Chapter 28: The Nervous System

Neurons

Organization of Nervous System

3 interconnected functions

• Sensory receptor Sensory input Integration (interpretation of the sensory signals and the formulation of response) motor input effector cells

Reflexes

• Automatic responses• Sensory neurons:

convey signals from sensory receptors to CNS

• Interneurons: in the CNS and relay appropriate signals to other neurons

• Motor neurons: convey signals from the CNS to effector cells

Neurons

• Cell body: contains most of the neurons organelles• Dendrites: receives signals from other neurons and

convey info to cell body• Axon: transmits signals to other cells• Glia: supporting cells• Myelin sheath: insulating material (a chain of Schwann

cells)• Nodes of Ranvier: spaces between Schwann cells• Synaptic terminal: at the end of the neuron• Synapse: site of communication between a synaptic

terminal and another cell

Myelinated motor neuron

Membrane potential

• Potential energy of a resting neuron

• The fluid inside membrane is a negative charge

• The fluid outside the membrane is a positive charge

• Resting potential: the voltage of a resting neuron

Action potential

1. Resting potential2. Stimulus applied, if strong

enough it rises to threshold3. Once threshold is reached,

action potential triggered, membrane polarity reverses so interior becomes more postive

4. The membrane repolarizes5. Undershoots the action

potential6. Returns to action potential

Action potential

1. resting membrane is + charged on the outside and the cytoplasm is – charged

2. Stimulus triggers the opening of a few Na channels in the membrane and a tiny amount of Na enters the axon making the inside slightly less –

3. Once threshold is reached, more Na channels open allowing more Na to move in, as more Na moves in the voltage soars to its peak

4. the peak triggers the closing of Na and the inactivation of Na channels, K channels open and K rapidly diffuses out

5. K channels close slowly causing an undershoot6. Return to resting state

Action potential

Propagation of action potential

1. Na channels open Na rushes in action potential is generated

2. K channels open K diffuses out of the axon (Na channels are closed)

3. Resting potential

Synapse

1. Electrical~electrical current passes

directly from one neuron to another

~in the human body electrical synapses are found in the heart and the digestive tract where nerve signals maintain steady rhythmic muscle contractions

1. Chemical~have a narrow gap called

the synaptic cleft separating the synaptic terminal of the sending (presynaptic) neuron from the receiving (postsynaptic) neuron

~the action potential is converted to a chemical signal consisting of neurotransmitters which can then generate an action potential in the receiving cell

Neuron Communication

1. An action potential arrives at the synaptic terminal2. The action potential causes chemical changes that

cause the synaptic vesicles to fuse with the plasma membrane of the sending cell

3. Fused vesicles release neurotransmitters into the synaptic cleft and they diffuse across the synaptic cleft

4. The neurotransmitters bind to receptor molecules on the receiving cell’s plasma membrane

5. Chemically gated ion channels open and ions diffuse into the cell causing new action potentials

6. Neurotransmitter is broken down by an enzyme or the neurotransmitter is transported back into the signaling cell and ion channels close

Neuron communication

Types of neurotransmitters

1. Acetylcholine~ important in the brain and at synapses between motor neurons and muscle cells~ex: botox-inhibits the release of acetylcholine so it disables the facial muscles therefore eliminating wrinkles around the eyes or mouth

More neurotransmitters

2. Biogenic amines~derived from amino acids

~ex: norepinephrine, epinephrine, serotonin and dopamine

~ affect the CNS

~serotonin and dopamine affect sleep, mood, attention and learning

~ Parkinsons is associated with a lack of dopamine in the brain

Even more neurotransmitters

3. asparate, glutamate, glycine and GABA (gamma aminobutyric acid)

~example: substance P- an excitatory neurotransmitter that mediates our perception of pain

~endorphins: decreases our perception of pain

Drugs act at chemical synapses

1. Stimulants: caffeine, nicotine, ~ activate acetylcholine receptors

~amphetamines and cocaine: increase the release of dopamine and norepinephrine

2. Depressant: alcohol3. Antidepressant medication: effects the action of

serotonin4. ADHD medications: believed to block the uptake of

dopamine and norepinephrine5. Marijuana: seems to bind to the neurotransmitters that

play a role in pain, depression, appetite, memory and fertility

6. Opiates: bind to endorphin receptors

Invertebrate nervous systems

Invertebrate nervous system

• Hydra: nerve net• Bilateral symmetry:

key branch in evolution of nervous systems leading to Cephalization and centralization

• Flatworms: 2 parallel nerve cods

Human nervous system

CNS• Consists of the brain and spinal

cord• Blood-brain barrier-a series of

capillaries that allows for essential nutrients and oxygen to pass freely into the brain and keep other chemicals out of the brain

• Ventricles-fluid filled spaces in the brain and are continuous with the central canal of the spinal cord

• Cerebrospinal fluid-formed in the brain by the filtration of blood

• Meninges-protects the brain• White matter-mainly composed of

axons• Gray matter-mainly composed of

cell bodies and dendrites

Nervous System

PNS

Somatic nervous system Autonomic nervous system

• Somatic nervous system- voluntary-skeletal muscles

• Autonomic nervous system-involuntary-controls smooth and cardiac muscles and organs of the digestive, cardiovascular, excretory and endocrine systems

PNS

PNS (cranial and spinal nerves)

Sympathetic division (fight or flight response)prepares the body for intenseEnergy-consuming activities

Parasympathetic divisionPrimes the body for activities that gain

And conserve energy for the bodyEx: stimulating digestive organs

Enteric divisionConsists of networks of neurons in the

The digestive tract, pancreas and gallbladderControl secretions and activity of smooth muscle

That produce peristalsis

Parasympathetic vs. Sympathetic

Embryonic brain regions

1. forebrain: cerebrum, thalamus, hypothalamus, posterior pituitary, pineal gland

2. Midbrain: part of the brain stem

3. Hindbrain: pons, cerebellum, medulla oblongata

Major structures of the human brain

Brainstem: conducts data to and from the other brain centers, helps maintain homeostasis, coordinates body movement

~medulla oblongata: controls breathing, circualtion, swallowing, digestion

~pons: controls breathing~ midbrain: receives and

integrates auditory data, coordinates visual reflexes, sends sensory data to higher brain centers

Cerebellum

• Coordinates body movement

• Plays a role in learning and in remembering motor responses

Thalmus

• Serves as input center for sensory data going to the cerebrum, output center for motor responses leaving the cerbrum

• sorts data

Hypothalamus

• Functions as homeostatic controls cneter

• Contols pituitary gland

• Serves as biological clock

Cerebrum

• Performs sophisticated integration

• plays a major role in memory, learning, speech, emotions

• Complex behavioral responses

Lobes of the cerebral cortex

1. Frontal lobe: motor skills, speech

2. Motor cortex: sends commands to skeletal muscles

3. Somatosensory cortex: receives and partially integrates signals from touch, pain, pressure and temperature

4. Parietal lobe: speech, taste, reading

5. Occiptial lobe: vision6. Temporal lobe: smell

hearing

Limbic system

• Human emotion, learning and memory

• includes the thalamus and the hypothalamus

• Includes the amygdala and the hippocampus

• Primary emotions that produce laughing and crying

Amygdala and hippocampus

• Amygdala– Central in recognizing

the emotional content of facial expressions and laying down emotional memories

• Hippocampus– Involved in both the

formation of memories and their recall

Memory

• The ability to store and retrieve information derived from experiences

• Short term memory: lasts only a few minutes

• Long term memory: enhanced by rehearsal, positive or negative emotional states and association of new data

Neurological disorders

• Schizophrenia: a severe mental disturbance characterized by psychotic episodes in which patients lose the ability to distinguish reality

• Symptoms: hallucination, delusions, blunted emotions, distractibility, lack of initiative, difficulty with verbal expression

• Causes: unknown

Neurological disorders

• Depression: characterized by depressed mood and abnormalities in sleep, appetite and energy level

• 2 types: major and biopolar

-major: may experience persistent sadness, loss of interest in pleasurable activities, changes in body weight and sleeping patterns, loss of energy and suicidal thoughts

-bipolar or manic –depressive disorder

Neurolocial Disorders

• Alzheimer’s Disease: a form of mental deterioration or dementia, characterized by confusion, memory loss and a variety of other symptoms

Parkinson’s Disease

• A motor disorder characterized by difficulty in initiating movements, slowness of movement and rigidity.

• Patients have a masked facial expression, muscle tremors, poor balance a flexed posture and a shuffling gait