Chapter 2 Nerve Cells and Nerve Impulses

• Membrane : – separates the inside of the cell from the outside environment – comprised of two layers of lipids with proteins embedded

Animal Cells

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: neurons gap between • 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 sodium

Cellular mechanisms of the resting potential

• The sodium-potassium pump puts three sodium ions out of the cell while drawing in two potassium ions. – helps to maintain the electrical gradient • The electrical gradient and the concentration gradient work to pull sodium ions into the cell.

• The electrical gradient tends to pull potassium ions into the cells.

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 exits the soma).

(a swelling where the axon • 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

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.