N E U R O N S A R E O R G A N I Z E D I N T O

N E R V O U S S Y S T E M S3 4 . 5

INTRODUCTION

• The cnidarians have nerve nets, the most simple type of nervous system.

The sea anemone has a nerve net that serves simple behaviours such as contraction and relaxation

EVOLUTION

• Evolution of nervous systems followed two major trends:

– Centralization—integrating neurons became clustered together in centralized organs (e.g., brain and spinal cord).

– Cephalization—major integrating areas became concentrated toward the anterior end of the animal (head).

• The anterior end meets the environment first; sensory organs are also concentrated there.

CENTRAL NERVOUS SYSTEM

• Central nervous system (CNS)—brain and spinal cord

• Composed mostly of integrating neurons and glial cells; it must interact with sensors and effectors.

• Effectors are cells or tissues that perform actions, that “carry out orders,” such as muscle cells.

PERIPHERAL NERVOUS SYSTEM• Peripheral nervous system (PNS)—neurons outside the CNS

• They bring sensory information from sense organs to the CNS and carry orders from the CNS to effectors.

• Nerves are bundles of axons in the PNS.

TYPES OF NEURONS

• Interneurons—neurons confined to the CNS

• Sensory neurons—sensory receptor cells or neurons that carry signals from sensory cells to the CNS (afferent neurons)

• Efferent neurons—convey signals from the CNS to muscles or other effectors

• Motor neurons—carry signals to skeletal muscles

AUTONOMIC NERVOUS SYSTEM

• Autonomic nervous system (ANS)—controls effectors other than skeletal muscles (autonomic effectors)

• Controls smooth muscle in organs, exocrine glands, some endocrine glands, acid secreting cells in the stomach, and other effectors.

3 DIVISIONS

• Vertebrate ANS has three divisions:

• Enteric division—nerve cells internal to the gut wall

• Sympathetic division prepares the body for emergencies—“fight or flight”

• Parasympathetic division slows the heart, lowers blood pressure and increases digestion

AUTONOMIC NERVOUS SYSTEM

• Autonomic efferent pathways begin with preganglionic neurons with cell bodies in the CNS.• Axons of preganglionic neurons synapse on a second neuron outside the CNS in a collection

of nerve cell bodies called a ganglion.

• The second neuron is postganglionic—its axon leaves the ganglion and synapses in the target organs.

AUTONOMIC NERVOUS SYSTEM• Parasympathetic preganglionic neurons exit the CNS from the brain

and sacral region of the spinal cord.– The ganglia are near the target organs.

• Sympathetic preganglionic neurons exit the CNS at the thoracic and lumbar regions of the spinal cord.

– Most of the ganglia lie next to the spinal cord.

AUTONOMIC NERVOUS SYSTEM

• Sympathetic postganglionic neurons use norepinephrine as the neurotransmitter.

• Parasympathetic postganglionic neurons use acetylcholine.

• In organs that receive both inputs, the target cells usually repond in opposite ways.

AUTONOMIC NERVOUS SYSTEM• The sympathetic and

parasympathetic divisions often work in opposition; acting together, they can adjust effector functions up or down as needed.

• Example: Sympathetic stimulation of the pace maker causes the heart rate to increase, and parasympathetic stimulation causes it to decrease.

AUTONOMIC NERVOUS SYSTEM

• The fight-or-flight response is an effect of the sympathetic division.

• When activated, it increases the heart rate, force of contraction, and cardiac output; it dilates lung passageways and increases release of glucose from the liver.

• At the same time, it reduces less urgent activities such as digestion.

SPINAL REFLEXES• Many neurons that control

skeletal muscles enter or leave the CNS in spinal nerves.

• Spinal nerves have both sensory and motor neurons.

CONCEPT 34.5 NEURONS ARE ORGANIZED INTO NERVOUS SYSTEMS

• Spinal reflex—afferent information converts to efferent activity without going through the brain

• The knee-jerk reflex:

• Stretch receptors in the patellar tendon send action potentials to the spinal cord.

• The sensory neuron synapses with a motor neuron, sending an action potential to the leg muscle.

1. A hammer tap stretches the tendon in the knee, stretching receptors in the knee

2. Stretch receptors fire action potentials.

3. The sensory neuron synapses with a motor neuron in the spinal cord

4. The motor neuron conducts action potentials to the quadriceps, causing contraction

5. Simultaneously a spinal interneuron inhibits firing in the motor neuron for the antagonistic muscle.

VERTEBRATE EVOLUTION OF THE FOREBRAIN

• Vertebrate brains have three main regions: forebrain, midbrain, and hindbrain.

• The brain and spinal cord must pass through the medulla oblongata, the most posterior part of the hindbrain.

– This area has changed little over the course of vertebrate evolution.

VERTEBRATE EVOLUTION OF THE FOREBRAIN• In contrast, the cerebral

hemispheres have undergone dramatic changes.

– They are important in carrying out high-order sensory, motor, and integrative functions.

• The evolution of enhanced functionality in mammals and birds has gone hand in hand with large increases in the numbers of neurons and larger brain size.

VERTEBRATE EVOLUTION OF THE FOREBRAIN• However, it is important to note that some animals with small brains exhibit stunning

behavioral capacities.

• In humans, all available evidence indicates that individual intelligence is not correlated with individual brain size.

SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES • Cerebral cortex—outer-most layer of the

cerebral hemispheres, with many cell bodies

– It is folded into convolutions, which increases its size.

• The left side of the body is served mostly by the right side of the brain, and vice versa.

• In each cerebral hemisphere, specific regions are specialized to carry out specific sensory and motor functions.

SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES

• Combined sensory and motor functions often occur in localized brain areas.

• Imaging methods such as PET (Positron) allow us to visualize areas where neurons exhibit increased electrical activity correlated with specific activities, such as language functions.

SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES

• Functional magnetic resonance imaging (fMRI) is another technique to pinpoint brain activity.

• Example: In a person experiencing fear, increased activity is seen in the amygdala in the forebrain.

• Even memories of frightening situations can activate the amygdala.

SPECIFICITY IN MAMMALIAN CEREBRAL HEMISPHERES • Parts of the brain that

serve various anatomical regions of the body are physically related to each other in ways that mirror the rest of the body.

• Example: Map of the somatosensory (“body sensing”) part of the cerebral cortex

– The size of each body part in the drawing reflects the amount of cortical area devoted to the part.