Chapter 2Nerve Cells and Nerve

Impulses

Animal Cells• Membrane:

– separates the inside of the cell from the outside environment

– comprised of two layers of lipids with proteins embedded

Animal Cells• Nucleus refers to the

structure that contains the chromosomes

• Mitochondria perform metabolic activities and provide energy that the cell requires.

• Ribosomes: Sites at which the cell synthesizes new protein molecules

• Endoplasmic reticulum: Transports newly synthesized proteins

The Human Nervous System—2 Kinds of Cells

• Neurons

– Approx. 100 billion in brain

– Receive and transmit info

– Behavior depends upon their communication

• Glia

– 10X the number of neurons

– Support neural communication

Neuroanatomy Handout #1: The Motor Neuron• A motor neuron

– has its soma in the spinal cord– receives excitation from other neurons– conducts impulses along its axon to a muscle

or gland– is the largest of the nerve cells

Neuroanatomy Handout #1: The Motor Neuron• Neurons are similar to other cells of the body• All neurons have a cell body (soma, A):

– responsible for the metabolic work of the neuron

– surrounded by cell membrane (A1)– Containing a nucleus (A2), mitochondria (A3),

ribosomes (A4), endoplasmic reticulum (A5)

Neuroanatomy Handout #1: The Motor Neuron

• Neurons are different from other cells of the body because they have distinctive shape and function

Neuroanatomy Handout #1: The Motor Neuron

• The 4 major components of a motor neuron:– Soma/Cell body– Dendrites– Axon– Presynaptic terminals

Neuroanatomy Handout #1: The Motor Neuron

• Dendrites (B)- branching fibers responsible for receiving information from other neurons

• Dendritic spines (B1) further branch out and increase the surface area of the dendrite

Neuroanatomy Handout #1: The Motor Neuron

• Axon (C) - thin fiber responsible for sending impulses to other neurons, glands, or muscles

• Some neurons are covered with an insulating material called the myelin sheath (D) with interruptions in the sheath known as nodes of Ranvier (C2).

• Axon hillock (C1) – bulge in the cell body where axon begins

Neuroanatomy Handout #1: The Motor Neuron

• Presynaptic terminals (E) refer to the end points of an axon responsible for releasing chemicals (neurotransmitters) to communicate with other neurons

Neuroanatomy Handout #1: The Motor Neuron

• Axons from other neurons (F) converge on receiving neuron

• Synapse: gap between neurons

• Postsynaptic neuron (G) and dendrite (G1)

Sensory and Motor Neurons• A motor neuron receives

excitation from other neurons and conducts impulses along its axon to a muscle or gland– It carries information from the

brain to the perimeter of the body

Sensory and Motor Neurons

• A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation (touch, temperature, odor etc.)– It carries information from the

perimeter of the body to the brain

Other Cells of the Nervous System

• Terms used to describe the neuron include the following:

– Afferent axon - refers to bringing information into a structure.

– Efferent axon - refers to carrying information away from a structure.

– Interneurons or Intrinsic neurons are those whose dendrites and axons are completely contained within a structure.

Other Cells of the Nervous System• Glia are the other major

component of the nervous system and include the following:

– Astrocytes help synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon.

– Microglia - remove waste material and other microorganisms that could prove harmful to the neuron.

The Cells of the Nervous System

– Oligodendrocytes & Schwann cells- build the myelin sheath that surrounds the axon of some neurons.

– Radial glia- guide the migration of neurons and the growth of their axons and dendrites during embryonic development.

The Cells of the Nervous System

• Spaniard Santiago Ramon y Cajal (1852-1934) was the first to demonstrate that neurons do not touch one another.

• With this understanding came new ideas about how neurons communicate.

The Nerve Impulse• A nerve impulse is the electrical message that

is transmitted down the axon of a neuron.• The impulse is regenerated at points along

the axon.• The speed of nerve impulses ranges from

approximately 1 m/s to 100 m/s.

The Nerve Impulse

• The resting potential: state of the neuron prior to the sending of a nerve impulse

• Electrical gradient: a difference in the electrical charge inside and outside of the cell– At rest, the membrane is slightly negative with

respect to the outside (approximately -70 millivolts)

Competing forces maintain a -70mV resting potential

1. Concentration gradient: The difference in the distribution of ions between the inside and the outside of the membrane

– Sodium (Na+) more abundant outside cell than inside (10:1)

– Potassium (K+) more abundant inside cell than outside (20:1)

2. Negatively charged proteins inside cell

Cellular mechanisms of the resting potential

• Selective permeability of the membrane allows some molecules (e.g. water, oxygen) to pass more freely than others.

• Charged ions, like sodium (Na+), potassium (K+), calcium (Ca++) and chloride (Cl-) pass through channels in the membrane.

• When the membrane is at rest:– Na+ channels are closed– K+ channels are partially

closed allowing the slow passage of potassium

Cellular mechanisms of the resting potential

• The sodium-potassium pump puts 3 Na+ ions out of the cell while drawing in 2 K+ ions. – helps restore and

maintain resting potential• Electrical and concentration

gradients attract sodium ions into the cell.

• Electrical gradient pulls potassium ions into the cell

Animation of Sodium Potassium Pump

The resting potential allows a neuron to respond quickly to a stimulus

The Action Potential

• The resting potential (-70mV) remains stable until the neuron is stimulated.

• Electrical polarization: the difference in the electrical charge between two places

Two things can happen to a resting neuron:

• Hyperpolarization: increasing the difference (polarization) between the electrical charge of two places (less likely to fire)

• Depolarization refers to decreasing the polarization towards zero (more likely to fire)

• The threshold of excitement refers any stimulation beyond a certain level that results in a massive depolarization (action potential).

The Action Potential

• An action potential is a rapid depolarization of the neuron.

• Stimulation of the neuron past the threshold of excitation triggers a nerve impulse, action potential, or “firing”

• -70mV can become +50mV

The Nerve Impulse• Voltage-activated channels

are membrane channels whose permeability depends upon the voltage difference across the membrane.– Sodium channels are

voltage activated channels.• When sodium channels are

opened, positively charged sodium ions rush in and a subsequent nerve impulse occurs.

The Nerve Impulse

• Scorpion venom attacks the

nervous system by keeping

sodium channels open and

closing potassium channels• Local anesthetic drugs block sodium channels

and therefore prevent action potentials from occurring.– Example: Novocain

• General anesthetics open potassium channels wider than usual

The Nerve Impulse

• The all-or-none law states that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it.– Action potentials are

equal in intensity and speed within a given neuron.

The Nerve Impulse

• A refractory period happens after an action potential occurs, during which time the neuron resists another action potential.

• The absolute refractory period: the first part, when membrane cannot produce an action potential

• The relative refractory period: the second part, when it takes a stronger than usual stimulus to trigger an action potential.

The Nerve Impulse• In a motor neuron, the action potential begins

at the axon hillock (a swelling where the axon exits the soma).

• Propagation of the action potential is the term used to describe the transmission of the action potential down the axon.

The Nerve Impulse• The myelin sheath of axons are interrupted by short

unmyelinated sections called nodes of Ranvier.• At each node of Ranvier, the action potential is

regenerated by a chain of positively charged ions pushed along by the previous segment.

The Nerve Impulse• Saltatory conduction: the “jumping” of the action potential

from node to node.– Provides rapid conduction of impulses– Conserves energy for the cell

• Multiple sclerosis: disease in which myelin sheath is destroyed; associated with poor muscle coordination

• Animation of propagation of the action potential along unmyelinated and myelinated axons

The Nerve Impulse

• Not all neurons have lengthy axons.• Local neurons have short axons, exchange

information with only close neighbors, and do not produce action potentials.

• When stimulated, local neurons produce graded potentials which are membrane potentials that vary in magnitude and do not follow the all-or-none law,.

• A local neuron depolarizes or hyperpolarizes in proportion to the stimulation.