The Nervous System Chapter 9

Maintaining Homeostasis… Your body has physiological and biochemical mechanisms in place to help it maintain a stable internal environment. Your body has physiological and biochemical mechanisms in place to help it maintain a stable internal environment. Internal body temperature: 37ºC Blood pH : 7.4 Concentration of glucose in the blood: 100mg/mL Blood pressure : 120/70 mm Hg The Nervous System serves as a high- speed communication system that receives and interprets information about changes in your external and internal environment. This information helps it direct your body into making the changes needed to maintain homeostasis. The Nervous System serves as a high- speed communication system that receives and interprets information about changes in your external and internal environment. This information helps it direct your body into making the changes needed to maintain homeostasis.

Structure of the Nervous System Two Main divisions: Two Main divisions: Central Nervous System (CNS) Central Nervous System (CNS) Peripheral Nervous System (PNS) Peripheral Nervous System (PNS)

The Central Nervous System (CNS) Contains the nerves of the brain and spinal cord Contains the nerves of the brain and spinal cord It acts as a coordinating centre for incoming and outgoing information. It acts as a coordinating centre for incoming and outgoing information. Think of this like processor in your computer, it controls everything. Think of this like processor in your computer, it controls everything.

The Peripheral Nervous System (PNS) 12 Cranial nerves 12 Cranial nerves 31 Spinal nerves 31 Spinal nerves These nerves carry information from the organs of the body to the CNS These nerves carry information from the organs of the body to the CNS PNS is further divided into Somatic and Autonomic nerves. PNS is further divided into Somatic and Autonomic nerves.

What does this look like ?

Somatic Nervous System Contains Somatic nerves Contains Somatic nerves Control skeletal muscles, bones, skin and sensory organs Control skeletal muscles, bones, skin and sensory organs The SNS is viewed as voluntary control of your body. The SNS is viewed as voluntary control of your body. Helps keep the body in touch with it’s surroundings. Helps keep the body in touch with it’s surroundings. Therefore: broken down further into sensory neurons and motor neurons. Therefore: broken down further into sensory neurons and motor neurons.

Autonomic Nervous System Contains Autonomic Nerves Contains Autonomic Nerves Contain special motor nerves that control the internal organs of the body Contain special motor nerves that control the internal organs of the body Just like the SNS it too is further broken down into sympathetic nervous system and the parasympathetic nervous system. Just like the SNS it too is further broken down into sympathetic nervous system and the parasympathetic nervous system. Viewed as involuntary control (heartbeat, digestion, respiration) Viewed as involuntary control (heartbeat, digestion, respiration) Acts as an “on-off” switch in your body Acts as an “on-off” switch in your body

ANS Cont’d In other words: they act opposite each other In other words: they act opposite each other - Sympathetic NS -“fight or flight” reaction - prepares the body for danger (stress) - Parasympathetic NS - counters the effects of the Sympathetic NS - directs activities of internal organs when body is at rest - relaxes

Nervous System Breakdown…..

What is a Neuron ? A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling.

But First….Cells in the Nervous System Nervous system contains 2 types of cells: Glial cells – nonconducting cells which are important for structural support (Like glue). Neurons – Functional units of the nervous system (the nerve itself)

Neurons Neurons are subdivided into 3 categories: Neurons are subdivided into 3 categories: 1) Sensory (afferent) – sense & relay info (stimulus) to the CNS (afferent means towards) 2) Interneurons – link neurons within the body 3) Motor neurons (efferent) – relay info to the effectors (ex. of effectors are muscles, organs and glands). (efferent means away from)

Examples of Sensory and Motor Nerves

Remember… Sensory neurons (ex. You touching something hot) send a signal TOWARD (afferent) your spinal cord Sensory neurons (ex. You touching something hot) send a signal TOWARD (afferent) your spinal cord In the spinal cord the signal meets interneurons which relay information to… In the spinal cord the signal meets interneurons which relay information to… Motor neurons carry that signal AWAY (efferent) from your spinal cord, causing the effectors (in this case muscle in your hand to contract and pull away). Motor neurons carry that signal AWAY (efferent) from your spinal cord, causing the effectors (in this case muscle in your hand to contract and pull away).

What are Effectors ? An effector neuron is a gland or muscle (motor neuron) that causes a change or something to happen An effector neuron is a gland or muscle (motor neuron) that causes a change or something to happen They bring about a change They bring about a change

Structure of a Neuron All neurons contain: dendrites, cell bodies and axons. All neurons contain: dendrites, cell bodies and axons. Dendrites: receive information or impulses/stimuli (tree-like structure) Dendrites: receive information or impulses/stimuli (tree-like structure) Cell body: contains a nucleus as well as organelles like all cells. Regular cell processes occur here. Cell body: contains a nucleus as well as organelles like all cells. Regular cell processes occur here. Axon: an extension of cytoplasm that projects nerve impulses from the cell body (usually long and thin) Axon: an extension of cytoplasm that projects nerve impulses from the cell body (usually long and thin)

Structure of a Neuron Cont’d Many axons are covered by a myelin sheath Many axons are covered by a myelin sheath Myelin is composed of a fatty acid and protein Myelin is composed of a fatty acid and protein Formed by Schwann cells (special glial cells) Formed by Schwann cells (special glial cells) Myelin insulates the axon and prevents loss of charged ions from the nerve cell. Myelin insulates the axon and prevents loss of charged ions from the nerve cell. Myelinated Myelinated

Between each myelin sheath is an area called the nodes of Ranvier Between each myelin sheath is an area called the nodes of Ranvier Nerve impulses jump from one node to another Nerve impulses jump from one node to another Therefore, these nodes of Ranvier speed up the movement of nerve impulses Therefore, these nodes of Ranvier speed up the movement of nerve impulses

Did You Know ? More than 100 axons could be placed inside the shaft of a single human hair More than 100 axons could be placed inside the shaft of a single human hair Nerves are made of many neurons held together Nerves are made of many neurons held together

Myelin and Multiple Sclerosis Myelin sheaths (on the axons in the CNS) become damaged Myelin sheaths (on the axons in the CNS) become damaged This leads to demyelinization and scarring This leads to demyelinization and scarring What do you think happens ? What do you think happens ?

The Synapse  The synapse is the junction between the terminal branches of a neuron and another cell.  Impulses are transmitted across this gap  An axon may synapse with 1000 other neurons

Nerve Action – Synaptic Transmission A nerve impulse is both an electrical and a chemical event A nerve impulse is both an electrical and a chemical event Electrical synapse: occurs at “gap junctions” Electrical synapse: occurs at “gap junctions” Chemical synapse: Occurs between two Chemical synapse: Occurs between two neighbouring cells where chemical messengers relay the message to the neighbouring cell.

Nerve Action – Synaptic Transmission We have many more chemical synapses We have many more chemical synapses Not as fast as electrical Not as fast as electrical But are more specific But are more specific Ex. Electrical synapse occur in situations where you require the fastest possible response (defensive reflexes) Ex. Electrical synapse occur in situations where you require the fastest possible response (defensive reflexes)

Chemical Synapses A nerve at rest has a relative negative charge inside and a positive charge outside A nerve at rest has a relative negative charge inside and a positive charge outside K ions are concentrated inside the neuron and Na ions are concentrated outside the neuron K ions are concentrated inside the neuron and Na ions are concentrated outside the neuron

Chemical Synapse Cont’d This neg. charge is maintained by sodium- potassium pumps which control the movement of Na and K ions through the cell membrane. (ATP supplies the energy to fuel the pumps). This neg. charge is maintained by sodium- potassium pumps which control the movement of Na and K ions through the cell membrane. (ATP supplies the energy to fuel the pumps). The membrane is now said to be polarized (due to unequal distribution of positively charged ions inside and outside the nerve cell) The membrane is now said to be polarized (due to unequal distribution of positively charged ions inside and outside the nerve cell)

Movement of the Action Potential So What Happens ? So What Happens ? An external stimulus changes the ability of the membrane to keep out the Na + ions & they rush in An external stimulus changes the ability of the membrane to keep out the Na + ions & they rush in In other words sodium channels open allowing sodium to rush in In other words sodium channels open allowing sodium to rush in Voltage change across the membrane Voltage change across the membrane This is known as an action potential This is known as an action potential

This rapid inflow of Na causes a charge reversal – depolarization This rapid inflow of Na causes a charge reversal – depolarization

This change in charge stimulates adjacent points in the nerve cell & a wave of impulse passes along the nerve cell This change in charge stimulates adjacent points in the nerve cell & a wave of impulse passes along the nerve cell An impulse ends when K ions rush to the outside and repolarize the membrane An impulse ends when K ions rush to the outside and repolarize the membrane

A nerve impulse can travel at the rate of 100m/sec A nerve impulse can travel at the rate of 100m/sec A nerve cell needs time to “recuperate” or become repolarized (return to normal) A nerve cell needs time to “recuperate” or become repolarized (return to normal) This is called the refractory period and usually lasts 1 – 10 ms This is called the refractory period and usually lasts 1 – 10 ms

Saltatory conduction is when the impulse jumps from one node of Ranvier to the next – so this is faster in myelinated neurons Saltatory conduction is when the impulse jumps from one node of Ranvier to the next – so this is faster in myelinated neurons It shortens the distance the action potential has to travel It shortens the distance the action potential has to travel Saltatory Conduction Visual Aid

So then what ? Our action potential is traveling down the axon Our action potential is traveling down the axon It reaches the Synaptic terminal It reaches the Synaptic terminal The impulse causes chemicals called neurotransmitters to be released from the vesicles in the synaptic knobs or (presynaptic membrane). The impulse causes chemicals called neurotransmitters to be released from the vesicles in the synaptic knobs or (presynaptic membrane).

The chemicals travel across the synaptic cleft (gap) and cause an impulse to be started along the postsynaptic membrane The chemicals travel across the synaptic cleft (gap) and cause an impulse to be started along the postsynaptic membrane Synaptic Transmission Video

Neurotransmitter Acetylcholine – Found in many presynaptic terminals Acetylcholine – Found in many presynaptic terminals Acts as an excitatory neurotransmitter on many postsynaptic terminals Acts as an excitatory neurotransmitter on many postsynaptic terminals Causes Na + channels to open and… Causes Na + channels to open and… Tell me what happens…Start with creates a depolarization Tell me what happens…Start with creates a depolarization

What happens to left over neurotransmitter ? It simply cannot remain in the synaptic cleft or Na + channels will stay open It simply cannot remain in the synaptic cleft or Na + channels will stay open Cholinesterase – present, enzyme which breaks down acetylcholine Cholinesterase – present, enzyme which breaks down acetylcholine This causes sodium channels to close and the nerve cell returns to resting membrane potential This causes sodium channels to close and the nerve cell returns to resting membrane potential

Threshold Level Important to note: Important to note: A stimulus must be above a certain level to produce a response (Approx -50mV) A stimulus must be above a certain level to produce a response (Approx -50mV) The all or none response - neurons either fire or not at all ( e.g. like a trigger of a gun) The all or none response - neurons either fire or not at all ( e.g. like a trigger of a gun) Q: What causes a greater intensity of response? Q: What causes a greater intensity of response? A: Not a stronger stimulus, but more impulses (known as summation). A: Not a stronger stimulus, but more impulses (known as summation).

Threshold

Action Potential – How a stimulus moves down a neuron Action Potential Animation

Understanding… On pg. 426 – do questions 3,4,5,7,8,9 and 13 On pg. 426 – do questions 3,4,5,7,8,9 and 13 Helpful if you read pages Helpful if you read pages