A NERVE CELL?

Classification

Glial cells nonconducting cells important for structural support and metabolism of the nerve cells

Neurons Sensory neurons Interneurons Motoneurons Nerve cells that conduct nerve impulses Sensory neurons Neurons that carry impulses from sensory receptors to the CNS (aka. afferent neurons) Interneurons Link neurons within the body (primarily in CNS) Integrate and interpret information (aka. Association neurons) Motoneurons Neurons that carry impuses from the CNS to effectors (aka. efferent neurons)

Anatomy of a Motor Neuron

Dendrite Cell body (soma) Axon Projections of cytoplasm that carry impulses toward the cell body Cell body (soma) Location of the nucleus Axon Extension of cytoplasm that carries nerve impulses away from the cell body

Axons Myelin sheath Schwann cells Nodes of Ranvier Neurilemma Insulated covering over the axon of a nerve cell that increases the speed of the impulse Schwann cells Special type of glial cell that produces myelin sheath Nodes of Ranvier Regularly occurring gaps between sections of myelin sheath along the axon Action potential jumps the gap speeding up the travel of the impulse Neurilemma Surrounds the axon and promotes the regeneration of damaged axons Axons

A Neuron’s ‘Language’

An action potential (nerve impulse) is an ‘all or nothing’ response. A threshold must be reached before an action potential is sent. A weak stimulus will not trigger an impulse but a stronger stimulus will trigger the same size of impulse no matter how intense. Analogy: light switch Ions exist in the bloodstream and in and around all your cells (we will focus on potassium, sodium, and chloride) Before we begin…

Resting Potential More sodium outside the cell Overall, negative charge inside the cell (polarized membrane) = -70 mV Due to effect of: Sodium/potassium pump 3 sodium ions pumped out for every 2 potassium ions pumped in. Differences in permeability The cell is more permeable to potassium leaving the cell than to sodium entering the cell.

Depolarization Positive charge from soma reaches the axon hillock If threshold is reached, the sodium channels open and sodium goes into the cell down it’s concentration gradient and the electrical gradient. Cell becomes positive = +40 mV

Repolarization Sodium channels close. Potassium gates are opened (K+ leaves the cell) Sodium/potassium pump pumps 3 sodium ions out of the cell for every 2 potassium ions it pumps in (ATP fueled). Original polarity of the nerve membrane restored

Hyperpolarization/ Refractory period Resting potential is overshot and the neuron falls to a -90mV state. Neuron returns to resting potential Recovery time required before a neuron can produce another action potential.

Textbook Diagram Propagation Triggers Page 420 Page 421 pH Change in pressure Chemicals

Saltatory Conduction

Synaptic Transmission Synapse Regions between neurons, or between neurons and effectors 20 nm Neurotransmitters Chemicals released from vesicles into synapses Located in the end plates of axons Rely on diffusion (slow) Explains speed of reflex verses problem solving

Neurotransmitters Synaptic vesicles in the end plate of the presynaptic neuron release neurotransmitters by exocytosis. Neurotransmitters attach themselves to the postsynaptic membrane (causing either depolarization or hyperpolarization).

Summation The accumulation of messages received by a postsynaptic neuron A postsynaptic neuron may need more than one neuron worth of neurotransmitter to reach threshold and trigger an action potential. A postsynaptic neuron may receive inhibitory and excitatory messages that cancel the effect

Follow up Page 417 #5, 6 Page 426 #1, 5, 10 Handout