Nervous System

FUNCTIONS:

1. Sensory input.

2. Integration.

3. Homeostasis.

4. Mental activity.

5. Control of skeletal muscles.

The Nervous System

Organization of the Nervous System

• Central nervous system (CNS)– Brain and spinal cord

• Peripheral nervous system (PNS)– Neurons outside the CNS

– Sensory division• Afferent fibers transmit impulses from receptors to CNS

– Motor division• Efferent fibers transmit impulses from CNS to effector organs

Relationship between motor and sensory fibers of the PNS and the CNS

Autonomic Nervous System

• Sympathetic– Fight or flight, stress– Excitatory effects elicited by norepinephrine

activating beta receptors– Inhibitory effects elicited by activation of alpha

receptors

• Parasympathetic– Rest and digest– Digestive system activated, heart rate inhibited,

blood vessels dilated– Vagus nerve primarily responsible for

activating parasympathetic responses

Fig. 8.39

Synapse

Specialized site of intercellular communication.

3 Components:

1. Presynaptic terminal

2. Synaptic cleft

3. Postsynaptic membrane

Functional Organization of the Nervous System

The Neuron

Neuroglia

Neuroglia

• Accessory cells of the nervous system• Astrocytes

– Support tissue in the CNS form blood-brain barrier

• Ependymal– Produce and move cerebral spinal fluid

• Microglia– Remove cell debris and bacteria from CNS

• Oligodendricytes and Schwann cells– Provide insulation around axons of CNS and PNS

neurons

Myelinated vs. Unmyelinated Axons

Membrane Potentials

• Nervous system functions by establishing concentration gradients and electrical potentials across the membranes

• The resting membrane potential of a neuron is negative and is said to be polarized

• These gradients are maintained by the sodium potassium pump

Concentration Gradients and Nerve Cell Function

Action Potentials

• Muscle and nerve cells are exciteable

• When a muscle or nerve cell is stimulated Na+ channels open and Na+ rushes into the cell

• This causes a local potential

• This local potential may not result in action potential– Doesn’t cross the threshold

• If the stimulus is sufficient to cause the local potential to cross the threshold an action potential results

• The action potential is the complete depolarization of the cell

• The action potential is an all-or-nothing event– If the local potential meets threshold, the cell totally

depolarizes and the action potential results– If the potential does not meet threshold, no action

potential results

Fig. 8.9

Fig. 8.10

Action Potential Propogation

• Unmyelinated neurons propogate signals more slowly than myelinated neurons

• Myelination acts as an insulator– Electrical signal will jump from node of

Ranvier to node of Ranvier– This is called saltatory conduction– Requires less energy than direct propogation

Propagation of the Action Potential

Synapse

• Electrical --rare• Chemical

--communication occurs in one direction:

presynaptic membrane to postsynaptic membrane

--action potential is not always propagated.

Synapse

Synapses may occur:• neuron to neuron• neuron to another type of cell (neuroeffector)

--neuromuscular junction

--neuroglandular junction

Fig. 8.13The Synapse

Neurotransmitters

--packaged in synaptic vesicles.

Nerve endings of the ANS secrete:• Acetylcholine (ACh)--Cholinergic neuron

– Parasympathetic effector

• Norepinephrine (NE)--Adrenergic neuron– Sympathetic effector

• Neurotransmitters diffuse across the synaptic cleft and bind to receptor on the post-synaptic membrane

• This can cause membrane channels (Na+, K+, or Cl-) to open or close depending on the neurotransmitter

• If stimulatory, Na+ channels will open

• If inhibitory, K+ or Cl- channels will open– Cell becomes more negative, hyperpolarized

Receptors2 types of cholinergic receptors:• Nicotinic

– Preganglionic sympathetic and parasympathetic

• Muscarinic– parasympathetic

2 types of adrenergic receptors:• Alpha

– Generally inhibitory

• Beta– Generally excitatory

Autonomic Reflex Arc

1. Receptor

2. Sensory neuron

3. Association neuron

4. Autonomic motor neuron

5. Visceral effector

Reflex Arc

Knee Jerk Reflex

Converging Circuit

Central Nervous System

Adult:• Brainstem

--medulla oblongata

--pons

--midbrain

• Diencephalon

--thalamus

--hypothalamus

--epithalamus

• Cerebrum

• Cerebellum

Brainstem• Medulla oblongata

– Inferior portion– Regulation of heart rate, venoconstriction,

ventilation, swallowing, , etc..

• Pons– Superior to medulla– Bridge between cerebrum and cerebellum

• Midbrain– Audio and visual processing

Cerebellum

• Integrates motor signals from cerebral cortex with feedback from PNS

• Proprioception

• Learning tasks

Dienchephalon• Thalamus

– Sensory input from PNS passes through thalamus (relay station)

• Epithalamus– Pineal gland – sleep cycle, puberty

• Hypothalamus– Master gland– Attached to pituitary by infundibulum– Controls much of homeostasis by stimulating or

inhibiting pituitary

Cerebrum

Brain Protection:

• cranial bones

• cranial meninges

• cerebrospinal fluid

• neuroglia (astrocytes)

The Brain

CEREBRUM

• Largest part of the brain; thinking part

• Markings:

Gyrus (gyri)-- wrinkle, raised area

Fissure(s)-- deep, wide groove(s)

Sulcus (sulci)-- shallow groove(s)

CEREBRUM

Lobes:

1) Frontal

2) Parietal

3) Temporal

4) Insular

5) Occipital

CEREBRUM

Displays lateralization:

• left hemisphere

language; math/science; reason

• right hemisphere

music/art; spatial relations; insight/imagination

CEREBRUM

• sensory areas

• motor areas

• association areas

Spinal Cord-- Composition

white matter (myelin)

dorsal column

ventral column

lateral column

gray matter (non-myelin)

posterior horn

ventral horn

lateral horn

Spinal Cord-- White Matter

• myelinated axons that travel along the spinal cord.

Ascending-- up cord to higher levels

Descending-- down cord from brain

Across the cord

Spinal Cord

• Dorsal roots (sensory)

• Ventral roots (motor)

combine to form spinal nerve.

• Dorsal Root Ganglion

Peripheral Nervous System

Cranial Nerves

• 12 pr.-- I to XII (anterior to posterior)

3 functions:

1) sensory

2) somatic-- control of skeletal muscle

3) parasympathetic--regulation of glands, smooth muscle, and cardiac

muscle.

Peripheral Nervous System

Spinal Nerves

• 31 pr. 8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

SPINAL NERVESrootlets roots spinal nerve ramus

Dorsal rami

Ventral rami--

Distributed 2 Ways:

Intercostal nerves (T1-T12)

Plexuses (5): cervical plexus (C1-C5)

brachial plexus (C5-T1)

lumbar plexus (L1-L4)

sacral plexus (L4-S4)

coccygeal plexus (S4, S5, Cx)