Neurons

Systems coordinate cell activities e.g. endocrine system - specialised to secrete hormones in blood Slow, overall coordination of ativities. Nervous system - transmit electrical impulses between 2 or more cells. Fast and precise coordination.

Functions of neurons:

• 1. Generation & transmission of electrical impulses  2. Electrical impulses reach specific targets
• 3. Modifies activity of target cells and allows selective control of specific target structures

Special requirements: No possibility to store energy, so glucose/oxygen must be in constant supply. Without supply, neurons stop working within seconds and die within minutes. Neurons develop from neural stem cells.

Why are nervous systems needed? Cells on inside aren't in contact with outside world. Cells live in different environments and have become specialised. Activities must be coordinated.

Signal transmission: Based on movement of electrically charged particles. Positive ions enter, negative ions leave and membrane depolarises (less negative inside). Negative ions enter, positive ions leave and membrane hyperpolarises (more negative inside).

Sodium-potassium pump/membrane potential: Active channels work against equillibrium. Need energy to maintain resting potential. 3 Na+ in, 2 K+ out.

Electrotonic transmission: Passive - ions move inside cell along electrical gradient. Ions may get lost along the way and signal decays over time.

Action potential = Active, self-replicating. Ions move through cell membrane, Generated at axon hillock, moving down axon towards terminal buttons.

Voltage-gated membrane channels: Na+ channels open/close in response to electrical changes at membrane. 1. Membrane depolarised  2. Na+ channels open  3. Na+ ions enter cell   4. Membrane depolarises further   5. Threshold potential reached (-50mV).

Depolarisation causes...closing Na+ channels, opening K+ channels so K+ ions leave cell and membrane is repolarised. K+ channels close when resting potential is restored, Briefly, there are less K+ ions inside than outside (hyperpolarisation).

• Membrane potential below 50mV = resting potential returns
• Membrane potential depolarises further = more Na+ channels open, more depolarisation
• Membrane potential reaches threshold, all Na+ channels open simulateanously/action potential

Direction of action potential -> Doesn't travel backwards. Difficult to depolarise during hyperpolarisation. Adjacent part is easy to depolarise.

Saltatory conduction: Myelination results in faster transmission and prevents inflow/outflow of ions. Myelin interrupted by Nodes of Ranvier. Action potential jumps from node to node.

Ion gradients/protein channels: Ion concentrations differ between inside/outside. If membrane was non-permeable, electrical potential would remain static.

Protein channels: Allow ions to enter/leave. If membrane channels were passive holes, membrane would depolarise.

When threshold is reached: Positive/negative ions enter or leave cell. So many Na+ enter makes the inside more positive.

Properties of action potentials: a. Does not decay   b. All-or-nothing   c. Not produced continously  d. Fast

• Glia cells: Provide protected environment for neurons survival. Developed from neural stem cells. 10x as many glia as neurons, but 1/10th size of a neuron (same volume)
• Microglia: Small, defensive function, Produce chemicals to aid repair of damaged neurons. Digest dead neurons (phagocytosis).
• Oligodendriglia: Large, flat branches wrapping selves around axons. Fatty substances insulating myelin sheath. Soma = their body.
• Astrocytes: Star-shaped. Physical/nutritional support (blood-brain-barrier). Transports nutrients from blood vessels to neurons, holds them in plae, takes part in neural signalling.