1

Nerve Cells

2

Nerve cells

• Around 100 billion neurons in the brain initially– Adult stage 15 billion

• Means of communication in the nervous system

• Excitatory and inhibitory in nature

3

Nerve cells

• Neurons– Sensorimotor activities– Higher mental functions

• Neuroglia cells– Support and protect neurons

4

Neurons

• Three parts1. Soma: body with nucleus

2. Dendrite: receives inputs

3. Axon: sends information– Signal is transmitted

electrochemically down shaft of axon.– Connects to target cells (neurons,

muscles, glands) at synapse.– Can either inhibit or excite target

cells.– Can be up to x10,000 length of soma

5

Soma

• Protoplasm– Nucleus– Cytoplasm

• Protein filled liquid• Microscopic Organelles

– neurofibril– mitochondria – powerhouse of the cell– ribosomes– lysosomes– Nissl bodies– Golgi com

6

Metabolic activity

• Firing requires oxygen and glucose– Constant supply is required (via capillaries of

vascular system).– If supply is disrupted:

• Meaningful neural activity will cease immediately• Neurons will die within hours if supply is not restored

7

Intracellular transport• Proteins generated and degraded in the

soma. – Proteins must by moved to axons/synapse

• Proteins transported via Microtubules in Axon– Anterograde Transport – toward synapse (new

proteins, transported by protein kinesin)– Retrograde Transport – toward cell body

(recycling, transported by protein dynein)– Suicide Transport (apoptosis, form of retrograde

transport)– Microtubules act as tracks for moving proteins

8

Nucleus

• Control Center

• Contains DNA

• Contains Nucleolus– Engaged in Protein Synthesis– Contains RNA - Code for Enzyme for Synthesis of

Specific Enzymes Needed

9

Nuclear structures and functions• Neurofibrils: Channels for Communication

in Cell. These filaments repulse each other, so their development enlarges the diameter of the axon and dendrite

• Mitochondria: Contain Enzymes for Metabolism: creates ATP, used for chemical energy.

• Free Ribosomes: Synthesize Proteins• Lysosomes: Intracellular Digestion (break down waste)• Nissl Bodies: Amino Acids to Proteins• Golgi Complex: Protein Secretion and its Transportation

(post office of the cell)

10

Neural communication• Dendrites:

– Afferent: influenced by other neurons/receptors.

– Branching and short

• Axons– Efferent: influence other neurons/receptors.– Dependent on electrical signal at axon hillock– End in axon terminals with synaptic knobs

that release neurotransmitters.

11

Myelin Sheath

• Myelin Sheath– Multilayer Lipid (fatty) Material – Insulates Nerve Fiber

• Analogy: plastic insulation on wire in house

– Regulation• Speed: Signals can jump between nodes• Efficiency: Escape of Electrical Energy

– Segmented: nodes of Ranvier with internodes

12

Saltatory conduction• Normally, chemical signal is

relatively slow down an axon.

• However, in mylinated segments the signal can jump between nodes.– Facilitates Speed of Fiber (120

msec)

13

Glial cells for myelin

• In CNS:– Oligodendroglia: single cell can support Myelin sheaths for

up to 50 axons– Multiple Sclerosis

• In PNS:– Schwann cells: each covers only about 1mm, so many cells

per axon. – Can Regenerate

• Injury to Motor Pathways• Amputation

14

Synapse• Connection Between Neurons• Three Parts

– Knob (Axon; presynaptic) • Contain Vesicles• Filled with Neurotransmitters

Released when Necessary• Chemically Stimulate Receiving

Nerve Cell Body

– Synaptic Cleft: gap between one neuron and the next.

• 2/100,000mm Wide

– Receptive Sites (Receiving Cell Axon or Dendrite; postsynaptic)

15

Action Potentials• When a neuron fires, it releases

neurotransmitters from terminals into synaptic cleft.

• Chemical in Neurotransmitter Stimulates or inhibits Postsynaptic Cell (Dendrite, Axon or Cell Body)

• If the postsynaptic cell receives enough stimulation, this neuron will fire.– Firing is an all-or-none response– After firing, neuron must wait a while

before it can fire again.

16

Types of neuron• Classified By Number of Processes Multipolar – many

dendrites, one axon (brain)– Bipolar – retina, smell, facial nerve– Unipolar – dorsal spine

17

Axonal Length Types

• Golgi Type I– Axons range from feet to inches.– E.G. Motor cortex

• Golgi Type II– Axons are very short– E.G. interneurons