Biology 12 – 3B

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The Nervous System Text Ref Pg 372-407 http://www.youtube.com/watch?v=JB7jSFeVz1U

Functions of the Nervous System

Uses sensory receptors to gather sensory input so it can monitor changes both inside and outside the body.

It processes and interprets sensory input and decides what type of response to elicit in a process called integration.

It causes a response, called motor output, by activating effector organs such as muscles or glands.

Neurons - “Nerve Cells”

Billions of neurons are the structural unit of the nervous system

highly specialized cells that conduct messages in the form of electrical impulses and chemical signals from one part of the body to another

The nervous system is the master controlling and communicating system of the body. Every thought, action and emotion reflects its activity. Neurons, specialized cells that serve as the functional unit of the nervous system, are connected to millions of sensory receptors that monitor changes occurring both inside and outside the body. The information collected by sensory receptors is called sensory input. During a process called integration, we either subconsciously or consciously decide on an appropriate motor response, or action, for each sensory stimulus received. For example, when you are driving and see a red light ahead (sensory input), your nervous system integrates this information (red light means “stop”), and your foot goes for the brake (motor output).

Neurons are interconnected in complex arrangements and use electrochemical signals and neurotransmitters to transmit signals called impulses from one neuron to the next. The interaction of different neurons from various neural circuits at any given time regulates our

perception of the world and what is going on within our body.

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Anatomy of a Neuron

Neurons are typically large, complex cells with 3 main parts:

1) Cell Body (Soma) – contains the nucleus and other cell organelles

Most are located in the CNS where they are protected by the bones of the skull and vertebral column.

Cell bodies that lie along the nerves in the PNS are called ganglia

2) Dendrites – the main input or receptive region of the neuron

Dendrites receive signals from other neurons and send them to the cell body

They provide an enormous amount of surface area for receiving signals from other neurons

3) Axon (“a” = away) – conducts nerve impulses away from the cell body toward other neurons or effectors

The dendrites and axons of a neuron are basically tubes constructed of cell membrane, called axomembrane, which are filled with cytoplasmic fluid called axoplasm

The electrochemical signal or impulse that allows neurons to communicate travels along the axomembrane

Axons can be very short, or can be very long up to 3 or 4 feet! – axons of the motor neurons controlling the skeletal muscles of your big toe extend from the lumbar region of your spine to your foot.

Homeostatic Imbalance

Normally incoming sensory input or nerve impulses are received by the dendrites and travel through the cell body and down the axon to the axon terminal. Certain viruses such as polio, rabies, and the herpes simplex virus as well as bacterial toxins such as tetanus toxin, damage neural tissues by travelling “backwards” (retrograde axonal transport) up axons. Once these viruses or toxins reach the cell body they “highjack” or destroy the cell and the neuron can no longer function properly.

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Myelin Sheath

Schwann cell and Action Potential (also in Real)

The axons of many nerve cells, particularly those that are long or large in diameter, are covered with a white, fatty sheath called myelin

Myelin sheath is formed by Schwann cells, a type of neuroglia (nervous system support cells) that have the lipid substance, myelin, in their cell membranes

Schwann cells wrap around the axons of neurons in the PNS many times like a“jelly roll”, and as a result they lay down several layers of their lipid rich plasma membrane

A single Schwann cell myelinates only a fraction of an axon, but many are arranged side by side approximately 1mm apart.

The gaps where there is no myelin sheath between Schwann cells are called the“nodes of Ranvier”.

In the PNS, myelin gives nerve fibers a white, glistening appearance

Myelin has 3 important functions:

1) Protects and electrically insulates nerve fibers from one another

2) Significantly increases the speed of nerve impulses: (Unmyelinated fibers conduct impulses quite slowly.)

3) Plays an important role in nerve regeneration in the PNS

If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth

Homeostatic Imbalance

Multiple Sclerosis (MS) is an autoimmune disease where the immune system attacks the myelin sheath of nerve axons in the CNS. When myelin is lost, the axons can no longer effectively conduct signals. Lesions develop and become hardened scars that interfere with the ability of nerve cells in the brain and spinal cord to communicate with each other. Patients with MS can suffer almost any neurological symptom or sign, including changes in sensation, muscle weakness, muscle spasms, or difficulty in moving. Difficulties with coordination and balance problems in speech or swallowing visual problems fatigue, acute or chronic pain and bladder and bowel difficulties are also common. Some patients also experience cognitive impairment of varying degrees and emotional symptoms of depression or unstable mood.

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There are 3 classes of neurons:

1) Sensory neurons

Cell bodies lie outside the CNS in ganglia

Transmit impulses from sensory receptors in the skin or internal organs to the CNS

Sensory neurons possess sensory receptors that can detect changes in the environment e.g. changes in pressure, temperature, pH, etc

2) Interneurons

Lie entirely within the CNS between sensory and motor neurons

Receive input from sensory neurons and also from other interneurons in the CNS and transmit it to motor neurons

3) Motor Neurons

Cell bodies lie within the CNS

Takes messages away from the CNS to an effector (skeletal muscle, organ, or gland)

Effectors then carry out responses to environmental changes detected by sensory neurons in the PNS.

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Transmission of a Nerve Impulse – through a neuron

Neurons exist in either a resting state, or stimulated state

When a neuron is adequately stimulated, an electrical impulse called an action potential is generated and conducted along the neuron

Resting Membrane Potential

When a neuron is NOT transmitting an impulse i.e. when it is resting, the inside of the cell is NEGATIVELY charged compared to the outside due to the presence of negatively charged proteins in the axoplasm of the neuron

For most nerve cells the resting potential is between -65 mV and -70mV

Neurons develop their resting potential because:

1) Negatively charged proteins within the axoplasm are too large to diffuse out of the axon, and so they contribute to the overall negative charge inside a neuron

2) Sodium-potassium pumps in the axomembrane use ATP to maintain an uneven distribution of ions by active transport

This pump pushes two potassium ions (2 K+) into the axoplasm for every three sodium ions (3 Na+) it pumps out of the axoplasm

Action Potentials

Action potentials are said to be “all” or “none” responses to stimuli

When threshold potential is reached (-55mV) the distribution of charges on each side of the axomembrane changes (Na+ ions flow into the axoplasm)

Stimuli that are not sufficient enough to reach threshold potential do not result in the firing of an action potential.

Steps of an Action Potential

1) A neuron receives a stimulus

When a resting neuron is stimulated voltage gated Na+ channels open and Na+ begins to flow into the axon of a neuron.

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If a stimulus is strong enough, enough Na+ will flow into the neuron to raise its membrane potential from its resting potential of -70mV to its threshold potential of -55 mV

2) Depolarization “Upswing of the Action Potential”

Once threshold potential is reached (-55mV), an action potential is initiated and rapid depolarization occurs

During depolarization voltage gated sodium channels open and Na+ rushes into the axon, rapidly increasing the

membrane potential from -55mV +40 mV.

3) Repolarization “Downswing of the Action Potential”

Immediately after an action potential is fired, repolarization occurs

Na+ channels begin to close and the net influx of Na+ stops completely

K+ channels open and there is a net flow of K+ out of the axon

As a result the axoplasm becomes more negative again (+40 mV -70mV)

4) Refractory (Recovery) Period “Undershoot”

During the refractory period the resting potential is re-established.

K+ gates are slow to close and as a result excessive K+ leaves the axoplasm causing a temporary “undershoot” where the membrane potential becomes more negative than during resting state.

Over time Na+/K+ pumps restore ion gradients back to -70 mV as in the resting state.

In the refractory period, Na+ channels are inactivated and so the neuron cannot respond to any new stimuli.

http://highered.mcgraw-

hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

Homeostatic Imbalance – Pufferfish Tetrodotoxin

Tetrodotoxin, frequently abbreviated as TTX, is a potent neurotoxin with no known antidote. TTX blocks action potentials in nerves by binding to the voltage-gated, fast sodium channels in nerve cell membranes, essentially preventing any affected nerve cells from firing. The binding site of this toxin is located at the pore opening of the voltage-gated Na+ channel. The first recorded cases of TTX poisoning were from the logs of Captain James Cook from 1774. Cook recorded his crew eating some local tropic fish (pufferfish), then feeding the remains to the pigs kept on board. The crew experienced numbness and shortness of breath, while the pigs were all found dead the next morning. In hindsight, it is clear that the crew received a mild dose of tetrodotoxin, while the pigs ate the pufferfish body parts that contain most of the toxin, thus being fatally poisoned.

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In non-myelinated axons an action potential travels down an axon one small section at a time

In myelinated axons, the gated ion channels that produce an action potential are concentrated at the nodes of Ranvier

Since ion exchange occurs only at the nodes, an action potential travels faster in myelinated axons than in non-myelinated axons because it literally “jumps” from node to node

This is called saltatory transmission; “Saltare” is a Latin word meaning “to leap”.

Homeostatic Imbalance

A number of chemical and physical factors inhibit or impair the propagation of nerve impulses. Although their mechanisms of action differ, alcohol, sedatives, and injected anesthetics all block nerve impulses by reducing membrane permeability to ions, mainly Na+. As we have seen, no Na+ entry into the axoplasm = no action potential generation

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Transmission Across a Synapse – from one neuron to another

Every axon branches into many fine endings (1000 - 10,000) and each is tipped by a small swelling called an axon terminal

Each axon terminal lies very close to the dendrite or cell body of another axon – however, they DO NOT physically touch

This region of close proximity is called a synapse and the space between two neurons is called the synaptic cleft

At a synapse, the first neuron is called the presynaptic neuron, and the next neuron is called the postsynaptic neuron

http://highered.mcgraw-

hill.com/sites/0072495855/student_view0/chapter14/animation__chemical_synapse__quiz_1_.html

transmission of information from one neuron to another across a synaptic cleft is usually carried out by chemicals called neurotransmitters

Inside the axons of neurons, neurotransmitters are stored in membrane bound “packages” called synaptic vesicles

Neurotransmitters act as chemical messengers that deliver excitatory or inhibitory messages from one neuron to the next

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Process of the Transmission Travelling across the Synapse

1) When an action potential arrives at an axon terminal of a presynaptic neuron, gated channels for calcium ions (Ca2+) open, and calcium floods into the axon terminal

2) This influx of Ca2+ causes synaptic vesicles carrying neurotransmitters to migrate to, and fuse with, the presynaptic membrane

3) The vesicles then release their contents via exocytosis into the synaptic cleft

4) The neurotransmitters then diffuse across the synaptic cleft and reversibly bind to specific receptor proteins on the plasma membrane of a postsynaptic neuron

Excitatory neurotransmitters cause Na+ to flood into the dendrite of the postsynaptic neuron – this change in membrane potential drives the neuron closer to an action potential.

Inhibitory neurotransmitters produce a signal that drives the postsynaptic neuron farther from an action potential by inhibiting transport of Na+ into its axon

5) Only a fraction of a second after neurotransmitters bind to their receptors on the postsynaptic membrane, they are released back into the synaptic cleft

In some synapses the postsynaptic membrane contains enzymes that rapidly breakdown and inactivate neurotransmitters

Neurotransmitters can also be taken up and recycled or broken down by the axon terminal they were released from (presynaptic membrane) once they have done their job

The short existence of neurotransmitters in a synapse prevents continuous stimulation or inhibition of postsynaptic membranes

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Neurons and how they work (also saved in Real)

Neurotransmitters

At least 50 different neurotransmitters have been identified.

These molecules, along with electrical signals, are used by a neuron to send messages throughout the body

Sleep, thought, rage, hunger, memory, movement, and even smiling reflect the effects of various neurotransmitters

Neurons contain the enzymes they need to produce their own neurotransmitters

Acetylcholine (ACh)

Acetylcholine is a neurotransmitter that has functions in both the CNS and the PNS

1) ACh in the PNS:

ACh is a major neurotransmitter in the parasympathetic division of the autonomic nervous system - stimulates skeletal muscle contractions *Botox inhibits the release

2) ACh in the CNS:

Plays a vital role in learning and short-term memory

Once ACh has been released into a synaptic cleft and has initiated a response, it is quickly removed from the cleft by the enzyme acetylcholinesterase (AChE)

Norepinephrine (aka NE or Adrenaline)

Is an excitatory neurotransmitter in the sympathetic division of the autonomic nervous system

Involved with the fight-or-flight response – increases heart rate, increases blood flow to skeletal muscles and releases glucose from energy stores

When released by the adrenal glands on top of the kidney into the blood NE acts as a hormone

Drugs and the Nervous System http://learn.genetics.utah.edu/content/addiction/drugs/mouse.html

Many drugs affect the nervous system by interfering with or enhancing the action of

neurotransmitters. 1) Prevent the release of neurotransmitters into the synaptic cleft = antagonist

Most anesthetics, barbituates and opiates work by blocking the release of neurotransmitters from the presynaptic membrane

2) Enhance the release of a neurotransmitter by causing it to “leak” out of synaptic vesicles = agonists

In the CNS nicotine causes neurons to release dopamine, a “feel good” neurotransmitter Amphetamines such as speed cause the excessive release of norepinephrine

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3) Mimic the action of a neurotransmitter by binding to its receptor on the postsynaptic membrane

Heroin, morphine and methadone bind to receptors meant for endorphins, a class of neurotransmitters that inhibit the pain response and produce a feeling of tranquility

If prolonged use of these drugs continues, the body’s natural production of endorphins decreases

Tolerance to the drugs develops so that the user needs to take more of the drug just to prevent withdrawal symptoms

In the PNS nicotine stimulates the same postsynaptic receptors as acetylcholine. This leads to increased activity of the skeletal muscles, increases heart rate and blood

pressure, and stimulates the digestive tract

4) Block receptors in the membrane of the postsynaptic neuron

THC, a chemical found in marijuana binds to a receptor for anandamide, a neurotransmitter molecule that is important for short-term memory processing and creating the feeling of peaceful contentment

As a result, long-term use can lead to brain impairment and problems with memory

5) Interfere with the removal of a neurotransmitter from a synaptic cleft by blocking the enzyme that causes the breakdown of the neurotransmitter

6) Prevent the re-uptake of the neurotransmitter by the presynaptic membrane

Cocaine prevents the presynaptic uptake of dopamine a “feel good” neurotransmitter and enhances excitatory responses

As a result cocaine users experience a “rush” sensation and enhanced arousal for several minutes after the rush experience

Cocaine causes extreme physical dependence. With continued use, the body begins to make less dopamine and users experience intense cravings for the drug.

Organization of the Nervous System

Homeostatic Imbalance

Norepinephrine, along with dopamine, has come to be recognized as playing a large role in attention and focus. For people with ADD/ADHD, psychostimulant medications such as Ritalin and Dexedrine, and are prescribed to help increase levels of norepinephrine and dopamine.

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The nervous system has 2 major divisions: 1) Central Nervous System (CNS)

o Consists of the brain & spinal cord o The integration and command center of the nervous system o Interprets sensory input and dictates motor responses based on past experiences,

reflexes and current conditions 2) Peripheral Nervous System (PNS)

Consists mainly of the nerves (bundles of axons) that extend from the brain and spinal cord of the CNS

o 31 pairs of spinal nerves carry info to and from the spinal cord (cervical, thoracic, lumbar, sacral, coccygeal).

o 12 pairs of cranial nerves carry info to and from the brain. These nerves:

1) Carry sensory messages to the CNS 2) Carry motor commands from the CNS to the muscles and glands

Peripheral nerves serve as the communication lines that link all parts of the body to the CNS

Peripheral Nervous System (PNS)

Can be divided into:

1) Somatic Nervous System “Voluntary Nervous System” Conducts impulses from the CNS to skeletal muscles. Through the somatic nervous system we can consciously control our skeletal

muscles 2) Autonomic Nervous System (ANS) “Involuntary Nervous System”

Consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands. Plays a role in moving food through the digestive tract, pumping blood through the heart, releasing secretions from glands, breathing, etc.

The ANS has 2 functional subdivisions:

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a) Parsympathetic Division – “Rest & Digest” nervous system o “Housekeeper” or “vegetative” system o Promotes all internal responses associated with being in

a relaxed state i.e. salivation, digestion, urination, defecation, crying, etc.

b) Sympathetic Division – “Fight or Flight” nervous system

o The fight or flight response is activated when our brain perceives sensory input (i.e. a stimulus) as a threat

o We become alert and attentive to our environment o The “stress hormones” adrenaline, noradrenaline and cortisol are

released from the adrenal glands (on top of the kidney) into our bloodstream

o The body responds by: a. increasing heart rate b. triggering the release of glucose from glycogen in the liver c. widening air passageways and increasing breathing rate d. dilating the pupils to maximize visual alertness e. altering blood flow patterns – reducing blood flow to surface

tissues and digestive system and increasing blood flow to the skeletal muscles

f. decreasing salivary secretion and tear production g. inhibits the bladder contractions and evacuating the colon

The parasympathetic and sympathetic divisions of the nervous system typically work in opposition to each other – what one subdivision stimulates, the other inhibits

Homeostatic Imbalance

Although the stress response is valuable potentially vital, for many individuals, psychological stressors are responsible for inappropriate or prolonged activation of the stress response. As a consequence they experience many negative physiological and psychological effects.

The stress response halts or slows down various processes such as sexual responses and digestive systems to focus on the stressor situation. People typically experience negative effects like, constipation, anorexia, erectile dysfunction, difficulty urinating, and difficulty maintaining sexual arousal. These are functions which are controlled by the parasympathetic nervous system and therefore suppressed by sympathetic arousal. Prolonged stress responses may result in chronic suppression of the immune system, leaving the body open to infections. In fact, many individuals that suffer from chronic stress disorders are also prone to more infectious diseases

than healthy individuals.

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Anatomy of the Central Nervous System (CNS)

The central nervous system is comprised of the brain and spinal cord

The brain is protected by the skull

The spinal cord lies within the spinal canal where it is protected by 21 vertebrae

Both the brain and spinal cord are wrapped in 3 protective membranes called meninges

In between the pia mater and arachnoid (1st and 2nd layers of the meninges) cerebrospinal fluid (CSF) circulates throughout the CNS

Essentially, the brain “floats” in CSF and it acts as a "cushion" by protecting the brain and spinal cord from jarring movements

CSF can be tested to diagnose a variety of neurological diseases.

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National Geographic Brain Games (also in Real)

The Brain

Comprised of millions of neurons, the brain looks somewhat like a giant pinkish-gray walnut that has the consistency of cold oatmeal

The average adult brain weighs between 3-3.5 pounds, and by mass, males and females have equivalent brain sizes

The brain (and spinal cord) is composed of 2 types of nervous tissue:

1) Gray Matter – contains cell bodies of neurons and short un-myelinated axons

2) White Matter – contains myelinated axons that are gathered together in bundles called nerve tracts

http://science.nationalgeographic.com/science/health-and-human-

body/human-body/brain-article/

Homeostatic Imbalance

Meningitis is an infectious disease that causes inflammation of the meninges around the brain and spinal cord. Most cases of meningitis are caused by bacteria, viruses or other microorganisms but, in some rare cases, it has also been caused by some medications. Meningitis occurs most often in children, teens, young adults, and people with weakened immune systems. Viral meningitis is fairly common, but it usually does not cause serious illness. Bacterial meningitis is not as common, but is very serious and can lead to brain damage and even death. Meningitis is contagious. The germs that

cause it can be passed from one person to another through coughing, sneezing and close contact.

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The Cerebrum - The “Conscious Brain”

Largest portion of the brain in humans - 85% of the brain by mass.

Carries out higher thought processes for memory, learning, language, voluntary movements and speech

The cerebrum is divided into 2 hemispheres that are divided by deep groove called the longitudinal fissure

The left and right cerebral hemispheres are connected by a bridge of white matter (myelinated axons) called the corpus callosum

The left cerebral hemisphere controls the right side of the body

The right cerebral hemisphere controls the left side of the body.

The outermost layer of the cerebrum is called the cerebral cortex

It contains billions of neuron cell bodies, dendrites and un-myelinated axons (gray matter)

This is the region of the brain that accounts for sensation, voluntary movement, and all conscious thought processes

Shallow grooves called sulci (sing. sulcus) divide the cerebral cortex into lobes.

1) Frontal Lobe – conscious thought and movement of voluntary skeletal muscles

2) Parietal Lobe – taste, sensation of temperature, touch, pressure and pain from skin

3) Temporal Lobe – hearing and smelling

4) Occipital Lobe – vision

Longitudinal Fissure

Gray Matter

White Matter

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Cerebellum – “The Subconscious Brain”

Receives sensory input from the joints, muscles and other sensory pathways about the position of body parts

Also receives motor output from the cerebral cortex about where these parts should be located

Sends motor impulses to the skeletal muscles to help us maintain posture and balance and co-ordinates smooth voluntary movements

Assists in learning new motor skills such as playing the piano or playing baseball

Integrates signals from other parts of the brain so we can do multiple activities at once. i.e. walk and talk at the same time.

The Thalamus “Gateway to the Cerebral Cortex”

The thalamus is a large mass of gray matter located deep within the brain

It receives, sorts and edits auditory, olfactory (smell), somatosensory, and visual sensory input

Impulses having similar functions are relayed as a group to the appropriate area of the cerebral cortex

Plays a key role in mediating sensation, motor activities, learning and memory

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Hypothalamus

1) Regulates the Autonomic Nervous System (ANS)

Influences blood pressure, heart rate, digestion, breathing, dilation of the pupils, etc.

2) Controls Emotions

The hypothalamus lies in the heart of the emotional part of the brain

Here neurons are involved in the perception of pleasure, fear, rage, sex drive, etc.

Initiates most physical expressions of emotions through the ANS i.e. fight or flight response

3) Regulates Body Temperature

The body’s thermostat is in the hypothalamus

Initiates sweating to cool the body down or shivering to preserve body heat as needed to maintain a relatively constant body temperature

4) Regulates Hunger

In response to blood levels of certain nutrients and hormones (i.e. insulin) the hypothalamus regulates feelings of hunger and satiety (feeling full)

5) Regulates Water Balance & Thirst

When body fluids become concentrated, i.e. we are dehydrated; the hypothalamus triggers the release of ADH (antidiuretic hormone) from the kidneys

ADH causes the kidneys to retain water in an effort to prevent further dehydration

6) Neuroendocrine Control

Collectively the hypothalamus and pituitary gland are called the neuroendocrine control center. They work together to maintain homeostasis with the help of the ANS

The hypothalamus receives information about the status of things such as body temperature, water balance, and the levels of many hormones within the blood.

The hypothalamus then uses hormones to stimulate the pituitary gland to release other hormones that work to maintain homeostasis.

Anterior Pituitary – Produces and releases hormones that are involved with everything from reproduction to skeletal growth: FSH, LH, GH, TSH, Prolactin, Adrenocortictropic

Posterior Pituitary – Stores and releases ADH, to regulate water balance (kidneys), and oxytocin (uterine contractions).

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Medulla Oblongata “Subconscious Brain”

Located in the brain stem

Contains a number of reflex centers that regulate our heart beat, breathing and blood pressure

Also contains the reflex center for vomiting, coughing, sneezing, hiccupping, and swallowing

The Reflex Arc

Reflexes are rapid, automatic, involuntary responses that provide a mechanism for immediate withdrawal from dangerous or unpleasant stimuli.

Reflexes occur over highly specific neural paths called reflex arcs.

The key advantage of a reflex arc is that it can process sensory input and elicit a response directly from the spinal cord, without the need to wait for instructions from the brain

Eventually, all the sensory information is sent to the brain for analysis, but the reflex has already happened by this time.

There are 2 types of reflexes:

1) Spinal Reflexes – e.g. knee jerk reflex, pain withdrawal reflex.

2) Cranial Reflexes – e.g. pupil constriction in bright light.

Many are present at birth and some are learned throughout life through repetition.

E.g. You place your hand on a hot burner on the stove:

1) Pain receptors in the skin detect a stimulus.

2) Stimulus travels through sensory neuron (dendrite cell body axon axon terminal) to the spinal cord via the dorsal root ganglion.

3) The stimulus is interpreted in the spinal cord by an interneuron.

4) The interneuron connects with a motor neuron in the ventral root ganglion.

5) Impulse travels along the axon of the motor neuron to muscle fibers.

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6) Muscle fibers (effector) contract and hand is moved away from hot burner.

7) Brain receives message and may register a pain response

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Surviving the Teenage Brain