Human Anatomy / Physiology Nervous System

The human nervous system Composed of cells called neurons: Can carry rapid electrical impulses Two main parts: Central nervous system: brain, spinal cord Peripheral nervous system: somatic (sensory and motor neurons) and autonomic neurons Main functions: CNS: integrate information and determine reaction, thought, memory, learning, instinct, balance, emotion, etc. PNS: transfer information from body to CNS, and “decisions” to muscles, glands, etc.

Central Peripheral

Neurons Specialized cells that carry electrical impulses node of Ranvier Motor Neuron

Structure of a Motor Neuron Dendrites receive information Cell body / nucleus controls metabolism, functions Axon sends signal Myelin insulates neuron Nodes of Ranvier carry electrical signal (saltatory conduction) Motor end plate passes message to muscle cell (motor end plate) Node of Ranvier

Types of Neurons Sensory Neurons: transmit electrical impulses from sensory receptors to the CNS Relay Neurons (Interneurons): move impulses inside the CNS Motor Neurons: take impulses from CNS to effectors (glands/muscles) Sensory Neuron Motor Neuron

How we perceive information A simple reflex Stimulus receptor sensory neurons relay neurons (In the CNS) motor neurons Effectors response 3 2 4 1 5 7 6

Reflex

Resting potential The electrical potential across the membrane of a neuron that is NOT conducting an impulse The neuron is “waiting” The cell is negative inside -70 millivolts (mV) Plus Cl- and other negative ions 

Resting Potential and Active Transport Sodium-potassium (Na+/K+) pump maintains the electrical potential requires constant ATP active transport, against concentration gradient ~20% of ATP is used to maintain readiness of neurons (resting potential) SODIUM OUT! POTASSIUM IN!

How the impulse is transmitted Impulse begins when a neuron is stimulated by another neuron or by the environment Electrical impulse moves in one direction: Dendrites → Cell Body → Axon Synapse: gap between 2 neurons Neurotransmitters send the signal to the following neuron No myelin = 5-25m/s With myelin = 10-120m/s

Action Potential The reversal (depolarization) and restoration (repolarization) of the electrical potential across the membrane of a nerve cell as an electrical impulse passes along it Shows the electrical potential at one point of the axon over time.

Passage of a nerve impulse: RESTING First the cell is at resting potential The Na+/K+ pump is maintaining -70mV

Passage of a nerve impluse: DEPOLARIZATION The cell becomes less negative Diffusion of Na+ ions from the preceding portion of the cell or signal from cell body Na+ channels open ONLY when -70 mV rises to -55mV (threshold) As Na+ enters, the potential reaches +30mV

Passage of a nerve impulse: REPOLARIZATION K+ channels open AFTER Na+ channels. Diffusion of K+ out of the cell makes the potential negative again (-70mV) and Na+ channels close For a short time the cells goes beyond -70mV to about -80mV (hyperpolarization) During this time, the Na+ gates will not open (refractory period). This prevents action potential from traveling backwards.

Passage of a nerve impulse: Back to RESTING The Na+ that entered will diffuse and open the next Na+ channels, and the signal continues The resting potential is restored by Na+/K+ pump and K+ leakage

Passage of a nerve impulse: summary

Practice: Action Potential Harvard Outreach Animation Many animations available for this unit

Myelinated v. Nonmyelinated

Synaptic transmission Along the axon, the signal is electrical To pass from one neuron to another, it must cross a small space at a neural junction (the synapse); this signal is chemical

The Synapse Synapse = gap between neurons Action potential cannot cross gap: neurotransmitters carry the impulse Neurotransmitters: stored at the end on axons (glutamate, GABA, acetylcholine, norepinephrine, dopamine, serotonin, nitric oxide, etc) Voltage Ca+2 gated ions open → calcium flows inside neuron Calcium help vesicles fuse with membrane → neurotransmitters are released These bind with neuroreceptors Voltage gated ions are activated = depolarization Impulse is passed on to post-synaptic neuron Neurotransmitters = broken by enzymes and reabsorbed by pre-synaptic neuron

Synaptic transmission

Synaptic transmission (common example) Action potential opens Ca2+ gates Ca2+ cause vesicles filled with neurotransmitter (ex. Acetylcholine, dopamine, seratonin) to release them through exocytosis The neurotransmitters diffuse across the synapse and bind to receptors. Na+ gates open in the post-synaptic neuronand start a new action potential The neurotransmitter is broken down (ex. By cholinesterase) and recycled and Ca2+ is removed This would excite the post-synaptic neuron, but other systems can inhibit / hyperpolarize it.

Visual summary of synaptic transmission