Nervous System (Chapter 37)
The nervous system
- helps seek food and avoid danger
- monitors internal and external conditions
-makes changes for homeostasis
Invertebrate nervous organization
- Osculum (central opening) closure in sponges in response to stimuli
- Cniadarians’ ability to contract and extend bodies, move tentacles and capture prey
- Have a nerve net composed of neurons; some have two such as sea anemones and jellyfish
- Allows major responses in times of danger and other has slower more delicate movements
-Planarians: Bilateral symmetry with ladder like nervous system
- Two ventrally located lateral or longitudinal nerve cords (bundles or nerves) from cerebral ganglia
to posterior
-transverse nerves connect the nerve cords and cerebral ganglia to the eyespots
- Cephalization: having a well-recognized anterior head with a brain and sensory receptors.
- Ganglion: cluster of neurons
- Anterior cerebral ganglia receive sensory information from photoreceptors in eyespots and sensory
cells in auricles
-Lateral nerve cords for rapid transfer of information; transverse nerves keeps movements of two sides
coordinated
NB* Bilateral symmetry and cephalization are two significant trends in the development of nervous
organization
- Annelids (earthworms), Arthropods(crabs), and molluscs(squid) have true nervous systems with brain
and ventral nerve cord.
Vertebrate Nervous organization
- paired cranial and spinal nerves contain numerous nerve fibers
- Central Nervous system: consists of the spinal cord and the brain develops from an embryonic dorsal
nerve tube
- ascending tracts carry sensory information to the brain
Nervous System (Chapter 37)
-descending tracts carry motor commands to the neurons in the spinal cord that control the muscles
-Three regions
-hindbrain: oldest part of the brain. Regulates motor activities below our level of consciousness
(our lungs and heart function when we’re sleeping)
- The medulla controls heart rate and breathing
- Movement, posture and balance: cerebellum
- midbrain: contain the optic lobes, which was originally a center for coordinating reflexes involving
the eyes and ears
- Forebrain: processes sensory information
- Thalamus: receives sensory input from the mindbrain and the hindbrain and passes it to
cerebrum, the anterior part of the forebrain in vertebrates
-hypothalamus: homeostasis, communicates with the medulla and pituitary gland
- cerebrum: highly developed in mammals, integrates sensory and motor input and is associated
with higher mental capabilities
The human nervous system
- Three main functions:
- receives sensory input, performs integration, and generates motor output
- muscle contractions and gland secretions are responses to stimuli received by sensory receptors
- in humans, the CNS consists of the brain and spinal cord. Spinal cord housed in vertebral column.
- PNS: consists of all the nerves and ganglia that lie outside CNS
- paired cranial and spinal nerves are part of PNS
- somatic nervous system has sensory and motor functions that control skeletal muscles.
- autonomic nervous system controls smooth muscle, cardiac muscle, and glands and is divided into
sympathetic and parasympathetic nervous systems.
- The CNS and PNS have to work together to maintain homeostasis
Nervous System (Chapter 37)
Nervous Tissue
-neurons are the functional units of the nervous system: receive sensory information and convery
information to the brain.
Neuroglia serve as supporting cells, providing support and nourishment
Neurons and Neuroglia
- Neurons have three major parts:
- cell body with nucleus and organelles
- dendrites: short highly branched processes that receive signals from the sensory receptors or
other neurons and teansmit them to the cell body
- axon: portion of the neuron that conveys information to another neuron or to other cells. Can be
bundled together to form nerves. Thus, axons are called nerve fibers
- insulating sheet called myelin sheath
- Neuroglia: glial cells; outnumber neurons in the brain
-microglia: help remove bacteria and debris
- astrocytes: provide metabolic and structural support directly to the neurons
- myelin sheath is formed by tightly spiraled neuroglia
NB* in the PNS, Schwann cells perform this function and create gaps called Nodes of Ranvier, while in
the CNS, neuroglial cells called ogliodendrocytes perform these same functions.
Types of neurons
- motor (efferent) neurons take nerve impulses from the CNS to muscles or glands.
- said to have multipolar shape because they have many dendrites on a single axon.
- cause muscle fibers to contract or glands to secrete
- sensory(afferent) neurons: take nerve impulses from sensory receptors to CNS
- Interneurons: occur entirely within CNS. Multipolar in shape and convey nerve impulses between
various parts of the CNS.
- some lie between sensory neurons and motor neurons; can take messages from one end of spinal
cord to other or brain to cord and vice versa
Nervous System (Chapter 37)
- from complex pathways in the brain where processes account for thinking, memory, and
language occur.
Transmission of Nerve impulses
- an electrical potential difference across a membrane is called the membrane potential
Resting potential
- the inside of the neuron sis more negative than the outside
- this difference in polarity is correlated with a difference in ion distribution on either side of the axonal
membrane
- the resting potential is 65 mV
Action Potential
- rapid change in polarity across a portion of an axonal membrane as the nerve impulse occurs
- uses two types of gated ion channels: in axonal membrane, sodium passes through the membrane.
Another path allows potassium.
- gated ion channels open and close in response to a stimulus such as a signal from another neuron.
- threshold is the minimum change in polarity across the axonal membrane that is required to generate
an action potential
- depolarization: the inside of the neuron becomes positive because of a sudden entrance of sodium
ions.
- an action potential only takes 2 milliseconds
Propagation of action potentials
- Saltatory conduction: action potential jumps from node to node
- a fiber can conduct a volley of impulses but only a small number of ions are exchanged with each
impulse.
- as soon as an action potential has moved on, the previous section undergoes a refractory period, so
the sodium ion gates are unable to open.
- thus, the action potential always moves down an axon towards its terminals.
Transmission across a synapse
- a synapse is a region of close proximity between the axon terminal of one neuron and the dendrite of
another
Nervous System (Chapter 37)
- the gap between the neurons is called the synaptic cleft
- neurotransmitters help carry out transmission across a synapse, and are stored in synaptic vesicles.
- excitatory neurotransmitters that use gated ion channels are fast acting.
-other neurotransmitters affect the metabolism of the post synaptic cell and are slow acting.
Neurotransmitters and neuromodulators
- Ach: acetylcholine; Norepinephrine, dopamine, serotonin: present in both CNS and PNS
- Ach: excited skeletal muscle but inhibits cardiac muscle
- NE in CNS is involved in dreaming, waking and mood
- dopamine: emotions, learning, and attention
- serotonin: thermoregulations, sleeping, emotion, and perception
NB* the short existence of neurotransmitters at a synapse prevents continuous stimulation or
inhibition of postsynaptic membranes
- Parkinson: lack of dopamine; Levodopa is used to treat this
- Many drugs affecting the nervous system act by interfering with or potentiaing the action of
neurotransmitters
- neurotransmitters can either mimic or block the receptor sites
- neuromodulators are molecules that block the release of a neurotransmitter or modify a neuron’s
response to a neurotransmitter.
Endorphins: pain killers produced by the brain when there is either physical or emotional stress
Synaptic integration
- neuron is on the receiving end of many excitatory and inhibitory signals
- an excitatory neurotransmitter produces a potential change called a signal that drives the neuron
closer to the action potential and in inhibitory neurotransmitter produces a signal that drives the neuron
farther from an action potential.
-Excitatory neurons have a depolarizing effect; inhibitory signals have a hyperpolarizing effect
- integration is the summing up of excitatory and inhibitory signals from different synapses or at a rapid
rate from one synapse.
- if a neuron receives both inhibitory and excitatory signals, the summing up of these signals may
prohibit the axon from firing.
Nervous System (Chapter 37)
Central Nervous System-Brain and spinal cord
-Brain and spinal cord
- Sensory information is received and motor controls are initiated
- Both are protected by bone:
- Vertebrae
- Skull
- Both are wrapped in 3 protective membranes: Meninges
- Meningitis- Bacteria/virus in the meninges
- Cerebrospinal fluid- spaces between meninges to cushion and protect the CNS
Spinal Cord
- Bundle of nervous tissue enclosed in the vertebral column
- extends from the brain to just below the rib cage
- 2 main functions:
1. Reflex actions
-automatic responses to external stimuli
2. Communication
- between brain and the spinal nerves
Gray and White Matter
-Gray: Cell bodies and unmyelinated fibers
- 2 dorsal and 2 ventral horns surrounding a central canal
- sensory neurons, motor neurons, and SHORT interneurons that connect them
- White: myelinated lon fibers called tracts
- connect spinal cord to brain
- information is continuously passing through
- dorsal (ascending information TO the brain) and ventral (descending infrormation
kjkjhjhjhjhjhjhjhjhjjFROM the brain) cross over --> Left and right hemispheres of brain control opposite
jhgjkgkjgkjghfkjhfhgsides of body
-Paralysis- spinal cord is severed
- Quadriplegia- injury in neck that paralyzes all 4 limbs
Brain
- 4 ventricles (interconnected chambers)
- cerebrospinal fluid is continuisly produced and circulated throughout by flowing out between
meninges
Cerebrum
- Also known as the "telencephalon"
Nervous System (Chapter 37)
- Largest portion of the brain in humans
- coordinates activities of the other parts of the brain
- 2 Parts: Cerebral hemispheres and cerebral cortex
Cerebral Hemispheres
- Longitudinal fissure, creating the left and right hemispheres
- corpus callosum allows communication between the two hemispheres since they receive and
control information for opposite sides of the body
- Also, because RH is more musical, artistic, emotional, spatial relations, and pattern
recognition/ LH deals with math, language, and analytical reasoning
- Sulcus- shallow grooves that divide each hemisphere into lobes
-Frontal lobe- reasoning, judgment, fliter (fire vs. stairs)
-Broca's area- produces speech
-Parietal lobes
- sensory reception and integration
- Temporal lobes
- contain primary auditory areas
- Occipital lobes
- Contain primary visual areas
Cerebral Cortex
- Thin outer layer of gray matter that covers the cerebral hemisphere
- Tens of billions of neurons
- Accounts for: sensation, voluntary movement, and thoughts
- Allows learning, thinking, and memory formation
- 2 primary regions: motor area, somatosensory are
Primary Motor Area
- Voluntary commands to skeletal muscles begin here
- Each skeletal muscle has a a section proportional to its size (i.e. tongue, face, arms, legs)
Primary Somatosensory
Sensory information from the skin and skeletal muscle
Nervous System (Chapter 37)
- Basal Nuclei (part of cerebral cortex): Deep rooted gray matter in white matter. tracts
- Ensures that the proper muscle groups are being activated
- Malfunction is associated with Huntington and Parkinson disease
Other Parts of the Brain
- Diencepholon- Contains the hypothalamus and thalamus
- Thalamus- Receives sensory information from all senses but smell
- comes via cranial nerves and tracts from the spinal cord
- known as the "gatekeeper" - integrates information to the appropriate part in the cerebrum
- arousal of cerebrum, higher mental functions (memory, emotions, etc.)
- Hypothalamus- maintains homeostasis
- controls pituitary gland, creating a link between endocrine system and nervous system
- Pineal gland- Secretes melatonin (Associated with sleep, jet lag, and insomnia)
- Possibly the onset of puberty
- Cerebellum- Largest part of hind brain
- Movement- sensory information from eyes, ears, joints, etc --> motor impulses by way of brain
stem to skeletal muscles
- maintains posture and balance
- Brain stem- contains:
- pons- breathing rate, head movement in response to visual and auditory stimuli (reflex)
- medula- heart beat, breathing, blood pressure
-Reticular Actviating system (RAS)
- complex network of nuclei and nerve fibers
- Receive sensory stimuli and send them to higher centers THEN sends motor
signals to spinal cord
Limbic System
- Complex network of tracts and nuclei
- blends higher level thinking with primitive emotions (i.e. -mental stress increasing BP, eating being
considered pleasurable)
Nervous System (Chapter 37)
- Hippocampus- in temporal lobe
- allows memory formation
-Memory- ability to hold a thought or mind
- Long- term memory- a mixture of semantics (numbers, words) and
of episodic (persons, events)
- skill memory- free of associations to episodic memories (i.e. riding a bike)
- Amygdala- responsible for emotions, especially fear
- fear conditioning and associating danger with sensory
information