The Nervous System

Human Anatomy and Physiology

McElwee

The Nervous System

• The master controlling and communicating system of the body.

• 3 Overlapping Functions a. Sensory input—monitoring stimuli (collecting

information from sensory receptors that monitor the body’s internal and external conditions).

b. Integration—interpretation of sensory input (decides what should be done at each moment)

c. Motor output—response to stimuli (activates muscles or glands; stimulate efferent neurons)

The Nervous System’s Functions

Organization (Central vs. Peripheral)

• Central Nervous System (CNS) – Brain and Spinal Cord – Integration and Command center

• Peripheral Nervous System (PNS) – Paired spinal and cranial nerves – Carries messages to and from the spinal cord and

brain – There are 12 pairs of cranial nerves emerging from

brain. – There are 31 pairs of spinal nerves emerging from

spinal cord.

CNS vs. PNS

Functional Organization • Functional classification groups neutrons according to the

direction the nerve impulse is traveling relative to the CNS. • Sensory (afferent) division

– Sensory neurons detect information about stimuli, such as light. – Sensory afferent fibers carry impulses from skin, skeletal

muscles, and joints to the brain. Receives initial stimulus from a sense organ.

– Afferent literally means “to go toward”.

• Motor (efferent) division – Relays new signals to effectors—muscles and glands—that carry

out responses. – Ex: Stimulate digestive gland to release an enzyme or stimulate

a muscle to contract.

• Interneurons or association neuron – resides within the spinal cord and brain – Accepts sensory input, processes it, and signals other neurons.

The motor division (efferent) has 2 subdivisions:

• The somatic nervous system allows us to voluntarily (consciously) control our skeletal muscles. Aka voluntary nervous system.

• The autonomic nervous system (ANS) regulates events that are automatic or involuntary. Ex: smooth and cardiac muscle and glands.. – Sympathetic

– Parasympathetic

There are 2 divisions of the autonomic nervous system (involuntary):

sympathetic vs. parasympathetic

• Sympathetic—the body’s “fight or flight” response to stress, danger, excitement, exercise, emotions, and embarrassment. – Increases breathing and heart rate – releases adrenaline and other stress hormones – decreases digestion to cope with these situations.

• Parasympathetic—the body’s “rest and digest” response when the body is relaxed, resting, or feeding. It works to undo the work of the sympathetic division after a stressful situation. – Decrease respiration and heart rate – Increase digestion – Permit the elimination of waste – Returns body to homeostasis

Concept Check

• What is the difference between peripheral and central nervous systems?

• Explain the difference between somatic and autonomic nervous systems.

• What are the 2 divisions of PNS?

Histology of Nerve Tissue

• The two principal cell types of the nervous system are:

– Neurons—excitable cells that transmit electrical signals.

– Supporting cells– cells that surround and wrap neurons. These are called neuroglia (nerve glue) or glial cells. They include many types of cells that generally support, insulate, and protect the delicate neurons.

I. CNS-- Glial Cells A. Astrocytes—largest, most abundant, star-

shaped. Only found in CNS, provides nutrition to neurons; maintains blood-brain barrier, repairs damaged neural tissue.

• B. Microglial—small, ovoid cells with spiny processes; engulf bacteria and cell debris (phagocytosis)

• C. Ependymal cells—range in shape from squamous to columnar.

– Epithelial cells that line the central cavities of the brain and spinal column.

– Some have cilia.

– Function: Assist in secretion

and circulation of CSF

(cerebral spinal fluid).

• D. Oligodendrocytes—form myelin (neurilemma) on axons in CNS.

– Few branches; holds nerves together

– Structural Framework

II. PNS Glial Cells • Schwann cells (neurolemmocyte)- form myelin in PNS

only. • Like oligodendrocytes • Support, protect (insulte), and nourish neurons. Increase speed of impulse. Nodes of Ranvier

• Satellite cells surround neuron cell bodies with ganglia.

• Ganglia = bunched up cell bodies in PNS

Myelinated Vs. Unmyelinated

CNS Glial cells

Glial Cells

Concept Check

• What is the difference between Schwann cells and oligodendrocytes?

Neurons (nerve cells)

• Highly specialized to transmit messages (nerve impulses) from one part of the body to another.

• Structural units of the nervous system

– Composed of a cell body, axon, and dendrites.

• Cell body contains the nucleus and is the metabolic center.

– Long-lived, amitotic, and have a high metabolic rate.

• Their plasma membrane function in:

– Electrical signaling

– Cell-to-cell signaling during development.

Neurons

• 1. Cell body

• 2. Processes

• 3. Myelin Sheaths

• 1. Cell body contains the nucleus and acts as the metabolic center.

Nerve Processes (axons and dendrites)

The armlike processes vary in length from microscopic to 3-4 feet.

A. Dendrites receive informations (electrical signals) sends toward the cell body.

B. The cell body and dendrites are input zones, where signals arrive and cause an electrical disturbance across the plasma membrane.

C. A large disturbance may spread to the trigger zone, an adjoining patch of the membrane where information about a stimulus becomes encoded in action potentials.

Axons Action potentials usually propogate themselves along an

axon, which is a neurons conducting zone. Most axons have branched endings that are output zones. Here, the action potentials become transduced into signals that can be sent on to neighboring cells.

Axons conduct them away from the cell body.

Neurons may have hundreds of branching dendrites, but only one axon.

Each axon terminal is separated from the next by a tiny gap called the synaptic cleft or synapse, so they never touch each other.

Axons

• Axon arises from the axon hillock. • Axons start as a single fiber, but may

give off branches called axon collaterals

• Axon and collaterals end in axon terminals which end in synaptic end bulbs – end bulbs contain synaptic vesicles

that contain neurotransmitters (chemical messengers)

Dendrites and Axons

Neuron

3. Myelin Sheaths

• Axons are covered with a whitish, fatty material called myelin, which has a waxy appearance.

• It protects and insulates the fibers and increases the speed of nerve impulses.

• These cells wrap around the axon in a jelly-roll fashion. The tight coil of wrapped membranes is the myelin sheath, enclosing the axon.

• The regular gaps between the sheaths are called nodes of Ranvier.

• Axons inside the CNS are myelinated by oligodendrocytes, and axons outside the CNS are myelinated by Schwann cells.

Myelin Sheaths

Neuron

White Matter vs. Gray Matter

• Myelinated axons appear white due to the presence of lipids and form the white matter of the inner brain and outer spinal cord. White matter is specialized for carrying information quickly through the brain and spinal cord.

• The gray matter of the brain and spinal cord are the unmyelinated integration centers where information is processed.

Types of Neurons (Structural)

• Multipolar – most numerous

• Bipolar – fewest (eye, ear, nose)

• Unipolar – sensory neurons in PNS

Neuron Classification (Functional)

• Sensory Neurons (afferent)—carry information about changes in the internal and external environments to the CNS. – Ex. Touch, pressure, vibration, temp, & pain.

– Special senses: smell, taste, vision, hearing, equil.

• Association Neurons (interneurons or connecting neurons) analyze sensory information and decide how to respond. Located in CNS.

• Motor Neurons (efferent neurons) carry instructions from CNS to muscles and glands.

Check for understanding: Label the neuron:

Nerve Impulse Physiology

• Neurons have 2 major functional properties:

• 1. irritability: the ability to respond to a stimulus and convert it into a nerve impulse.

• 2. conductivity: the ability to transmit the impulse to other neurons, muscles or glands.

Types of Conduction.

• Continuous conduction – unmyelinated axon conducts an impulse over its entire surface

• Saltatory conduction – myelinated axon “jump” from one node to next – much faster

Resting neurons (Unmyelinated impulse)

• The plasma membrane of a resting, or inactive, neuron is polarized, meaning there are fewer positive ions on the inside of the neuron’s plasma membrane than on the outside.

• The positive ions inside the cell are K+1.

• The positive ions outside the cell are Na+1.

• As long as the inside remains more negative than the outside, the neuron will stay inactive.

Resting/ Inactive Neurons

• Many different types of stimuli excite neurons to become active and generate an impulse.

– Light excited eye receptors.

– Sound excites ear receptors.

– Pressure excites skin receptors.

• Most neurons are excited by neurotransmitters released by other neurons.

• Either way, the result is the permeability properties of the cell’s plasma membrane change for a brief period of time.

Action Potential Initiation • Normally, sodium ions cannot diffuse through

the plasma membrane , but when the neuron is stimulated, the “gates” of sodium channels in the membrane open. Because sodium is in a much higher concentration outside the cell, it will diffuse quickly into the neuron, causing depolarization.

• The inside is now more positive, and the outside is less positive. (Na and K inside cell)

How is this maintained?

• Gated ion channels (K+ and Na+ ) 1. K+ out/Na+ in by

diffusion (K+ more) 2. Na+/K+ pump

• If the stimulus is strong enough, and the influx of Na is high enough, (enough depolarization), a threshold potential is reached, and a nerve impulse (action potential) is sent down axon. This long distance impulse is an all-or-none response, like firing a gun. It is either propagated (conducted) over the entire axon, or it doesn’t occur at all. The nerve impulse never goes part way along an axon’s length, nor does it die out with distance.

• Almost immediately after the Na rush into the neuron, the membrane permeability changes again, becoming impermeable to Na, but permeable to K.

• So K diffuse out of the neuron into the tissue fluid very rapidly. This restores the electrical conditions to the polarized or resting state.

• Repolarization--K+ rush out of neuron after Na+ rush in, which repolarizes membrane back to resting potential.

• Sodium-potassium pump—requires ATP

• Until repolarization occurs, a neuron cannot conduct another impulse.

Na+/K+ Pump

• Active transport • 3 Na+ out/2 K+ in • Keeps inside of membrane

less positive than outside

Nerve Impulse Propogation

The Action Potential

The Nerve Impulse

Summary: Action Potential (aka nerve impulse)

1. Stimulus-gated Na+ channels open (neurotransmitters)

2. Na+ enters (depolarization)

3. Threshold potential (-59mV) reached if stimulus strong enough

4. If threshold reached voltage-gated Na+ channels open and Na+ enters cell (voltage to +30mV). Gates close quickly.

5. K+ channels also open, but slower to respond – K+ leaves cell to restore resting potential (repolarization)

Synapses

• Presynaptic neuron

• Postsynaptic neuron (or an effector such as a muscle)

• Synaptic knob

• Synaptic vessicles

• Neurotransmitter

• Synaptic cleft

• Receptors

SYNAPSE

• Gap between neuron

• presynaptic neuron – neuron before the synapse

• postsynaptic neuron - neuron after synapse

–Nerve impulse travels from presynaptic to postsynaptic neuron

• An impulse traveling down an axon of a presynaptic neuron reaches synaptic end bulb, opens Ca++ channels

• Ca++ rushes into cell causes synaptic vesicles to fuse with cell membrane and release their neurotransmitter into the synaptic cleft

• Neurotransmitter binds with receptors on the postsynaptic neuron’s cell membrane

How?

• AP reaches synaptic knob - Ca++ gates open

• NT vessicles migrate then fuse with plasma membrane

• NT release

• NT bind to receptors (tied to ion channels)

– Excitatory (EPSP) or inhibitory (IPSP)

• Summation of EPSPs and IPSPs determine whether neuron will fire or not (or muscle contract)

NEUROTRANSMITTERS

• At least 30 – 50 different neurotransmitters identified

• Synaptic end bulbs may contain 2 or 3 different neurotransmitters

• Needs to be removed or will influence nerve indefinitely

–Some degraded by enzymes

–Some reuptake into cells

Synapse

Transmission at Synapses

Neurosynapse