The Nervous System at Work

1. The Nervous System at Work Lecture 2

2. The Nerve Impulse At Rest • The cell membrane of an non-stimulated neuron carries an electric charge. • This charge is maintained by ions concentrated on either side of the membrane. • The inside of the membrane is negative while the outside is positive. • In this state, the membrane is polarized. • A nerve impulse starts with a local reversal in charge, which then spreads along the membrane in one direction. • This sudden electrical change is called an action potential.

3. Nerve Impulse • When the reversal occurs, the membrane depolarizes and within a thousandth of a second returns to its resting state, repolarizing. • Depolarization and repolarization are brought about by rapid shifts in sodium and potassium ions across the cell membrane. • A stimulus is any force that can start an action potential. • In an unmyelinated fiber, the action potential spreads continuously along the membrane of the cells. • Whe myelin is present, it insulates the fiber against the spread of the current causing the action potential to jump from node to node along the sheath. This type of conduction is actually faster than continuous conduction.

4. The Synapse • A synapse is a point of junction between two neurons or between a neuron and an effector. • Information must be passed across the synaptic cleft to the next cell. Within the branched endings of the axon are small vesicles containing neurotransmitters. • Once an action potential reaches the end of the axon, the axon releases the neurotransmitter into the synaptic cleft. The neurotransmitter then acts as the chemical signal to stimulate the next cell. • The cell that releases the neurotransmitter is the presynaptic cell and the cell that is stimulated by the neurotransmitter is the postsynaptic cell. • Receptors on the next cell influence how or if that cell will respond to a given neurotransmitter.

5. Neurotransmitters • The main neurotransmitters are epinephrine (adrenaline), norepinephrine (noradrenaline), and acetylcholine. • All three function in the Autonomic Nervous System, however, acetylcholine is the main neurotransmitter for stimulating skeletal muscles. • Neurotransmitters may follow several paths for removal: • They may slowly diffuse away from the synapse. • They may be destroyed rapidly by enzymes in the synaptic cleft. • They may be taken back into the presynaptic cell to be used again.

6. The Reflex Arc • A complete pathway through the Nervous System from stimulus to response is termed a reflex arc. • The basic parts are • Receptor – the end of a dendrite or some specialized receptor that detects a stimulus • Sensory neuron – cell that transmits impulses toward the CNS • CNS – impulses are coordinated and a response is organized. One or more interneurons may carry impulses to and from the brain, may function within the brain, or may distribute impulses to different regions of the spinal cord. • Motor neuron – cell that carries impulse away from the CNS • Effector – muscle of gland outside the CNS that carries out a response