concentration around neurons.

2- Regulating extracellular neurotransmitter concentration.

Glutamic acid -aminobutyric acid (GABA)

• Astrocytes also interact with brain capillaries. It is thought that they help in the formation of blood-brain barrier.

Blood-Brain Barrier Brain capillaries have tight junctions between their endothelial cells separating blood from brain tissue forming the blood-brain barrier (BBB).

BLOOD

Capillary endothelial Cells at the cerebral capillaries

Brain TissueTight junctions

Non-cerebral capillary endothelial cells

Other TissuesGaps

BLOOD

Action potential in Neurons

Neurons communicate by producing action potential and propagatingit along their axon to other neurons or cells.

Conduction of Action Potentials

- Electrical signals in the nerve axon are generated and transmitted in the form of action potential.

- A nerve signal maybe initiated by a stimulus which can be:

chemical, electrical, physical, or change in temperature.

Characteristics of Nerve Action Potential and its Conduction

1. Threshold: Is the minimum stimulus required for generation of action potential.

2. All-or None Law: The amplitude of the action potential is constant regardless of the strength of stimulus.

Q- How the nerve can transmit different strengths of stimuli?

Characteristics of Nerve Action Potential and its Conduction

3. Refractory Period: During the time of action potential area of axon which the action potential is occurring can not produce another action potential.

Characteristics of Nerve Action Potential and its Conduction

Types of Nerve Fibers1- Unmyelinated Nerve Fibers:-most of the axon is exposed and without myelin covering. Transmits signals slower than myelinated fibers, e.g. used in slow response such as control of blood vessels.

2- Myelinated Nerve Fibers (saltatory conduction):

-Their axon is covered with myelin sheath, it transmits nerve signals very fast the larger the diameter of axon and myelin sheath around it the faster nerve conduction, e.g. fast sensorySignals from skin to brain or spinal cord.

Synapse

Synapse is the functional connection between a neuron and a second cell.

- Synapse can occur between:

- Neurons (neuron-neuron)- A neuron and an effector cell in muscle/gland

-Two types of synapses:

- Electrical synapse- Chemical synapse

Electrical Synapse

Gap Junction

Chemical Synapse

Neuromuscular Junction

Neurotransmitters

Chemical synapses transfer information from neuron to the next cell across the synapse by chemical compounds called neurotransmitters.

There are many chemicals which act as neurotransmitters in the nervous system, here is a list of some of the most abundant ones:

1- Amines, e.g. Acetylcholine (Ach)

2- Cathecolamines, e.g Norepinephrine and Epinephrine

3- Amino acids, e.g Glutamic acid, and GABA

4- Polypeptides, e.g Glucagone, insulin

5- Nitric oxide