Biology – Premed Windsor

University School of Medicine

Premed I Biology

January 2015

Pre Med – Biology Chapter 13

The Nervous System

There is more to lectures than the power point

slides!

Engage your mind

KEYWORDS

Gland, Hormone, Impulse, Nervous system, Effector, Stimulus, Receptor, Neuron, Synapse

Nervous System

• Central Nervous System

• Peripheral Nervous System

Parts of the nervous system

The nervous system is made up of three main parts;

• The brain (CNS)• The spinal cord (CNS)• Nerve fibres (PNS)

It detects stimuli such as light, sounds, temperature, pressure, pain and co-ordinates the bodies response.

EFFECTORSAn effector is any part of the body that produces the response.

Here are some examples of Effectors:

• a muscle contracting to move the arm • a muscle squeezing saliva from the salivary gland • a gland releasing a hormone into the blood

CNS

• Brain contains approximately 100 billion neurons (nerve cells)

• Function of the brain is to process information arriving from other areas of the central nervous system and the peripheral nervous system via the spinal cord.

CNS

• Function of the spinal cord is to relay messages from the brain to the peripheral nerves and from peripheral nerves to the brain

• Spinal cord also controls reflex arcs or involuntary responses that bypass the brain

PNS

• The peripheral nervous system is made up of the nerves and ganglia (groups of cell bodies of neuron) that are found outside of the CNS

• Sensory neurons transmit impulses from the PNS to the CNS and motor neurons transmit impulses from the CNS to the PNS

• Peripheral nerves that communicate directly with the brain are called cranial nerves (12 pairs - connect sensory receptors in nose, eyes, ears, tongue, etc.)

• Peripheral nerves that communicate with the brain via the spinal cord are called spinal nerves (31 pair - muscles of the body and various glands and organs)

Parts of the nervous system

For example;If you smell something burning…• Your nose (receptor) detects the stimulus

(smell)• Nerve fibres send the message to the brain• Your brain then sends a message to move your

body away or to put out the fire! SIMPLE RIGHT!

Lets look at some bits more closelyRECEPTORS

• Receptors are sensors on the body that detect stimuli• They convert stimuli into electrical signals (messages) called

impulses.

NERVE FIBRES• Nerve fibres are bundles of nerve cells (neurons)

that pass on electrical signals (impulses) to the brain.

• From the brain, nerve fibres send impulses to effectors (muscles).

NEURONS• Brain is made of 100 billion neurons

• Basic functional unit of nervous system

• Neurons are nerve cells that transfer information within the body

• Each neuron has a vast number of “connections” (synapses) with other neurons

• Transmit electrochemical signals along the long length of communication with other neurons

NEURONSThere are three types of neurons:

1. Sensory neuron – Transmit information about external stimuli (carries impulses from the receptors to the spinal cord following stimulation)E.g. Respond to pressure, hot, cold, light, mechanical vibration, etc or internal stimuli such as blood pressure, muscle tension

2. Interneuron (Relay Neuron) – Located in CNS(Analyze and interpret the sensory information)Receives messages from other neurons by dendrites. Relays message to motor neuron through axons

3. Motor Neuron – Receives signals from interneuron at dendrite, relays message to cell body in CNS. Impulse then travels to effector (muscle or gland activity)eg. Motor neurons transmit signals to muscle cells causing contraction

• Neurons that carry out integration (interneurons) are organized in a Central Nervous System (CNS)

• Neurons that carry information into and out of the CNS are the Peripheral Nervous System (PNS)

• When bundled together, Axons of neurons form the NERVES

• Sensory neurons have long dendrites and short axons

• Motor neurons have short dendrites and long axons.

NERVOUS SYSTEM

CNS

PNS

ANS

SNS

Parasympathetic

Sympathetic

Nervous system releases electrical impulses that are quick but the effects are short lived and it involves the CNS (either just spinal cord or both spinal cord and brain) messages are carried by neurons

Endocrine system releases chemical signals that are slow to react but the effects are long lasting. This does not involve the CNS. Messages are carried by the blood.

CONTROL SYSTEMSThere are two main control systems in your body.

1. Nervous system – which has two main sections• Central or CNS that is made up of the spinal cord

and brain and functions to coordinate all actions of the body

• Peripheral or PNS that is made up of the bodies nerves and functions to connect the CNS to the rest of the body through neurons. PNS is further divided into

2. Peripheral Nervous System• Somatic or SNS which is associated with the voluntary control of body

movements and is made up of all neurons, sense organs, skin, skeletal muscles

• Autonomic or ANS which is associated with the involuntary control of body movements such as reflex and controls such things as heart rate, body temperature, digestion etc. The ANS is further divided into

» Parasympathetic nervous system works in actions that do not require a fast response (rest and digest response)

» Sympathetic nervous system works in actions that do require a fast response (fight or fight response)

Somatic Nervous System

• Somatic nervous system describes peripheral nerves that receive and send signals to skeletal muscle, skin, and tendons

• Sensory receptors in skin, muscle and tendons send information to the CNS about body position and environmental conditions

• CNS relays signals to motor neurons that control the contraction of skeletal muscle and the movement of the body

• Somatic nerves control voluntary movements of the body such as walking, jumping, writing, typing, etc

Autonomic Nervous System

• Autonomic nervous system controls involuntary responses to stimuli by the body

• Autonomic nerves serve cardiac muscle, smooth muscle, glands, and all of the internal organs

• ANS acts on these various effectors to maintain homeostasis within the body – Parasympathetic branch - acetylcholine neurotransmitter– Respond to stress through sympathetic branch - norepinephrine

neurotransmitter

Endocrine system – which is a system of glands that release a number of signalling chemicals known as hormones.

Neuron

• Neurons have numerous long processes and transmit nerve impulse

• Neuron has a large Cell body (soma) with two or more thin protoplasmic processes extending from it

• One of the processes called the “AXON”, long and conducts nerve impulses away from the cell body

• Second process conduct nerve impulses towards the cell body and are called “DENDRITES”

• Neurons are of 3 types based on the number of nerve processes:

• 1. Unipolar Neurons --- Only one axon, a single process leaves the cell body

• 2. Bipolar Neurons --- Two processes leave the cell body: an axon and a dendritic branch

• 3. Multipolar Neurons --- Many processes arise from cell body

• A neuron may have many Dendrites (extensions) that receive signals from other neurons and transmit the impulse to the cell body

• Cell bodies contain cell organelles, including the nucleus, and carry out normal cell functions

• Axons receive signals from the cell body and transmit the impulse away from the cell body

• Glia or Glial cells – Supporting cells that nourish and insulate the axons of neurons

• Extended axon or dendrite of a neuron is called a Nerve fiber

• Nerve fibers are of two types:

• Covered by lipid-rich insulating layer called “Myelin Sheath” or Myelinated

• Not covered by myelin sheath, called “Non-myelinated”

• Myelin sheaths (neuron wraps) are formed by Schwann Cells in the PNS (What forms myelin sheath in CNS??)

• Schwann cells form multiple layers of membrane around the neuron and insulate it

• In between the areas of myelin sheath, Nodes of Ranvier or bare patches exist

• Nerve impulse or action potential will jump form node to node greatly increasing the speed of nerve transmission.

• This node to node transmission, called saltatory conduction, can produce transmission speeds of up to 200 meters per second and explains the speed at which we can react to potentially harmful stimuli.

Action Potential

• The dendrites and axons of a neuron are cell membrane, called axomembrane, that are filled with cytoplasmic fluid called axoplasm

• Electrochemical signal or impulse that allows neurons to communicate travels along the axomembrane.

• When the dendrite of a neuron receives sufficient excitatory stimulation, called threshold, an action potential results

• Action potential is an "all or none response“• If stimulation exceeds the threshold, an impulse will be

generated• Sub-threshold stimulation will not elicit an action

potential• The stimulation that initiates an action potential usually

will be generated by sensory receptors for sensory neurons and at a synapse for interneurons and motor neurons.

Action PotentialResting potential:• The axoplasm is kept negative relative to the outside of the

neuron as 3 sodium ions are pumped out for every 2 potassium ions that are pumped in

Action Potential:• When the dendrite of a neuron receives stimulation exceeding

threshold, an action potential is generated in the axomembrane of the neuron and quickly moves along the dendrite to the cell body and axon

• An action potential is produced by the action of gated channel proteins embedded in the axomembrane. There are gated protein channels for both sodium ions and potassium ions.

Action potential has three phases--• 1.Depolarization (facilitated diffusion of sodium ions)• Sodium gated channel proteins open and sodium ions rush from

the outside of the axomembrane to the inside. This changes the polarity across the neuron from -65mV (resting potential) to +40mV

• 2.Repolarization (facilitated diffusion of potassium ions)• Potassium gated channels open and potassium ions rush to the

outside of the axomembrane. This makes the outside of the membrane positively charged relative to the inside once again. The potential across the membrane returns from +40mV back to -65mV.

• 3. Recovery/Refractory Period (active transport of sodium and potassium ions) The sodium-potassium pump is busy re-establishing the resting potential by pumping sodium ions out and potassium ions back in through the axomembrane.

The Synapse

SYNAPSES

Where two neurons meet, there is a tiny gap called a synapse.

Signals cross this gap using chemicals. One neuron releases the chemical into the gap. The chemical diffuses across the gap and makes the

next neuron transmit an electrical signal.

“The communication between neurons is chemical in nature”

• A synapse is composed of a presynaptic membrane on the axon bulb of the first neuron and a postsynaptic membrane on the dendrite or cell body of the second neuron

• A very small gap, called the synaptic cleft, lies between the presynaptic and postsynaptic membranes.

What is a synapse?A synapse is a junction between two neurons across which electrical signals pass. The human body contains up to 500 trillion synapses.

presynaptic cell

postsynaptic cell

The release of neurotransmittersWhen a nerve impulse arrives at the end of one neuron it triggers the release of neurotransmitter molecules from synaptic vesicles.

synaptic vesicle

neurotransmitter molecules

Continuing the impulseThe neurotransmitters diffuse across the synaptic cleft and bind with receptors on the next neurone, triggering another impulse.

nerve impulse

receptor

synaptic cleft

• Neurotransmitters can cause excitation or inhibition at the postsynaptic membrane

• E.g. Acetylcholine and Norepinephrine

• A single neuron will have many dendrites and many synapses with other neurons

• Many excitatory signals will depolarize the neuron and cause an action potential, while inhibitory signals super polarize the neuron and prevent an impulse being generated

Command & Control Center

Pairs of Cranial nerves, spinal nerves and ganglia make up most of the peripheral nervous system

Gray Matter – Made up of neuron cell bodies

White Matter – Made of bundled axons

REFLEX ARC• When your body needs to react to something very

quickly (to protect itself) it uses a reflex arc (spinal reflex).

• Instead of sending impulses from the receptorSpinal cord brain spinal cord effector

• It sends impulses from the receptor spinal cord effector…Yup it bypasses the brain!

• Reflexes = Body’s automatic responses to certain stimuli

• Reflex protects the body by providing a rapid, involuntary response to a particular stimuli

Peripheral Nervous System

• Transmits information to and from the CNS• Afferent – Sensory information reaches CNS• Efferent – Once processed within CNS, instruction

travels to muscles and glands.

• PNS has 2 Efferent components: • 1. Motor system – Carry signals to skeletal system

( Voluntary and Involuntary)• 2. Autonomic Nervous System (Involuntary)

3 divisions of Autonomic Nervous System: 1. Sympathetic (“Fight or Flight Response”)2. Parasympathetic ( “Rest & Digest”)3. Enteric – Digestive tract

Sympathetic – heart beats faster, digestion is slowed or inhibited, adrenal medulla secretes epinephrine (adrenaline), glucose production

Parasympathetic – heart rate decreases, digestion increases, glycogen production

ANSWERS• Electrical impulses along neurons, chemical

(neurotransmitters) across the synapse.• The microscopic gap between two neurones.• By chemicals called neurotransmitters.• To pass the impulse onto the correct motor

neurone.• Glands or muscle that produce the effect or make a response.

The sequence of a reflex arc

REFLEX ACTION

The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.

In bright light

In dim light

•Radial muscles of the iris relax. •Circular muscles of the iris contract. •Less light enters the eye through the contracted pupil.

•Radial muscles of the iris contract. •Circular muscles of the iris relax. •More light enters the eye through the dilated pupil.

The iris reflex

Inside the eye

lensfocuses lighton retina

irisregulates amountof light entering eye

ciliary muscleschange shape of the lens

optic nervetransmits impulses to the brain

corneaprotects eye surface and focuses light rays

retinasenses lighthold lens in place

suspensory ligaments

BRAIN

• The brain is divided into a left and a right hemisphere

• Each hemisphere is further divided into four lobes

• The left hemisphere of the brain controls the right side of the body and the right hemisphere controls the left side of the body

• The two hemispheres are connected by the corpus callosum which allows communication between the two sides of the brain

Cerebrum

• The cerebrum is the largest part of the human brain and gives us consciousness and the ability to think, wonder, ponder, etc.

• It is highly convoluted in structure• The cerebrum is responsible for receiving

sensory signals (ex. touch) and controlling voluntary responses by the body such as movement

Thalamus

• The thalamus is called the "gatekeeper" for the cerebrum

• All information received by the cerebrum is routed through the thalamus (except smell).

• The thalamus also plays a role in the reticular activating system (RAS) that makes the cerebrum aware of important stimuli (perhaps awakening you from sleep) and filters out unimportant stimuli (background noise).

Hypothalamus

• The hypothalamus lies below the thalamus• It functions in maintaining homeostasis in the body• Some of the bodily functions controlled by the

hypothalamus are sleep, hunger and thirst, water balance, blood pressure, and body temperature (thermostat). The mechanism by which it controls these functions often involves hormones released from the pituitary gland that have effects on target organs in the body such as the thyroid gland, adrenal glands, and kidneys

Medulla Oblongata• The medulla oblongata is located in the brain stem at

the base of the skull• It has neurons that communicate with both the brain

and the spinal cord.• The medulla oblongata controls involuntary functions

vital to life and is the site of the respiratory (breathing) and cardiac (heartbeat) control centers mentioned earlier in the course

• Other reflexes controlled by the medulla oblongata include; vomiting, coughing, sneezing, hiccupping, and swallowing.

Cerebellum • The cerebellum lies behind the brain stem. It is

responsible for muscular coordination and balance

• Receives information about positions of the joints and lengths of muscles

• Hand-eye coordination

Pituitary GlandThe pituitary gland is made up of the anterior pituitary and posterior pituitary (one lies in front of the other)

REFLEX ACTION

NERVE PATHWAY… Have a go at completing this worksheet

NERVE PATHWAY… answers

SENSORY NEURONTakes impulses from the sense organ to the relay neuron.

RELAY NEURONTakes impulses to the brain and from the brain.

MOTOR NEURONTakes impulses from the relay neuron to the effector.

BRAINReceives impulses from the spinal cord and sends out new impulses.

SENSE ORGAN/ RECEPTORSenses stimuli from our surroundings.

EFFECTORThe muscle that receives the impulse from motor neuron.

SPINAL CORDWhere the relay neurones are found.

Summary/Terms (1/2)accommodation – The reflex reaction that keeps the

lens the right shape to focus light on the retina.

CNS – The central nervous system, consisting of the brain and spinal cord.

iris – The part of the eye that limits the amount of light entering the eye.

lens – The part of the eye that focuses light on the retina.

motor neuron – A neuron that carries electrical impulses from the CNS to muscles and glands.

neuron – A specialized cell that carries electrical impulses.

Summary/Terms (2/2)neurotransmitter – A chemical that diffuses across

synapses to continue an impulse in a connecting neurone.

reaction time – The time taken to respond to a stimulus.

reflex – A fast, automatic protective response.

retina – The part of the eye that contains light receptors.

sensory neuron – A neuron that carries electrical impulses from sense organs to the CNS.

synapse – The gap between two connecting neurons.

STARTER

Name the hormones –write down the names of the hormones secreted by the glands.

Name the hormones – Have you thought of any others?

Time: 10 minutesWorking Groups: Your choice of 1, 2 or 3

STARTER answers

Pituitary gland – FSH, LH (stimulates the release and maturity of follicles during menstruation)

Thyroid gland – Thyroxine (regulates the rate of metabolism)Adrenal gland - corticosteroids and catecholamines including

cortisol and adrenaline and small amounts of testosterone (regulates stress levels)

Pancreas – peptides (regulates the production of shorter active digestive enzymes)

Ovary – oestrogen, progesterone and small amounts of testosterone (regulates the growth of eggs and stabilises the growing foetus during pregnancy)

Testis – testosterone (plays a key role in the health and well-being of the man)