Nervous System

The Nervous System• Parts:

– Brain, spinal cord, nerves, sensory receptors• Subdivisions:

– Central Nervous System (CNS) – Peripheral Nervous System (PNS)

• Autonomic – involuntary– Sympathetic – “fight or flight” responses – Parasympathetic – “rest & relax” responses– Enteric- “brain in the gut”

• Somatic - voluntary• Functions…

– Sensory perceptions, mental activities, stimulating muscle movements, regulates secretions of glands

Central Nervous System• Consists of brain

and spinal cord

• The structural & functional center of the entire nervous system which integrates incoming pieces of information & initiates an outgoing response

Peripheral Nervous System

• All bodily nerves– Afferent (sensory)– Efferent (motor)

• All pathways going toward and away from the CNS

Nervous System Organization

Cells of Nervous System• Neurons or nerve cells

– Receive stimuli and transmit action potentials (electrical current)

• Organization– Cell body or soma– Dendrites: input– Axons: output– Myelin or No myelin

• Glia cells ( 5 types)– Support, clean, provide

nutrition and protect neurons

Did you know? There are 100 billion neurons in the brain, but there are about 10 to 50 times that many glial cells in the brain

Types of Neurons• Functional

Classification– Sensory or afferent:

action potentials toward CNS (receives stimuli; could be a special sense organ)

– Motor or efferent: action potentials away from CNS (attached to a muscle or gland)

– Interneurons or association neurons: within CNS from one neuron to another

• Structural Classification– Bipolar: a single axon and dendrite

arise at opposite poles of the cell body. Found only in sensory neurons, such as in the Retina, Olfactory and auditory systems

– Unipolar: a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). These are also sensory neurons

– Multipolar: has numerous cell processes (an axon and many dendrites). These are Motor neurons

More about the neuron… 1 substance effects the Speed of the Impulse!

• Myelinated axons– Myelin protects and

insulates axons from one another

– Not continuous• Nodes of Ranvier• Impulse “jumps” from

node to node• Fast impulse

• Unmyelinated axons– Slower impulse

“Saltatory Conduction”

How Neurons Send An Impulse

• Cells produce electrical signals called action potentials

• Transfer of information from one part of body to another

• Electrical properties result from ionic concentration differences across plasma membrane and permeability

Nerve Impulses – Making the ElectricityResting Vs. Action Potential

Resting Potential•Concentration of K+ higher inside than outside cell•Na+ higher outside than inside

Nerve Impulses: Flow of the Electrical Current

• Propagation of Action Potential– Ions (charged particle)

• Na+ (moves inside only)

• K+ (moves outside only)

– A wave of electrical fluctuation that travels along the plasma membrane; due to changes in chemical concentrations by ions

Action Potentials• Series of permeability

changes when a local potential causes depolarization of membrane

• Phases– Depolarization

• More positive

– Repolarization• More negative

• All-or-none principle– Neuron will fire or it

won’t

The Synapse

• Junction between two cells

• Electrical message transferred across the synapse by chemicals called neurotransmitters

How is an electrical impulse ignited?

One Word:Stimulus• Any change in your

environment.– Temp, sound, smell

• You may or may not respond to a specific stimulus

What receives the Stimulus? Sensory Receptors

• In order for a stimulus to be detected, it must be strong enough to elicit an impulse– It must be at the threshold level- the

minimum stimulus to start an impulse

• The all-or-none response means that either a neuron will fire or it won’t, there is no partial impulse

• Sensation- the brain’s interpretation of what the stimulus is

Classification of Receptors1. Mechanoreceptors- activated by

mechanical stimuli or deformation of the receptor

2. Chemoreceptor- changing of the chemical concentrations around the body

3. Thermoreceptors- detect hot and cold

4. Nociceptors- any stimuli that can cause tissue damage; sensation of pain

5. Photoreceptors- respond to light

Receptors, Stimulus, & Body Actions

• A Stimulus ignites a Receptor• The Receptor receives the message that the Stimulus is sending• The message is sent and a Body Action occurs

• There are three kinds of actions that our body carries out, namely voluntary actions, involuntary actions or reflex actions and conditioned reflex actions. All three actions involve stimuli, an impulse, neurons and effectors. However they do have their differences.

• A reflex is a direct connection between stimulus and response, which does not require conscious thought.

– Voluntary Reflex is one of your own free will or design; done by choice; not forced or compelled.

Body Actions• Voluntary Actions:

A voluntary action is basically an action in which you initiate by your own conscious.

– For example, when you see a friend over at the other side of the room, you wish to attract the attention of that friend of yours, hence you may want to wave your hand and call out his name. This is done by your brain by sending impulses from it to the effectors or in this case, your biceps and triceps muscles and also your larynx, via relaying neurons, the spinal cord, synapes and motor neurons. The impulses upon reaching their respective effector muscles cause the waving of your hand and you to shout out your friend's name. This action is under the control of the will thus is known as a voluntary action.

• Involuntary Actions (Reflex Actions):A voluntary action is under the control of one’s will, involuntary actions as their name suggest, are total opposites of voluntary actions. • In this case, your spinal cord or your brain takes total control, without your own

conscious, depending on where the stimuli originate. Reflex actions controlled by the spinal cord, example scratching, are called spinal reflexes while those by the brain, example blinking, are called cranial reflexes. Blushing, sneezing and salivation are also reflex actions however, salivation is also known an conditioned reflex action.

Reflexes• A reflex is a response to a perturbing stimulus that acts to return the

body to homeostasis. This may be subconscious as in the regulation of blood sugar by the pancreatic hormones, may be somewhat noticeable as in shivering in response to a drop in body temperature; or may be quite obvious as in stepping on a nail and immediately withdrawing your foot.

• Reflexes require a minimum of two neurons, an sensory neuron (input) and a motor neuron (output).

The sensory neuron (such as a pain receptor in the skin) detects the stimuli and sends a signal towards the CNS. This sensory neuron synapses with a motor neuron which innervates the effector tissue (such as skeletal muscle to pull away from the painful stimuli). This type of reflex is the "withdrawal" reflex and is monosynaptic, meaning only one synapse has to be crossed between the sensory neuron and the motor neuron. It is the simplest reflex arc and the integration center is the synapse itself. Example: Tendon Jerk

–

Reflexes• Polysynaptic reflexes are more complex and more common. They

involve interneurons which are found in the CNS. More complex reflexes may have their integration center in the spinal cord, in the brainstem, or in the cerebrum where conscious thoughts are initiated.

In humans: the polysynaptic reflex is the sudden movement to protect life and limb. An example usually given is walking in a shallow pond and stepping on a sharp object. The foot immediately raises before you are voluntarily aware of pending danger. It is more complex than the monosynaptic reflex because prolonged output from the spinal cord is needed to process “Am I in danger? or Am I hurt?” The monosynaptic reflex doesn’t need the process time to initiate a “stretch or jerk”.

Reflex ArcA predictable response to a stimulus which may or may not be

conscious– A reflex consists of either muscle contraction or glandular secretion– Neurons involved in reflex

• Afferent neuron- sensory• Interneuron• Efferent neuron- motor

Process:1) Detection of signals from outside environment or detection of deviation

change) from homeostasis from internal environment.2) Integration of multiple signals from outside and inside to produce

appropriate response.3) Response to counteract stimulus being detected

• Many people consider only the simplest types of responses as "reflexes", those that are always identical and do not allow conscious actions.

• We must not confuse these with "reactions", which are different from reflexes in that they are voluntary responses to a stimulus from the environment.

– For example, while the body has various subconscious physiological responses to mitigate cold, as humans we can simply choose to put on more clothes. This is a conscious order made by the cerebrum, not an involuntary response to a stimulus. This is a very complex response involving millions of neurons and some time to process the voluntary response.

– In contrast, spinal reflexes occur much faster, not only because they involve fewer neurons, but also because the electrical signal does not have to travel to the brain and back. Spinal reflexes only travel to the spinal cord and back which is a much shorter distance. Because of this and the complexity of conscious reactions, they take more time to complete than a reflex. On average, humans have a reaction time of 0.25 seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus. Did your reaction time fall within the average during the Reaction Time Lab?

– Reaction times vary from individual to individual. Because of the higher degree of neural processing, reaction times can be influenced by a variety of factors. Reaction times can decrease with practice, as often times athletes have faster reaction times than non-athletes. Sleepiness, emotional distress, or consumption of alcohol can also impact reaction time.

Reflexes Vs. Reactions

What about the processing/integration center OR better known as the BRAIN!

Organization of the Brain• Hindbrain

– Adjacent to top part of spinal cord

• Midbrain– Rises above hindbrain

• Forebrain– Uppermost region of brain

Hindbrain• Medulla

– Controls vital functions, such as breathing and heart rate

– Regulates reflexes

• Cerebellum– Plays important role in motor coordination

• Pons– Involved in sleep and arousal

Midbrain

• Brain stem– Includes much of hindbrain (but not

cerebellum) and midbrain– Determines alertness– Regulates basic survival functions

• Reticular Formation– Involved in stereotyped patterns of behavior,

such as walking and sleeping

Forebrain: Limbic System, Thalamus, & Basal Ganglia

• Important in both memory and emotion

• Two principal structures

– Amygdala

• Involved in discrimination of objects necessary for survival

– Hippocampus

• Has special role in storage of memories

• Thalamus

– Serves as relay station for information

• Basal Ganglia

– Works with cerebellum and cerebral cortex to control and coordinate voluntary movements

Forebrain: Hypothalamus

• Monitors . . .

– eating, drinking, sex

– emotion, stress, reward

• Helps direct endocrine system

• Regulator of body’s internal state

• Involved in pleasurable feelings

Forebrain: Cerebral Cortex

• Occipital lobes– Responding to

visual stimuli

• Temporal lobes– Hearing, language

processing, memory

• Frontal lobes– Personality,

intelligence, control of voluntary muscles

• Parietal lobes– Registering spatial

location, attention, motor control

Somatosensory Cortex

Located at front of parietal lobesProcesses information about body sensations

Motor CortexLocated just behind frontal lobesProcesses information about voluntary movement

Association CortexMakes up 75% of cerebral cortexIntegrates information

Last but NOT Least…. Spinal Cord

PORTAL for the nerve pathways that carry information from the arms, legs, and rest of the body, and carries signals from the brain to the body.