The Nervous System

BRAIN, SPINAL CORD, AND NERVES

Functions of the Nervous System

• 1. Communication and coordination system in the body

• 2. Seat of intellect and reasoning• The goal is to monitor changes in the

environment inside and outside the body, interpret the changes, and initiate a response in an effort to maintain homeostasis.

• Electrochemical messages called Nerve impulses race through your body every moment, traveling along special routes, or Nerves, at high speeds.

• This is how communication happens

Divisions of the Nervous system

• Two main groups• Central Nervous System (CNS) which includes

the brain and spinal cord• Peripheral Nervous System (PNS) which

includes the nerves and sensory receptors– The PNS is divided into the Autonomic Nervous

System (ANS) which controls unconscious activities and the Somatic Nervous System (SNS) which oversees conscious activities

Nervous tissue

• The functional cells of nervous tissue are called neurons, which receive support from nearby neuroglial cells (connective part)

• Each neuron consists of a cell body and branches. The cell body contains the nucleus and most of the cytoplasm, and the branches include many dendrites which carry impulses toward the cell body, and a single axon which carries impulses away.

Nervous tissue

Nervous tissue

• In many neurons, the axon is covered with numerous neuroglial cells known as Schwann cells, which provide a white-colored protective sheath that is mostly fat.

• This fat layer is called the myelin sheath and it insulates and protects the axon (some axons are nearly one meter – about 3 feet – long).

Types of neurons

• Sensory (afferent) neurons carry nerve impulses from body parts into the brain or spinal cord. They have receptors at the end of their dendrites

• Motor (efferent) neurons carry nerve impulses out of the brain or spinal cord to effectors

• Interneurons (mixed nerves) lie within the brain or spinal cord and link other neurons

Central Nervous System

• The “central station” for incoming and outgoing nerve impulses

• Includes the brain and spinal cord• Both are protected by bones (cranium and

vertebral column) and a thick set of membranes called the meninges (located between the soft nervous tissue and the hard bones, and is several layers thick)

Meninges

• Outer layer is tough, white fibrous connective tissue and contains many nerves and bv - dura mater

• As it continues down the spinal cord it does not attach directly to the vertebrae but is separated by an epidural space (filled with adipose and connective tissue to pad around the spinal cord)

• Middle layer is thin weblike arachnoid (lacks nerves and bv)

• Inner layer is thin and plastered to the nervous tissue pia mater (contains many nerves and bv)

• Between middle and inner is subarachnoid space that is filled with a slightly yellowish fluid called cerebrospinal fluid (CSF)

• CSF is also found in the four ventricles of the brain (small spaces “little bellies” within the brain’s center)

The Spinal Cord

• The spinal cord extends from the medulla of the brain (at the foramen magnum) down the back to L1 and L2 (about 18 inches) where it “splits”

• It is divided into 31 segments and each segment includes a pair of spinal roots, which form spinal nerves as they leave the spinal cord (so there are 31 pairs of spinal nerves)

• It consists of both white and gray matter – gray is in the center of the cord, white is on the outside (opposite of the brain)

• The function of the spinal cord is a reflex center and two-way conduction pathway (ascending and descending tracts)

The Brain

• The brain receives sensory information, interprets and integrates this info, and controls muscle and glandular responses

• Its nerve impulse activity also provides your memory, thoughts, dreams, and personality

• It receives a large blood supply to fuel its constant activity (10s, 30s, few mins, 4-8mins)

• Located in the cranial cavity and weighs 3 lbs

• The brain is composed of both gray and white matter

• Gray matter is mainly neuron cell bodies and dendrites (integrative centers)

• White matter is the axons covered with myelin (carries nerve impulses)

• The brain includes four main parts: cerebrum, cerebellum, diencephalon, and brain stem

The Cerebrum

• Consists of two large masses called cerebral hemispheres (mirror images of each other)

• The hemispheres are connected by a deep bridge of nerve fibers called the corpus callosum

Hemisphere Dominance

FYI

• 90% of the population is left brain dominant – others are right or both equal

• Deep within each cerebral hemisphere are several masses of gray matter called basal ganglia. Their neuron cell bodies serve as relay stations for motor impulses originating in the cerebral cortex and passing into the brain stem and spinal cord. They send impulses that inhibit motor functions using dopamine.

• The surface has many ridges, or convolutions (gyri), separated by grooves. A shallow groove is a sulcus, and deep groove is a fissure.

• The most important functional part of the cerebrum is the cerebral cortex, which is an outer fringe of gray matter (contains nearly 75% of all the neuron cell bodies in the nervous system)

• The cerebral cortex is divided into functional zones known as lobes

The Lobes

• Frontal lobe – controls voluntary muscles (R to L and L to R), emotions, personality, morality, intellect, speech (Broca’s area – you know the word but can’t say it)

• Parietal lobe – receives and interprets nerve impulses from the sensory receptors for pain, touch, heat, and cold. Also helps in determining distances, sizes, and shapes

• Occipital Lobe – visual area, controlling eyesight

• Temporal Lobe – auditory area (upper) and olfactory (smell) area is anterior

Diencephalon

• Located beneath the cerebrum and anterior to the cerebellum

• Means “double brain” and it contains 2 impt areas

• 1. Thalamus – inner chamber, a relay station that redirects nerve impulses to and from the cerebrum

• 2. Hypothalamus – located below thalamus, regulates involuntary activities (list of 7)

• 1. heart rate and BP• 2. body temp• 3. water and electrolyte balance• 4. control of hunger and body weight• 5. control of mvmts and gland secretions• 6. production of substances that stimulate the

pituitary gland to secrete hormones• 7. sleep and wakefulness

Brain Stem

• A bundle of nervous tissue that connects the cerebrum to the spinal cord

• Parts include the midbrain, pons, and medulla oblongata

• Midbrain serves as a reflex center• Pons relays impulses to and from the medulla

oblongata and cerebrum and regulates the rate and depth of breathing

• Medulla oblongata connects the brain and spinal cord and controls vital visceral activities– Heart rate, contract and dilate certain blood

vessels, regulates breathing

Cerebellum

• Located below and posterior to the cerebrum• Coordinates muscle responses, manages

equilibrium, and maintains muscle tone

Cell membrane potential

• The surface of a cell membrane is usually electrically charged, or polarized, with respect to the outside

• This polarization is due to an unequal distribution of positive and negative ions between sides of the membrane

• Important in the conduction of muscle and nerve impulses

• Because of the active transport of sodium and potassium ions, cells throughout the body have a relatively greater concentration of sodium ions (Na+) outside and a relatively greater concentration of potassium ions (K+) inside

• The cytoplasm of these cells has many large negatively charged particles that cannot diffuse across the cell membranes.

• Sodium and potassium ions follow the laws of diffusion and show mvmt from high to low concentration as permeability permits

• The difference in electrical charge between two regions is called a potential difference and in a resting nerve cell this is called resting potential

• When permeability changes in the region of cell membrane that is being stimulated, channels open and allow sodium to diffuse inward

• This mvmt is aided by the negative electrical condition on the inside of the membrane, which attracts the positively charged sodium ions

• Now the membrane loses its negative charge and becomes depolarized

• At almost the same time, membrane channels open and allow potassium ions to pass through and diffuse outward so the inside again becomes negatively charged and repolarized

• This rapid sequence of depolarization and repolarization takes about one-thousandth of a second and is called Action Potential

• www.youtube.com/watch?v=yQ-wQsEK21E• A wave of action potentials moves down the

fiber to the end. This delivers a nerve impulse

The Synapse

• Neurons have the ability to conduct nerve impulses very quickly, but how does one cell communicate with another cell?

• Adjacent neurons communicate by releasing chemicals across tiny gaps that separate them, called synapses (synaptic cleft)

• The chemicals, known as neurotransmitters, are released by a neuron when a nerve impulse reaches its distal end

• The neurotransmitters then diffuse across the synaptic cleft to contact the adjacent cell

• Contact with the next neuron may stimulate it to trigger a nerve impulse, or it may inhibit it (pain meds)

http://www.youtube.com/watch?v=p5zFgT4aofA

• A simple reflex is one in which there is only a sensory nerve and a motor nerve involved

• Classic example is the knee-jerk reflex• Also the withdrawal reflex

The Peripheral Nervous System

• Consists of nerves that branch out from the CNS and connect it to other body parts– Cranial nerves, from the brain– Spinal nerves, from the spineAlso includes ganglia and sensory receptors

• 12 pairs of cranial nerves come from the underside of the brain

• Most are mixed nerves, but some are only sensory (smell and vision)

• <see list in handout>

• 31 pairs of spinal nerves originate from the spinal cord

• They are mixed nerves that provide two-way communication between the spinal cord and the neck, trunk, and limbs

• 8 pairs cervical, 12 pairs thoracic, 5 pairs lumbar, 5 pairs sacral, 1 pair coccygeal

• A network of spinal nerves combine to form a plexus except in the thoracic region

• Ganglia are clusters of neuron cell bodies that are outside the brain and spinal cord, appearance is a swelling or a knot

• These are centers where nerve impulses are passed from one neuron to another across a synapse

• Sensory receptors are nervous structures that respond to temperature, pressure, touch, or pain

• They are embedded in the skin, but also within the walls of some visceral organs

The Autonomic Nervous System

• The portion of the PNS that functions without conscious effort

• Controls visceral functions to maintain homeostasis

• Further divided– The sympathetic system consists primarily of 2

cords, beginning at the base of the brain and proceeds down both sides of the spinal column and has nerves to all the vital internal organs

– This is referred to as the “fight or “flight” system– When the danger passes, the parasympathetic

system will help restore the balance to the body by the vagus and pelvic nerves

– Both systems are strongly influenced by emotions– This leaves us with high levels of “stress

hormones”

The Limbic System

• Amygdala is the integrative center for emotions, emotional behavior, and motivation.

• Just like with the hippocampus, major pathways communicate bidirectionally and contain both efferent and afferent fibers.

• The hippocampus is associated mainly with memory, in particular long-term memory. The organ also plays an important role in spatial navigation.

Limbic system structures

The amygdala and hippocampus play important roles in memory. The amygdala is responsible for determining what memories are stored and where the memories are stored in the brain . It is thought that this determination is based on how huge an emotional response an event invokes. The hippocampus sends memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrieves them when necessary. Damage to this area of the brain may result in an inability to form new memories.

The Reflex Arc

The Reflex Act

• The simplest type of nervous response is the reflex act, which is unconscious and involuntary <examples>

• Every reflex act is preceded by a stimulus (any change in the environment)

• Special structures called receptors pick up these stimuli

• Afferent neurons take the message to the interneuron

• The interneuron interprets the info and decides the action

• The efferent neuron takes the message to the responding organ (effectors)

• Reaction to the stimulus is called the response and if there is mvmt the muscles are the effectors

The Reflex Arc