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.

3.

Synaptic cleft 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 Chemical --rare --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

The Synapse Fig. 8.13

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

Receptors

• •

2 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.

2.

3.

4.

5.

Receptor Sensory neuron Association neuron Autonomic motor neuron 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) 3) Parietal Temporal 4) 5) Insular 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 NERVES

rootlets



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)