Nerve physiology Physiology of Nerves n There are

Nerve physiology

Physiology of Nerves n There are two major regulatory systems in the body, the nervous system and the endocrine system. n The endocrine system regulates relatively slow, long-lived responses n The nervous system regulates fast, short-term responses

Neuron structure n Neurons all have same basic structure, a cell body with a number of dendrites and one long axon.

Types of neurons

Divisions of the nervous system

Non-excitable cells of the nervous system

Structure of gray matter

Signal transmission in neurons

Resting potential

Ionic basis of Em Na. K-ATPase pumps 3 Na+ out for 2 K+ pumped in. n Some of the K+ leaks back out, making the interior of the cell negative n

Electrochemical Gradients Figure 12. 12

Ion channels n n Remember Ohm’s Law: I=E/R When a channel opens, it has a fixed resistance. Thus, each channel has a fixed current. Using the patchclamp technique, we can measure the current through individual channels

Gated channels: ligand-gated

Gated channels: voltage-gated

Gated channels: mechanically-gated

Graded potential n A change in potential that decreases with distance n Localized depolarization or hyperpolarization

Graded Potentials

Graded Potentials

Action Potential Appears when region of excitable membrane depolarizes to threshold n Steps involved n Membrane depolarization and sodium channel activation n Sodium channel inactivation n Potassium channel activation n Return to normal permeability n

The Generation of an Action Potential Figure 2. 16. 1

Graded potentials vs Action Potential

Characteristics of action potentials Generation of action potential follows all-or -none principle n Refractory period lasts from time action potential begins until normal resting potential returns n Continuous propagation n spread of action potential across entire membrane in series of small steps salutatory propagation

The Generation of an Action Potential

Induction of an action potential I

Induction of an action potential II

Voltage-gated Na+ channels n n These channels have two voltage sensitive gates. At resting Em, one gate is closed and the other is open. When the membrane becomes depolarized enough, the second gate will open. After a short time, the second gate will then shut.

Voltage-gated K+ channels n n n Voltage-gated K+ channels have only one gate. This gate is also activated by depolarization. However, this gate is much slower to respond to the depolarization.

Cycling of V-G channels

Action potential propagation n n When the V-G Na+ channels open, they cause a depolarization of the neighboring membrane. This causes the Na+ and K+ channels in that piece of membrane to be activated

AP propagation cont. n n n The V_G chanels in the neighboring membrane then open, causing that membrane to depolarize. That depolarizes the next piece of membrane, etc. It takes a while for the Na+ channels to return to their voltagesensitive state. Until then, they won’t respond to a second depolarization.

Propagation of an Action Potential along an Unmyelinated Axon

Saltatory Propagation along a Myelinated Axon

Saltatory Propagation along a Myelinated Axon

Schwann cells cont. n In unmyelinated nerves, each Schwann cell can associate with several axons. n These axons become embedded in the Schwann cell, which provides structural support and nutrients.

Synaptic transmission

g Aminobutyric Acid n Also know as GABA n Two know receptors for GABA n Both initiate hyperpolarization in the post-synaptic membrane n GABAA receptor allows an influx of Cl- ions n GABAB receptors allow an efflux of K+ ions

Transmitter effects on Em n Most chemical stimuli result in an influx of cations n n This causes a depolarization of the membrane potential At least one transmitter opens an anion influx n This results in a hyperpolarization.

EPSPs and IPSPs If the transmitter opens a cation influx, the resulting depolarization is called an Excitatory Post Synaptic Potential (EPSP). n These individual potentials are sub-threshold. n If the transmitter opens an anion influx, the resulting hyperpolarization is called an Inhibitory Post Synaptic Potential (IPSP n All these potentials are additive. n

Post-synaptic integration

Signal integration

Signal integration cont.

Presynaptic inhibition

Presynaptic facillitation

Neural circuits I

Neural circuits II

Myelination I n In the central nervous system, myelin is formed by the oligodendrocytes. n One oligodendrocyte can contribute to the myelin sheath of several axons.

Myelination II n In the peripheral nervous system, myelin is formed by Schwann cells. n Each Schwann cell associates with only one axon, when forming a myelinated internode.

White and gray matter in the nervous system

Structure of the spinal cord I n The CNS is made up not only of the brain, but also the spinal cord. n The spinal cord is a thick, hollow tube of nerves that runs down the back, through the spine.

Structure of the spinal cord II

Structure of the spinal cord III

Structure of the spinal cord IV