The Nervous System

Function• the major controlling, regulatory, and

communicating system in the body.• the center of all mental activity including thought,

learning, and memory.• together with the endocrine system, the nervous

system is responsible for regulating and maintaining homeostasis.

• Through its receptors, the nervous system keeps us in touch with our environment, both external and internal.

• The activities of the nervous system can be grouped together as three general, overlapping functions: – Sensory – Integrative – Motor

Sensory:– millions of sensory receptors detect

changes, called stimuli, which occur inside and outside the body.

– They monitor such things as temperature, light, and sound from the external environment.

– Inside the body (the internal environment), receptors detect variations in pressure, pH, carbon dioxide concentration, and the levels of various electrolytes.

– All of this information is called sensory input.

Integrative:– Sensory input is converted into

electrical signals called nerve impulses that are transmitted to the brain.

– There the signals are brought together to create sensations, to produce thoughts, or to add to memory.

– Decisions are made each moment based on the sensory input. This is integration.

Motor:– Based on the sensory input and

integration, the nervous system responds by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions.

– muscles and glands are called effectors because they cause an effect in response to directions from the nervous system. This is the motor output or response

Structure of the Nervous System

The Nervous system has two major divisions

1. The Central Nervous System (CNS) – consist of the Brain and the Spinal Cord. – The average adult human brain weighs 1.3 to 1.4

kg .The brain contains about 100 billion nerve cells,called Neurons and trillons of "support cells" called glia.

– The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams.

2. The Peripheral Nervous System (PNS) – consists of the neurons NOT Included in the

Brain and Spinal Cord.– Is divided into two divisions:

• Somatic nerves ( voluntary)• Autonomic nerves (involuntary) 

• The somatic nerves– Controls the skeletal muscles, bones and skin– Some neurons collect information from the

Body and transmit it TOWARD the CNS.  These are called AFFERENT NEURONS.

– Other neurons transmit information AWAY from the CNS.  These are called EFFERENT NEURONS.

• The autonomic nerves– Control the internal organs of the body– Regulatory system that works with the

endocrine system to maintain homeostasis– Consist of motor neurons that operate without

conscious control.– Organized into the sympathetic nervous system

and the parasympathetic nervous system– Autonomic regulation involves constant

interplay of balance between sympathetic and parasympathetic

• The sympathetic nervous system– Prepares the body

for stress: increases heart rate, increases the release of glucose, dilates the pupils, increases blood flow to the skin, causes release of epinephrine

The parasympathetic nervous system– Restores normal

balance: decreases heart rate, stores glucose, constricts pupils, decreases blood flow to the skin.

Activity of autonomic system is regulated by neurons in brain and spinal cord – brainstem contains centers necessary for control

of heart rate, blood pressure, and body temperature

– control of brainstem regions exerted by hypothalamus

– hypothalamus, in turn, influenced by limbic system structures

– emotional responses accompanied by extensive changes in autonomic function

The Neuron

The Neuron

The Neuron• is the functional unit of the nervous system.

• Humans have about 100 billion neurons in their brain alone!

• While variable in size and shape, all neurons have three parts:– Dendrites– The cell body– The axon

Dendrites– Bring information to the cell body (incoming)– Rough Surface (dendritic spines)– Usually many dendrites per cell ( can be up to a

thousand)– No myelin sheath– Branch near the cell body

• The Cell Body (Soma)– contains the nucleus,

mitochondria and other organelles typical of eukaryotic cells.

– Is the metabolic control center of the neuron and its manufacturing and recycling center: it is in the cell body that neuronal proteins are synthesized

The Axon:– Take information away from the cell body – Smooth Surface – Generally only 1 axon per cell – Can have myelin – Branch further from the cell body

Types of Neurons

• Three types of neurons:– Sensory neurons– Interneurons– Motor neurons

Sensory Neurons– typically have a long dendrite and

short axon– carry messages from sensory

receptors to the CNS. – The cell bodies of the sensory

neurons leading to the spinal cord are located in clusters, called ganglia, next to the spinal cord.

– The axons usually terminate at interneurons.

Interneurons-Interneurons are found only in the central nervous system where they connect neuron to neuron.

-They are stimulated by signals reaching them from sensory neurons, other interneurons or both.

-are also called association neurons.

-It is estimated that the human brain contains 100 billion (1011) interneurons averaging 1000 synapses on each or some 1014 connections

Motor Neurons– Typically have a long

axon and short dendrites– Transmit messages from

the central nervous system to the muscles (or to glands).

– The axons connecting your spinal cord to your foot can be as much as 1 m long (although only a few micrometers in diameter).

The Nerve Impulse.

The Neuron at Rest

• The plasma membrane of neurons contains many active Na-K-ATPase pumps.

• These pumps shuttle Na+ out of the neuron and K+ into the neuron when ATP is hydrolyzed.

• Three Na+ are pumped out of the neuron at a time and two K+ ions are pumped in

• This creates a concentration gradient for Na+. As Na+ accumulates on the outside of the neuron, it tends to leak back in.

• Na+ must pass through proteins channels to leak back through the hydrophobic plasma membrane. These channels restrict the amount of Na+ that can leak back in.

• This maintains a strong positive charge on the outside of the neuron

• The K+ inside the neuron also tends to follow its concentration gradient and leak out of the cell.

• The protein channels allow K+ to leak out of the cell more easily.

• As a result of this movement in Na+ and K+ ions, a net positive charge builds up outside the neuron and a net negative charge builds up inside.

• This difference in charge between the outside and the inside of the neuron is called the Resting Potential.

• The resting potential in most neurons is

–70 mV.

• When the neuron is at rest, it is polarized

Initiation of the Action Potential

• A change in the environment ( pressure, heat,sound, light) is detected by the receptor and changes the shape of the channel proteins in part of the neuron –usually the dendrites.

• The Na+ channels open completely and Na+ ions flood into the neuron. The K+ channel close completely at the same time and K+ ions can no longer leak out of the neuron in that particular area.

• The interior of the neuron in that area becomes positive relative to the outside of the neuron.

• This depolarization causes the electrical potential to change from –70 mV to + 40 mV

• The Na+ channels remain open for about 0.5 milliseconds then they close as the proteins enter an inactive state.

• The total change between the resting state (-70 mV) and the peak positive voltage ( +40mV) is the action potential ( about 110 mV)

• The spike in voltage causes the K+ pumps to open completely and K+ ions rush out of the neuron. The inside becomes negative again. This is repolarization.

• So many K+ ions get out that the charge goes below the resting potential. While the neuron is in this state it cannot react to additional stimuli.

• The Refractory period lasts from 0.5 to 2 milliseconds.

• During this time, the Na-K-ATPase pump reestablishes the resting potential.

Transmission of the impulse

• The stimulus induces depolarization in a very small part of the neuron, at the dendrites.

• The sequence of depolarization and repolarization generates a small electrical current in this localized area.

• The current affects the nearby protein channels for Na+ and causes them to open.

• When the adjacent channels open, Na+ions flood into that area of the neuron and an action potential occurs. This in turn will affect the areas next to it and the impulse passes along the entire neuron.

• The electric current passes outward over the membrane in all directions BUT the area to one side is still in the refractory period and is not sensitive to the current. Therefore the impulse moves from the dendrites toward the axon.

Threshold stimulus

• Action potentials occur only when the membrane in stimulated (depolarized) enough so that sodium channels open completely.

• The minimum stimulus needed to achieve an action potential is called the threshold stimulus.

• If the membrane potential reaches the threshold potential (generally 5 - 15 mV less negative than the resting potential), the voltage-regulated sodium channels all open. Sodium ions rapidly diffuse inward, & depolarization occurs.

All-or-None Law

• Action Potentials occur maximally or not at all.• In other words, there's no such thing as a partial or

weak action potential. Either the threshold potential is reached and an action potential occurs, or it isn't reached and no action potential occurs.

• However, different neurons have different densities of Na+ channels and therefore have different APs

• The AP remains constant as it travels down the neuron. Its amplitude is always the same because it corresponds to wide open Na+ channels.

• The frequency of the AP can change.

Conduction Velocity

• impulses typically travel along neurons at a speed of anywhere from 1 to 120 meters per second

• the speed of conduction can be influenced by:– The diameter of a fiber. Velocity increases as diameter

increases.

– Temperature. As temperature increases, the velocity increases. Axons of birds and mammals can be very small because of the high body temperature.

– the presence or absence of myelin.

• Neurons with myelin (or myelinated neurons) conduct impulses much faster than those without myelin.

• Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse.

• So, in a myelinated neuron, action potentials only occur along the nodes and, therefore, impulses 'jump' over the areas of myelin - going from node to node in a process called saltatory conduction (the word saltatory means 'jumping')

Summary

• The Action Potential, or nerve impulse is an electrochemical event involving the rapid depolarization and repolarization of the nerve cell membrane.

• The axon terminals of one neuron do not touch the dendrites of other neurons. What happens when the impulse reaches the axon terminal?